RRDI

Rice cultivation

Rice
Rice is the single most important crop occupying 34 percent (0.77 /million ha) of the total cultivated area in Sri Lanka. On average 560,000 ha are cultivated during maha and 310,000 ha during yala making the average annual extent sown with rice to about 870,000 ha. About 1.8 million farm families are engaged in paddy cultivation island-wide. Sri Lanka currently produces 2.7 million t of rough rice annually and satisfies around 95 percent of the domestic requirement. Rice provides 45% total calorie and 40% total protein requirement of an average Sri Lankan. The per capita consumption of rice fluctuates around 100 kg per year depending on the price of rice, bread and wheat flour.

It is projected that the demand for rice will increase at 1.1% per year and to meet the rice production should grow at the rate of 2.9% per year. Increasing the cropping intensity and national average yield are the options available to achieve this production targets.

The current cost of production of rough rice is Rs. 8.57 per kg. The cost of labor, farm power and tradable inputs constitutes 55%, 23% and 23% respectively. The labor cost has risen at a higher rate than other costs over the last few years.

While the global demand for rice will increase at 1.95% the production will increase at 1.62% per annum making the tradable rice volume to be doubled in another 20 years time. As a result the rice price would decline at 0.73% per year. On the other hand the domestic price of rice on par with Thai A1 super (the cheapest in the world market) would be higher by 50 -70 USD per t than the internationally traded rice. This situation will place Sri Lanka under increase pressure to produce cheaper and high quality rice in the coming years.
Climate
Despite its relatively small aerial extent, Sri Lanka exemplifies a variety of climatic conditions depending on the geographical settings of respective locations. The average annual rainfall of the island varies from about 900 mm (Maha Lewaya, Hambantota) to over 5,500 mm (Kenilworth Estate, Ginigathhena). Being located in the low latitudes between 6 and 10 N and surrounded by the Indian Ocean, Sri Lanka shows very typical maritime-tropical temperature conditions. These conditions are characterized by greater daily than annual temperature ranges and moderate average temperatures in comparison with the more continental tropics. Temperature conditions in Sri Lanka are also characterized by a significant temperature decrease in the central highlands according to the vertical atmospheric lapse rate.

Sri Lanka has traditionally been generalized in to three climatic zones in terms of Wet Zone in the southwestern region including central hill country, and Dry Zone covering predominantly, northern and eastern part of the country, being separated by an Intermediate zone, skirting the central hills except in the south and the west (Map 1). In differentiating aforesaid three climatic zones, rainfall, contribution of southwest monsoon rains, soils, land use and vegetation have been widely used. The Wet zone receives relatively high mean annual rainfall over 2,500 mm without pronounced dry periods. The Dry zone receives a mean annual rainfall of less than 1,750 mm with a distinct dry season from May to September. The Intermediate zone receives a mean annual rainfall between 1,750 to 2,500 mm with a short and less prominent dry season.

As low temperature is an important climatic factor affecting plant growth in the Wet and Intermediate zones of Sri Lanka, a sub-division based on the altitude takes into account the temperature limitations in these two climatic regions. In this delineation, the Low-country is demarcated as the land below 300 m in elevation and the Mid-country with elevation between 300 - 900 m while the Up-country is the land above 900 m elevation (Map 2). Both Wet and Intermediate zones spread across all three categories of elevation while the Dry zone is confined to the Low-country resulting seven agro-climatic zones covering the entire island (Map 3). These seven agro-climatic zones have further sub-divided into Agro-Ecological Regions (AER) with a total of 46 AERs covering the entire island (Map 4).

The delineation of AER boundaries of Sri Lanka has been based on the rainfall regime, terrain characteristics, predominant soil type, land use and vegetation so that each AER represents an uniform agro-climate, soils and terrain conditions and as such would support a particular farming system where certain range of crops and farming practices find their best expression.

Detailed studies on climatology of Sri Lanka has identified that "climatic year" or "hydrological year" of the island begins in March and not in January so that seasonal weather rhythm or more specifically the rainfall seasons ranges from March to February. It is generally accepted that there are four rainfall seasons in Sri Lanka:

March - April -- First Inter Monsoon (FIM) rains
May - September --South West Monsoon (SWM) rains
October - November -- Second Inter Monsoon (SIM) rains
November - February -- North East Monsoon (NEM) rains

These rainfall seasons do not bring homogeneous rainfall regimes over the whole island and it is the main cause to exhibit such a high agro-ecological diversity of the country despite its relatively small aerial extent. Out of these four rainfall seasons, two consecutive rainy seasons make up the major growing seasons of Sri Lanka, namely Yala and Maha seasons. Generally Yala season is the combination of FIM and SWM rains. However, since SWM rains are not effective over the Dry zone it is only the FIM rains that fall during the Yala season in the Dry zone from mid March to early May. Being effective only for two months, the Yala season is considered as the minor growing season of the Dry zone. The major growing season of the whole country, Maha begins with arrival of SIM rains in Mid September/October and continues up to late January/February with the NEM rains.

Rice is grown under more diverse environmental conditions than any other major food crop in the world and the situation remains as the same in Sri Lanka too. Except in almost all AERs in the Up country Wet and Intermediate zones where minimum temperature at nighttime is limiting, paddy is the most common land use in valley bottoms in the all other AERs of the country. Solar radiation is not a limiting factor for rice growth in almost all rice growing regions of Sri Lanka. However, when all other conditions such as water, nutrients and temperature are non-limiting, the intensity of sunlight may determine the yield level depending on the location and season. For example, in the Wet zone, solar radiation may limit the rice yield during Yala season due to high cloud cover arising from the southwest monsoonal circulation whereas a similar situation could expect in the Dry zone during Maha season due to overcast conditions that may result due to weather systems formed in the Bay of Bengal and northeast monsoonal circulation.

Climate of the Low Country Wet Zone

This agro-climatic region has been sub-divided in to five AERs (Map 4) where rice is the main land use in inland valleys and flood plains. The expected annual rainfall at the 75% probability level in this region ranges from 1,700 to 3,200 mm depending on the agro-ecological region (Table 1). Its average maximum temperature ranges from 32 to 35 0C. The highest values are being recorded during the period of late February to early May. The average minimum temperature is ranged from 22 to 24 0C where the lowest values are generally observed during the period of December to February, the winter months of the island. The day time relative humidity is generally ranged from 60 to 75 percent where as nighttime values may reach even up to 90 per cent at anytime of the year.

Table 1. Agro-ecological regions of Low Country Wet Zone

Agro-ecological Region

Expected annual rainfal-mm

Major land use

WL1a

> 3,200

Tea, Rubber, MHG, Paddy, EAC

WL1b

> 2,800

Rubber, MHG, Paddy

WL2a

> 2,400

Rubber, Tea, Coconut, MHG, Paddy, EAC

WL2b

> 2,200

Rubber, Coconut, MHG, Paddy

WL3

> 1,700

Coconut, Fruits, MHG, Paddy

MHG: Mixed Home Gardens, EAC: Export Agricultural Crops

Climate of the Mid Country Wet Zone
Even though this agro-climatic region has been sub-divided in to six AERs (Map 4), rice is predominantly found only in four AERs (Table 2). The two AERs located in higher elevations, namely WM1a and WM1b are not suitable for rice as cool injuries are likely to occur. The expected annual rainfall at the 75% probability level in this region ranges from 1,400 to 3,300 mm depending on the agro-ecological region. Its average maximum temperature ranges from 27 to 33 0C. The highest values are being recorded during the period of late February to early May. The average minimum temperature is ranged from 18 to 22 0C where the lowest values are generally observed during the period of December to February, the winter months of the island. The further low values of night temperature are likely to experience in higher elevations of the region (i.e., WM1a and WM1b regions). The day time relative humidity is generally ranged from 55 to 80 percent where as nighttime values may range from 75 to 85 per cent.

Table 2. Agro-ecological regions of Mid Country Wet Zone

Agro-ecological Region

Expected annual rainfall - mm

Major land use

WM1a

> 3,300

Tea, Natural forests

WM1b

> 2,900

Tea, Natural forests MHG,

WM2a

> 2,200

Tea, MHG, EAC, Natural forest, Paddy

WM2b

> 1,800

MHG, Paddy, EAC, Tea

WM3a

> 1,600

MHG, Paddy, EAC, Tea

WM3b

> 1,400

MHG, EAC, Tea, Paddy, Rubber

MHG: Mixed Home Gardens, EAC: Export Agricultural Crops

Climate of the Up Country Wet Zone
This agro-climatic region has been sub-divided in to four AERs (Map 4) and rice is hardly found in this region. As the elevation of this agro-climatic region is well above the 900 m, low temperature has become a limiting factor for growth of rice plants.

Table 3. Agro-ecological regions of Up Country Wet Zone

Agro-ecological Region

Expected annual rainfall - mm

Major land use

WU1

> 3,100

Tea, Forest plantations, Natural forests

WU2a

> 2,400

Tea, Forest plantations,

WU2b

> 2,200

Tea, Forest plantations, Vegetables

WU3

> 1,800

Tea, Vegetables, MHG, Forest plantations

MHG: Mixed Home Gardens

Climate of the Low Country Intermediate Zone
This agro-climatic region has been sub-divided in to five AERs (Map 4) where rice is the predominant land use in valley bottoms and terraced upland slopes in some areas. The expected annual rainfall at the 75% probability level in this region ranges from 1,100 to 1,600 mm depending on the agro-ecological region. Its average maximum temperature ranges from 29 to 35 0C. The highest values are being recorded during the period of late February to early May. The average minimum temperature is ranged from 20 to 26 0C where the lowest values are generally observed during the period of December to February, a common phenomenon for the entire island. The day time relative humidity is generally ranged from 55 to 75 percent where as nighttime values may reach even up to 90 per cent especially during winter months of the year.

Table 4. Agro-ecological regions of Low Country Intermediate Zone

Agro-ecological Region

Expected annual rainfall - mm

Major land use

IL1a

> 1,400

Coconut, MHG, EAC, Paddy, Rubber

IL1b

> 1,100

Coconut, Paddy, MHG, EAC

IL1c

> 1,300

MHG, Rubber, Paddy, Sugarcane

IL2

> 1,600

MHG, Paddy, RUC, Scrub, Sugarcane, Citrus

IL3

> 1,100

Coconut, Paddy, MHG,

MHG: Mixed Home Gardens, RUC: Rainfed Upland Crops, EAC: Export Agricultural Crops

Climate of the Mid Country Intermediate Zone
Although this agro-climatic region has been sub-divided in to eight AERs (Map 4), rice is being cultivated only in five AERs (Table 5). In these AERs rice is the major land use in valley bottoms and terraced slopes at least in one season out of two seasons in a year. In the other season, farmers may switch in to vegetable cultivation depending on the land suitability. The expected annual rainfall at the 75% probability level in this region ranges from 1,100 to 2,000 mm depending on the agro-ecological region. Its average maximum temperature ranges from 28 to 330C. The highest values are being recorded during the period of late March to early May. The average minimum temperature is ranged from 18 to 23 0C where the lowest values are generally observed during the period of December to February. The day time relative humidity is generally ranged from 55 to 75 percent where as nighttime values are generally around 75 to 85 per cent.

Table 5. Agro-ecological regions of Mid Country Intermediate Zone

Agro-ecological Region

Expected annual rainfall - mm

Major land use

IM1a

> 2,000

Tea, Vegetables, MHG, Paddy, Forest plantations

IM1b

> 2,000

Natural forests, MHG, Paddy, Grasslands

IM1c

> 1,300

Natural Forests, Vegetables

IM2a

> 1,800

EAC, MHG, Tea, Vegetables

IM2b

> 1,600

Natural forests, MHG, Paddy, Tea, Vegetables

IM3a

> 1,400

MHG, EAC, Paddy

IM3b

> 1,200

MHG, EAC, Rubber, Vegetables, Paddy

IM3c

> 1,100

Vegetables, Tea, MHG, EAC

MHG: Mixed Home Gardens, EAC: Export Agricultural Crops

Climate of the Up Country Intermediate Zone
Although this agro-climatic region has been sub-divided in to seven AERs, rice is being cultivated only in two AERs due to limitation of the temperature regime in the rest of AERs owing to their relatively higher elevations. In those two AERs rice may be the major land use in valley bottoms during minor rainy season (Yala). In the other season (Maha), farmers may switch in to high value temperate vegetable crops, especially for potato to harness the potential of low temperature regime prevailing in these regions. The expected annual rainfall at the 75% probability level in those two regions ranges from 1,400 to 1,600. Its average maximum temperature ranges from 22 to 290C. The highest values are being recorded during the period of late March to September. During the said period, high winds that blow from the southwest direction is a common weather phenomenon to experience in this region. The average minimum temperature is ranged from 13 to 18 0C where the lowest values are generally observed during the period of December to March. Hence, low temperature injuries in rice plants could be a recurrent problem if rice is grown in those regions during the major rainy season, Maha season. Relative humidity during day time in this agro-climatic region is generally ranged from 60 to 82 percent where as nighttime values may reach even up to 90 per cent especially during winter months of the year.

Table 6. Agro-ecological regions of Up Country Intermediate Zone

Agro-ecological Region

Expected annual rainfall - mm

Major land use

IU1

> 2,400

Tea, EAC, Natural forests, Forest plantations

IU2

> 2,100

Tea, Vegetables, MHG, Natural forests, MHG, Forest plantations

IU3a

> 1,900

Tea, Forest Plantations,

IU3b

> 1,700

Tea, Natural forests, Forest plantations

IU3c

> 1,600

Tea, Vegetables, Paddy

IU3d

> 1,300

Tea, Vegetables, Forest plantations, Natural forests

IU3e

> 1,400

Tea, Vegetables, Paddy, MHG

MHG: Mixed Home Gardens, MHG: Mixed Home Gardens

Climate of the Low Country Dry Zone
This agro-climatic region is the countrys driest part and it has been sub-divided in to 11 AERs. Even though water is a limiting factor in this part of the country for year round crop production, trans-basin diversion of some rivers of Wet and Intermediate zones and large number of tanks that were built during ancient times have made it possible to cultivate lowlands in to rice or rice based cropping systems. Out of 11 AERs in this region, rice is the predominant agricultural land use in 10 AERs except in DL3 AER (Table 7), the Oxisol belt which spreads from northwestern coastal region to northern peninsular (Map 4). The expected annual rainfall at the 75% probability level in this region ranges from 650 to 1,100 mm depending on the agro-ecological region. In some AERs monthly rainfall distribution depicts a bi-modal pattern where as AERs found in the northeastern and eastern parts of the Dry zone shows a uni-modal monthly rainfall distribution. Hence, unless irrigation water is supplied, cultivation of rice in lowland in those regions is possible only during the major rainy season (Maha season).

When the Wet zone of Sri Lanka experiences Southwest monsoon rains, the same monsoonal wind blows over the Dry zone as a warm and dry wind, a Fhn like wind locally known as Yal Hulang, Wesak hulang or Kachchan. Hence, crop water requirement during this period, May to September (Yala season) is very much higher than that of the other times of the year (Maha season). The general wind speed of the Dry zone is 3 5 km/hr. However, during said period, it may reach even 12 15 km/hr. The average maximum temperature in the Dry zone ranges from 29 to 38 0C depending on the AER. The highest values are being recorded during the period of late February to late September irrespective of the location. Thus, high temperature injuries are being experienced in rice grown during Yala season in the Dry zone, commonly known as the Ehela Pussa. Continuous weather observations have shown that it is becoming a more and more common feature in rice cultivation during recent times and it could be a repercussion of global warming. The average minimum temperature is ranged from 20 to 26 0C where the lowest values are generally observed during the period of December to February, a common phenomenon for the entire island. However, further low nighttime temperatures are experienced during winter months in the northern peninsular of the island due to the influence of the huge land mass of the Indian sub-continent making it possible to grow potato. However, rice is hardly grown in this region due to some other edaphic limitations. The day time relative humidity in the Dry zone is generally ranged from 50 to 75 percent where as nighttime values may reach even up to 90 per cent, especially during winter months of the year.

Table 7. Agro-ecological regions of Low Country Dry Zone

Agro-ecological Region

Expected annual rainfall - mm

Major land use

DL1a

> 1,100

MHG, Paddy, Forest plantations, Scrub, Sugracane, Natural forests,

DL1b

> 900

Rainfed Upland Crops, Paddy, Scrub, MHG, Forest plantations

DL1c

> 900

RUC, Paddy, Scrub, Natural forests, Forest Plantations, Sugarcane

DL1d

> 900

RUC, Paddy

DL1e

> 900

RUC, Paddy, Scrub

DL1f

> 800

RUC, Paddy, Scrub, Natural forests

DL2a

> 1,300

RUC, Paddy, Natural forests, Sugarcane, Scrub

DL2b

> 1,100

Paddy, RUC

DL3

> 800

Cashew, Coconut, Condiments, Scrub, Natural forests

DL4

> 750

Scrub, Paddy, RUC

DL5

> 650

Scrub, Natural forests, RUC, Paddy

MHG: Mixed Home Gardens, MHG: Mixed Home Gardens, RUC: Rainfed Upland Crops
Recommended Varieties
Five to Six month age group

Variety by mat.duration

Year released

Pedigree

Recommended for

Maturity duration (days)

Higher yield Recorded (t/ha)

Attributes

H-9

1968

C104/Mas//Panduruwee

ML 1

50-180

3.5

PS

Bg 3-5

1973

Panduruwee/Mas//Engkatek

ML

150-180

5.5

PS

Bg 407

1981

IR5/Panduruwee

ML

150-180

7.5

PS, resistant to BB

Bg 745

1981

71-554/Podiwee A8

ML

150-180

6.0

PS, samba grain

Bg 38

1981

Engkatek//H-4/Podiwee A8

ML

150-180

6.0

PS, samba grain

Four to Four & half month age group


Variety by mat. Duration

Year released

Pedigree

Recommended for

Maturity duration (days)

Higher yield recorded (t/ha)

Attributes

H-4

1958

Murungakayan 302/Mas

GC

135

4.5

Wide adaptability, Red Pericarp, resistant to BL

H-8

1966

H-4/Podiwee A8

GC

135

5.5

PS

Bg 11-11

1970

Engkatek/*2 H-8

GC

135

6.5

Samba grain

Bg 90-2*

1975

R262/Remaja

GC

120

8.5

High yield

Bg 400-1

1980

Ob 678//IR20/H-4

GC

130

8.5

Wide adaptability, resistant to iotype I and BB moderately tolerant to iron toxicity

Bg 379-2

1980

Bg 96-3*2/Ptb33

GC

135

8.5

Resistant to BPH and BB

Bg 380

1982

Bg 90-2*4/Ob 677

MI/DI

120

10.0

Very high yield, resistant to GM-I

Bg 450

1985

Bg 12-1*2/IR42

GC

130

6.0

Samba grain resistant GM-I

Bg 403

1983

83-1026/Bg 379-2

GC

120

8.0

WP resistant to BL, BLB

Bw 78

1977

H-501/Podiwee A8*2 H-5

LCWZ, Iron toxic soils

135

5.0

WP samba and ill drained soils rice resistant to BL, tolerant to salinity

Bw 100

1979

H-501/Podiwee A8/*2H-5

LCWZ, Iron toxic soils

135

6.0

WP samba grain, resistant to BL and Bronzing

Bw 451 (Bw 297-2)

1987

Bg 400-1/Bg 11-11

LCWZ, saline soils

135

6.0

WP replacement for Pokkali

Bw 400 (Bw 272-8)

1987

Bw 259-3/Bw 242-5-5

120

RP salinity, blast

Bw 452 (Bw 85)

1992

Hondarawala 502/C104

GC

135

5.0

RP an old improved variety, but higher response to added nitrogen and resistant to lodging tolerant to iron toxicity and submergence in LCWZ

Bw 453 (Bw 293-2)

1992

IR 2071-586/Bg 400-1

LCWZ

135

7.0

WP MR to leaf blast, R to GM-I and tolerant to iron toxicity

Ld 66

1971

H-501/Deo-Geo-Woo-gen

Iron toxic soils and acidic soils

135

5.0

WP more suitable for broadcasting

MI 273

1971

Gamma irradiated H4

GC

135

RP (H4 Dwarf) to BL. Has all desirable characters of H4. Dwarf, non lodging

At 401 (At 69-5)

1992

Bg 94-1/Pokkali

Costal saline areas

120

5.0

RP replacement for pokkali

At 405 Lanka Samurdhi

1997

At 402/Basmathi 442

Dry and Intermediate zones with assured supply of water

120

5.6 t/ha

MR to BPH Basmathi grain quality Long/Slender, aromatic


Three and half month age group

Variety by mat. Duration

Year released

Pedigree

Recommended for

Maturity duration (days)

Higher yield recorded (t/ha)

Attributes

H-7

1964

PP/ Mas//H-5

GC

105

3.5

Good grain quality

Bg 34-6

1971

IR8-246///PP/Mas//H-501

GC

105

6.5

RP

Bg 94-1

1975

IR 262/Ld 66

GC

105

8.5

High yield WP

Bg 94-2

1978

IR262/Ld 66

GC

105

8.5

High yield WP

Bg 350

1986

Bg 94-1///Bg 401-1/80-3717

GC

105

8.5

RP resistant to GM-I

Bg 352

1992

Bg 380/Bg 367-4

GC

105

6.0

Resistant to BL and BPH WP Intermediate bold type grains

Bg 357 (Bg 1639)

1997

Bg 797/Bg 300//85-1580/Senerang M-17

Islandwide cultivation

106

9.55

Resistant to BPH, Gall midge Biotype 2, MR to thrips, R/MR to Blast, MR to bronzing (Iron toxicity), NR to low temperature WP L/M

Bg 358

1999

Bg 12-1 / Bg 1492

GC

106

9.5

samba, resistant to BPH, BL and BLB, moderately tolerant to iron toxicity

Bg 359

1999

Bg 12-1/Bg 1492

GC

105

9.5

samba, resistant to BPH, GM 1 and 11, MR to thrips, R/MR to blast, MR to iron toxicity and low temperature, white pericarp, L/M

Bg 360

1999

88-5089/Bg 379-2

WZ

105

7.0

Resistant to GM 1 and 11, BPH, BL and BLB, moderately tolerant to Iron toxicity

Bw 266-7

1981

Bw 242-5-5//Ob 677/"Bg 90-2

As a regional release

105

4.5

Non lodging, WP long for Ratnapura slender) quality grains, resistant to GM-I

Bw 267-3

1981

Ld 125/Bw 248-I

LCWZ, Iron toxic soils

105

4.5

White pericarp, long slender grains, resistant to BL, Iron toxicity and seed spotting

Bg 351

1986

Bg 90-2/Bg 401-1

Bw 351 (Bw 288-1-3)

105

5.0

RP moderately resistant to sheath blight and iron toxicity

At 16

1977

IR 8/H-4

Southern province

105

RP resistant to lodging and BL

At 353 (At76-1)

1992

Bg 94-1 (R)/Bg 400-1//Bg 94-1

Saline area

90

6.5

RP MR to Blast and BB, good for potential acid, saline conditions

At 35
(At69-2)

1992

Bg 94-1/Pokkali

Saline areas

105

5.0

WP salline resistant, resistant to lodging

Ld 355

1994

Bw 451/IR50

Southern Province

105

4.5

Samba grain, resistant BL, BLB, WP

Ld 356

1994

Bw 451/Bw 351

Kalutara and Galle districts

100

4.5

Short round grain/moderately tolerant to iron toxicity resistant to seed spotting and rice GM

At 353 (At76-1)

1992

Bg 94-1 (R)/Bg 400-1//Bg 94-1

Saline area

90

6.5

Red pericarp, MR to Blast and BB, good for potential acid saline conditions found in Nilwala scheme


Three month age group

Variety by mat. Duration

Year released

Pedigree

Recommended for

Maturity duration (days)

Higher yield recorded (t/ha)

Attributes

H-10

1969

PP/Mas//H-5

GC

90

3.0

RP

62-355

1969

PP/H-5

RF/M

90

3.0

RP tolerant to drought

Bg 34-8

1971

IR 246///PP/Mas//H-501

GC

90

6.5

High yield

Bg 276-5

1979

Ob 678/*2 Bg 34-8

GC

90

7.0

Resistant to GM-1,

Bg 300

1987

Bg 367-7//IR 841/Bg 276-5

GC

90

7.0

Resistant to GM-1, BPH, BL and BB

Bg 301

1987

1280/H-4

RF/DI

90

6.0

RP tolerant to drought, BL and BB

Bg 304

1993

Co 10/IR 50//84-1587/Bg 731-2 GC

GC

85

7.4

WP resistant GM, BL and BLB

Bw 272-6B

1981

Bw 259-3/Bw 242-5-5

LCWZ (suitable for mineral, half bog and bog soils)

90

4.0

RP replacement for Herath Banda, Batapolal, resistant to BL, resistant to lodging

Bw 302 (Bw 272-3)

1987

Bw 259-3/Bw 242-5-5

Salines and acid

90

Salinity and acid sulphate soils

At 303
(At 77-1)

1990

At 66-2/Bg 276-5

GC

90

5.0

RP resistant to BL

Bg 305

1999

Bg 1203/Bg 1492

GC

90

8.0

White pericarp, resistant to GM-1 and 11, BPH, BL and BLB

Two & half month age group
Variety by mat. Duration
Year released
Pedigree
Recommended for
Maturity duration (days)
Higher yield recorded (t/ha)
Attributes
Bg 750
1981
Ainantsao//75-1870/PP
LCIZ
70
3.0
Ultra-short maturity
Crop Establishment

Land Preparation

Land preparation is defined as the physical preparation of soil to develop edaphic environment conducive for optimum plant growth after seed or seedling establishment . It also facilitates root growth, mixing of soil layers to increase availability of residual fertilizer, incorporation of residue to minimize weed growth, increasing soil organic matter content and reduce percolation losses of water and nutrient.

Paddy land preparation is basically divided into two as wet and dry land preparation. In wet land preparation the rice fields are first flooded with water before tillage while in dry land preparation the soil is not flooded before tillage .

The conventional land preparation practiced by majority of Sri Lankan rice farmers typically involves three steps namely primary, secondary and tertiary tillage. Draft buffalo, four or two wheel tractor, or a rotovator are used as power sources in land preparation.

Primary tillage

Initial plowing is done to deepen the rhizosphere and to incooperate weeds and crop residues to soil. The moisture content of the soil determine the time of tillage. The tillage is done when the soil has the correct water contact and is friable for plowing or rotovating and harrowing where soil moisture content reduce 25-50% of the field capacity. Since continuous cropping and use of inproper plowing equipment have created a sandy shallow root zone plowing to a depth of about 20 cm is recommended.

Moldboard plough fixed to four or two wheel tractors, disc plough, animal and manual power are used for this operation. Soil condition, amount of weeds and crop residue and farmers requirement determine the number of tillage to be done and the type of plough to be used. If the time period between tillage and crop establishment is extended tillage should be repeated to prevent soil compaction. Moldboard plough is used for deep plowing and disc or conventional plough is used for medium tillage. Deep plowing is important to control perennial weeds like “Atawara“ (Panicum repens Linn.) and Purple nutsedge “ Kaladuru” (Cyprus rotandus L). Primary tillage is done 10-14 days before the secondary tillage.

Secondary tillage

The objective of secondary tillage is to break the big clumps of soil remained after secondary tillage into small clumps and to incooperate the weeds germinated late and remaining crop residues to the soil. This is done across to the direction of primary tillage by using two wheel tractor or animals. Secondary tillage involves clearing , repairing and replastering of bunts and done 7-10 days before crop establishment.

Tertiary tillage

This involves pudlling and leveling. Tillage of flooded soil is referred to as puddling. Soil puddling destroys soil structure, which reduces percolation rates and loss of water. This results low porosity and permeability and formation of a soil plow pan. During this operation, basal fertilizer is added and soil is puddled several times until the fertilizer is mixed to the soil well. Puddling is very efficient in clay soils that form deep cracks penetrating the plow pan at about 15 to 20 cm soil depth during the period of soil drying before land preparation. Although puddling reduces percolation rates of the soil, the action of puddling itself consumes water.

Puddling is followed by leveling to have a good seedbed suited for plant growth. A well-leveled field is a prerequisite for good water and crop management. When field is not level, water may stagnate in the depressions whereas higher parts may fall dry. This results uneven crop emergence, early growth, ,fertilizer distribution and possibly additional weeds. Effective land leveling will improve crop establishment and care, reduce the time and water required to irrigate the field and ensure more uniform distribution of water in the field. The Field is then kept for one day before sowing or planting until fine clay particles migrate downward and fill up the cracks and pores in the plow pan.

It takes around 3-4 weeks to complete the and preparation by conventional manner while the labor consumption is also high. The main limiting factor is the consumption of large amount of water. It is estimated that conventional land preparation consumes 20-40% of the total water requirement of the crop. Due to these reasons farmers in some rice growing areas of Sri Lanka shift from conventional land preparation to non puddle land preparation methods such as minimum or zero tillage.

Minimum Tillage

In minimum tillage practice land preparation is kept to the minimum necessary for crop establishment and growth, thereby reducing time, labor and fuel costs and damage to soil structure. Primary tillage or row tillage is done and crop residues are kept in between rows. The bunds are not cleaned and weed control is done regularly by using herbicides. To compensate poor germination and seed lost by birds and rodents, 150-250kg/ha of seed paddy is used either as in dry or wet basis.

Zero Tillage

No Land preparation is practiced while pre emergent herbicides are used to control weeds before sowing. The seed paddy soaked in water for 48 hours and incubated with 24 hours are broadcasted to the moisten soil on the onset of rain. A higher seed rate (150-250 k/ha) iis used to maintain preferred pant density. Seven to ten days after germination fields are saturated with water and selected herbicides are used to control weeds later on.

Table 1. Advantages and disadvantages of different land preparation methods

Method of land preparation

Advantages

Disadvantages

Conventional

Good root growth

Water retention ability is high

High labor use

Time consumed

Need lot of water

Minimum and zero tillage

Save time and labor cost

Need less water

Soil degradation low

Poor seed germination

Need high seed rate

High use of herbicides

Crop Establishment Nursery management

Crop Establishment

Crop establishment can be defined as the establishment of seeds or seedlings in the field I order to grow it as a crop. Method of establishment of rice can be broadly divided into direct sowing of pre germinated or ungerminated seeds and transplanting of seedlings. The choice of the method of establishment depends on factors such as,

  • age of the variety
  • availability of moisture
  • climatic conditions
  • availability of inputs and labor

Broadcasting and transplanting are the main methods of crop establishment practiced in Sri Lanka (table 1).

Distribution of the Method of Establishment of Rice in Sri Lanka

2011/12 Maha

2012 Yala

Broadcasting

91 %

93%

Transplanting

9%

7%

Direct sowing

This method is becoming more popular among rice farmer as it is economical than transplanting. The yields are also comparable with transplanted rice if crop is properly managed. Direct seeding methods could be divided into Wet seedling and Dry seeding.

In Wet seeding pre-germinated seeds are broadcasted into puddled and leveled field which are free from standing water. At the time of puddling basal fertilizer mixture is added. After germination of seed, seedling desiccation due to water stress is avoided by intermittent wetting of the field. When the seedlings are of about 5 cm tall (about a week after sowing) water is impounded to prevent germination of weeds and desiccation of the seedlings. The stand establishment by this method varies with the quality of land preparation, weed competition, water management and the rainfall during the initial period after sowing. Row seeding of germinated seeds could also be done but it is practiced in limited scale because of the cost and the difficulty in obtaining implements. This method of sowing will help controlling weeds, especially mechanical control and management of the crop. This system will also help to maintain optimum density of seedlings whereas random broadcasting often lead to low or high seedling density. Selection of a suitable variety for direct seeding is important as there is a genotypic variability in germination under submerged conditions. However, if field can be maintained at or below field capacity for about 5 days, focus should be on varieties which process good initial seedling vigor. Seedling vigor is mainly determined by the seed quality and other cultural practices. Stand establishment is often poor with direct seeding because of poor quality seed paddy, poor land preparation, weed competition, poor water management, unfavorable environmental conditions and physical damages. Therefore seed rates should be adjusted accordingly to have the desired panicle number.
Components of yield could be divided into panicle number, seeds per panicle and seed weight. Panicle number is mostly determined by the tillering ability of a variety which is a function of the number of seedlings per unit area. In general a healthy crop of new improved rice variety, under optimum condition, should bear about 350-400 panicles per sq., meter. Thus seed rate should be adjusted accordingly to meet this requirement. A variety with a seed weight of about 23-25g/1000 seeds (medium grain size) have a seed rate of about 100 kg/ha. Seed rate decreases with seed weight thus, small grain type varieties ("Samba") have lesser seed rates (75-80 kg/ha). Decreasing seed rate would increase unproductive tillering. Increasing seed rate would also increase density, which increases unhealthy seedlings with small panicles due to competition for resources, and increase susceptibility to pest and diseases. A uniform plant density can be maintained by uprooting high density seedlings (thinning out) and transplanting them in places having low plant density.

Seeds can also be sown as ungerminated dry seeds in Kakulan or Manawari sowing. In this method dry seeds are sown to dry soil either in rows or in random. Seed rate generally vary with the severity of the environment and the type of physical damages to the seeds. Depending on the level of weed infestation in dry seeded rice the seed rate also varies from 150 kg/ha to 300 kg/ha. However if conditions for rice seed germination and subsequent operations are favorable the seed rate for dry seeding could be reduced.

Water seeding is newly introduced method of establishment in which 48 hours soaked and 24 hours incubated seed paddy is broadcasted to puddle soil with standing water. The seeds are slowly get deposited with a thin layer of mud and excess water is removed a day after the establishment. This method helps to protect the sown seed paddy from birds and to protect from lodging because of the deep root system developed.

Seedling broadcasting

The 12 days old seedlings with a ball of mud raised in parachute trays are sown uniformly throughout the puddle and leveled field. The seed paddy requirement is 20-30 kg/ha. Since the root system is not damaged there is no transplanting shock and seedlings are quickly established in the field. The seedling density is maintained 30-35/m2 for maximum yield. This method saves seed paddy and labor cost for 50% than that of in transplanting. It also produces high number of tillers while the harvest can be obtained 7-8 days before.

Transplanting

The extent of transplanted rice is decreasing due to the scarcity of labor and other resources and the decrease cultivation of 4-4½ month rice varieties. Transplanting will also decrease rice plants ability to withstand moisture stress. Transplanting is generally recommended for 4 - 4 ½ month varieties and if 2½ or 3 month variety is transplanted it should be planted with young (12-14 days old) seedlings. The physical and bio-chemical factors would set a minimum and maximum age for a particular nursery. Minimum age of a seedling for transplanting would be about 12-14 days. For a three month age crop seedling age should not increased beyond 15 days while for a 4 - 4 ½ month crop it is about 21 days. Seedling age of a dapog nursery should not exceed 14 days.

Transplanting is also recommended when land preparation is not up to the standard and water management is poor. It has been reported that transplanting increase the yield of long age varieties when compared with broadcasting because transplanting reduces the excessive build up of vegetative biomass due to transplanting shock. When row transplanting is practiced, spacing between hills vary with the age of the variety. A spacing of 20 x 20 cm2 and 20x15 cm2 is recommended for a long age (4-4 ½month) and short age (3-3 ½ month) varieties. The spacing of a 2½ month variety is 15x15 cm2. A hill should be planted with 2-4 healthy seedlings. Row transplanting is done either by hand or by using a transplanting machine. Ability to maintain an optimum plant density, ability to use weeder and easy application of fertilizer and chemicals are the advantages of row transplanting. If random transplanting is practiced hill density of about 25-30 seedlings /m2 for 4-4 ½ month varieties and 30-35 seedlings /m2 for 3-3 ½ month varieties is optimum.

For transplanted rice seedling age is the major factor in determining yield. Transplanting shock, which is the set back of growth due to uprooting and replanting of seedling, increases with the increase age of seedling and with decrease age of the variety. In general the effect of transplanting on yield increases with decreasing age. Seedling age (in calendar days) also vary with the environmental condition and the type of nursery.

The tiller numbers that can be obtained from transplanted rice get retard if transplanting depth exceeds 4-5cm since the internodes are underneath of the soil. In order to obtain a maximum number of tillers transplanting at a depth of 2-3 cm is recommended.

Nursery Systems

Raising seedlings for transplanting could be done in either wet bed, dapog or dry bed methods. For mechanical transplanting seedling boxes could be used. The choice of a particular nursery system depends on the availability of water labor, land and agricultural implements. Success in raising healthy seedlings depends on constant supervision of the seed bed and constant supervision.

Wet-Bed Method

Wet seed bed nursery is mainly used in areas where water is adequate for nursery establishment. Before sowing of germinated seeds, soil is thoroughly puddled and leveled and construct drainage canals between seed beds for proper removal of water. Addition of organic manure (decomposed) and small amount of inorganic fertilizer as basal dressing will increase easiness of uprooting of seedlings and seedling vigor. Total seed bed area is about 1/10 of the area to be transplanted and requires about 50 kg of seed paddy per ha. Seed rate is adjusted for small grain varieties. Nursery site is kept without shade and with adequate irrigation and drainage facilities. Quality rice seeds is soaked in clean water for a minimum period of 24 hours and incubate in a warm dry place for about 48 hours. Sprouted seed is then broadcasted uniformly on the nursery bed. Before seeding the nursery is drained completely. Thereafter nursery is maintained in moist condition for about 5 days. Once the seedlings are established, the nursery is impounded with water and raised the level gradually. The best stage of transplanting seedling is about 15-21 days. Nursery is maintained free from weeds, any pest or disease incidence and nutrient deficiencies. If such conditions occur it must be treated at the nursery level. The advantages of wetbed nursery are rapid growth and easy uprooting of seedlings.

Dry-bed method

This system of nurseries are prepared in dry soil conditions. Seed beds of convenient dimensions are prepared by raising the soil to a height of about 5-10 cm. A thin layer of half burnt paddy husk could be distributed on the nursery bed mainly to facilitate uprooting. In this method dry or in just sprouted seeds are sown in rows, which are about 10cm apart. Sowing of seeds could also be done as random but random sowing should be discouraged as the weed control is difficult. The site is kept free of shade and with adequate irrigation facilities. Nursery area should be about 1/10 of area to be transplanted. Seed rate is higher than for wet-bed (about 150 kg/ha) as the germination could be lower. Uprooting of seedlings is done between 15 - 21 days after germination. Nursery is needed to maintain without any moisture stress. A basal fertilizer mixture could be applied and incorporated between rows if the soil nutrient supply is low. The advantage of this method is that seedlings are short and strong, has longer root system than wet bed and can be raised even during heavy rains which are not possible with wet bed. However roots may get damaged during pulling. Seedlings of upland nurseries may also get infected with blast and are more prone to pests such as rodents etc.

Dapog method

Dapog nurseries could be located anywhere on a flat surface. However, if low land paddy field is used, water supply/control should be very reliable. Area needed is about 1/20 of the transplantable land which is much smaller than conventional nurseries. Seed rate is about 125 kg/ha. Seed bed is leveled and make the centre slightly higher than the edges to permit water to drain off the surface. The surface is covered with either banana leaves with the mid rib removed, poly ethylene sheets or any flexible material to prevent seedling roots penetrating to the bottom soil layer. Cemented floors can also be used for this purpose. The seed bed is covered with about 1/4" layer burnt paddy husk or compost. Pre-germinated seeds are sown uniformly on the seed bed to a thickness of 2-3 seeds. The germinating seeds are splashed with water and pressed down by hand or with a wooden flat board in the morning and afternoon up to 3-4 days to prevent uneven growth. Too much watering is prevented. More frequent irrigation is necessary if seed were sown without the bedding. The nursery can be transplanted in 12-14 days after germination of seeds. The advantage of the "dapog" over wet/dry bed nursery is that less area is needed and the cost of uprooting of seedling is minimal. However since the seedlings are small transplanting is difficult. Very young seedlings from dapog nurseries are subjected to less transplanting shock than of other nurseries, thus these seedlings are more suitable for short aged varieties. Other disadvantage of dapog seedling was the field should be very well leveled and free of water since the seedlings are very short. For mechanical transplanting, nurseries should be about 1.2m wide (may vary with the type of transplanter). A sheet of polythene is placed on the leveled nursery bed and a compost layer to a height of 1.5 - 2cm is placed on it. Sprouted seeds are then sown to a density of 700-1000 g/m2. Irrigation is done to prevent water stress. Seedlings are ready for transplanting after 14 days.

Parachute nursery

The parachute nursery is established in parachute trays in a size of 56 x 34 x 2cm (length x width x height). About 760-765 trays consisted with 434 small hills and are required to cultivate one hectare. The nursery can be established as a drybed or a wetbed nursery. A flat land having sufficient sunlight and accessibility to water supply is used to established a drybed nursery. The trays filled with ¾ of its hills with clay are placed in two rows. Either dry or 24 hours soaked and 24 hours incubated seeds are placed 2-3 seeds/hill and then a thin layer of soil is spread on it. If needed water is added and then the trays are covered with coconut or banana leaves to protect them from rain and also to conserve moisture.

A part of the field can be used to prepare a wetbed parachute nursery. The size of a nursery is about 10cm height and 75cm width with an appropriate length. It should be kept for 1-2 days before the trays are placed and the seeds are placed in the same manner as drybed nursery.

The coconut or banana leaves are removed after 2-3 days and water is added if needed. The 12 days old seedlings can be used for sowing .The seed requirement is 20-30 kg/ha. Less cost for seedpaddy and labor and the ability to supply fertilizer into the nursery are the advantages of parachute nursery.


Water management
Rice (Orysa sativa L) is cultivated either as a rain fed or as a supplementary or fully irrigated crop. The system of rice cultivation mainly depends on the available rainfall and its distribution. In general, except in semi arid areas where rice cultivation is marginal, average rainfall in rice growing areas of Sri Lanka can meet at least part of the water requirement for a rice crop during it cropping season. Thus in strict terms there is no fully irrigated rice cultivation in Sri Lanka. Rice is a semi aquatic plant and does not need standing water for a successful rice crop. However, uncertainty of water supply, either through irrigation or rain, and to reduce weed infestation rice is always cultivated as a crop with standing water. Response of the rice plant to water stress is varied with its growth stage and other agronomic practices. Direct sown rice crop is less prone to drought than a transplanted crop. Highest water use is during the preparation of land, thus land preparation with minimum timing and maximum use of rain water at the correct time of the season is recommended.

Effect of water deficit

Stress has been defined as "any environmental factor capable of inducing a potentially injurious strain in plants". Water is a major constituent of tissue, a reagent in chemical reaction, a solvent for and mode of translocation for metabolites and minerals within plant and is essential for cell enlargement through increasing turgor pressure. With the occurrence of water deficits many of the physiological processes associated with growth are affected and under severe deficits, death of plants may result. The effect of water stress may vary with the variety, degree and duration of water stress and the growth stage of the rice crop. Water requirement is low at the seedling stage. Unless there is severe water stress the effect during this stage could be recovered. Water stress during vegetative stage reduces plant height, tiller number and leaf area. However, the effect during this stage is varied with the severity of stress and age of the crop. Long duration varieties cause less yield damage than short duration varieties as long vegetative period could help the plant to recover when water stress is relieved. Leaf expansion during vegetative stage is very sensitive to water stress. Cell enlargement requires turgor to extend the cell wall and a gradient in water potential to bring water into the enlarging cell. Thus water stress decreases leaf area which reduces the intercepted solar radiation. Rice leaves in general have a very high transpiration rate thus under high radiation levels rice plant may suffer due to mid day wilting.

Rice plant can transpire its potential rate even when soil moisture was around field capacity. Therefore maintaining saturated water regime through the crop duration is best for saving water and increasing grain yield. However, if the weed pressure is high maintaining standing water until the closure of the canopy and then maintaining saturated soil conditions could increase water saving without reduction in yield. Soil cracking should be prevented to reduce percolation during subsequent irrigation. In general rice plant uses less than 5% of the water absorbed through roots from the soil. The rest is lost through transpiration which helps to maintain leaf energy balance of the crop. Decrease leaf water potential closes stomata and decrease transpiration which in turn increase leaf temperature. Stomatal closure could be due to the accumulation of Abscisic acid which is a drought tolerant mechanism. Even though closure of stomates improves water use efficiency under water stress conditions this decreases carbon assimilation due to reduction in physical transfer of CO2 molecule and increase leaf temperature reduces the biochemical processes. Decrease solubility of CO2 may also reduce CO2 assimilation. Translocation of assimilates could also decrease under desiccating conditions due to reduction is source and sink capacity and decrease water potential. There is a marked genotypic variation in rooting pattern in rice in response to water stress. Drought resistant varieties possess deep and thick roots in contrast to this and shallow roots. Thick roots in rice are positively correlated with xylem vessel area are vital to the conductance of water from soil to the upper parts of the plant to meet the evaporative demand. It has also observed that water stress reduces the uptake of nutrients which could be due to the fact that most of the elements are absorbed via roots through passive diffusion. Direct seeded rice are more tolerant to water stress than transplanted rice which could be due to its superior root system. Increase N nutrition increases the susceptibility of the rice plant to water stress. Rice is most susceptible to water stress during reproductive stage. Water stress at or before PI reduces panicle number most, but all stresses regardless of crop stage or duration significantly reduce panicle number. Water stress after PI reduces the potential spikelet number.

Water stress at heading reduces rate of exertion of the panicle. Anthesis and ripening stages are the most sensitive stage for water stress. Water stress during anthesis increased unfilled spiklets. Spikelet sterility decreases with decrease leaf water potential during meiotic stage of pollen development. Mild stress affect sink more than source, whereas severe stress affects both. Stress during grain filling decrease translocation of assimilates to the grain which decreases grain weight and increase empty grains. Increase canopy temperature above 280C due to water stress linearly increase relative spikelet sterility. The ability of a plant to grow satisfactorily when exposed to periods of water stress is called drought resistance. Mechanism of drought resistance in rice could be either tolerance, avoidance, escape or recovery. The "True" drought avoiding plants could posses mechanisms to maintain favourable water status, either by conserving water or by their ability to supply water to above ground organs even during drought stress. Root depth is a plant trait most strongly related to drought avoidance in upland rice culture which is an avoidance mechanism. Rice plant that can escape or evade drought through the adjustment of the life cycle is also an important trait for Drought resistance. Leaf rolling or reduced leaf area, stomatal closure and delayed flowering under water stress conditions compared to well watered condition could be escape mechanisms. Tolerance implies the plant tissues to withstand water stress. The degree of tolerance in rice vary among varieties and among growth stages within a variety. Osmorogulation in certain varieties of rice is a tolerance mechanism. Recovery of a rice plant after relieving drought stress is varied with the variety, the severity of stress and growth stage.


Excess water effects

To be developed

Water requirement of a rice crop in Sri Lanka
Water requirement for a successful rice crop varies with the method of land preparation, method of crop establishment and duration of the rice crop. It also varies with the soil, environmental conditions and the management of the subsequent rice crop.


Method of water loss

Water is lost through evaporation (E) from free water surface, transpiration (T) from the crop, seepage and percolation of the soil, bunt leakages and runoff from the field. Seepage and percolation vary with the edaphic environment which could be partially controlled through proper management. However, evapotranspiration is determined mainly by the vapor pressure deficit and the canopy size which is beyond the control of a farmer. Bund leakages and runoff from the field is totally under the farmers’ control. Therefore the main determinants of water requirement (WR) are evapotranspiration, seepage and percolation (S & P) rates, which could be summarized as follows.

WR = E + T + (S + P)

Water requirement for Land Preparation

Water requirement for land preparation could be minimal with dry land preparation which is popularly known as "Kekulan" or "Manawary" system of cultivation, which needs little or no supplementary moisture. However, majority of rice is cultivated as lowland crop. The duration of land preparation mainly determine the amount of water required which is dependent on the type of land class and the weed infestation. Water requirement for lowland land preparation is determine by the amount required for soil soaking, losses during operations and maintaining standing water in the field. Water requirement for soaking the land depends on the initial soil moisture content and surface conditions of the land and soil type. The requirement may vary from 30 mm. to 125 mm. of water as there may be losses through cracks and other ways. After first ploughing field is inundated with water to keep the soil and weeds under water which facilitates decomposition. During the period when standing water is maintained on the surface, water is lost through evaporation, seepage and percolation. Average rate of evaporation in a sunny day in the Dry Zone during "Maha" is about 3.5 mm and during "Yala" is about 6 mm. Seepage and percolation rates are highly variable depending on the soil type (porosity), topography and depth to the water table. Reddish Brown Earth (RBE) soil has an average S & P rate of 7-10 mm/day and Low Humic Gley (LHG) soils it is around 3-4 mm/day. Therefore to maintain standing water or to keep the soil saturated, water should be supplied to meet the S, P and evaporation requirements. Thus the water requirement increases with the increase in duration of land preparation. A minimum period of two weeks is required for conventional method of land preparation. In general water requirement for land preparation in the dry zone of Sri Lanka vary from 150 mm on LHG to 300 mm on RBE

Water requirement during crop growth

Water is lost from a rice field mainly through evapotranspiration, seepage, percolation, surface runoff & bund leakages which could vary depending on crop, environment and the field management factors. Evapotranspiration from a rice crop canopy is a function of the size of the crop (leaf area), water availability and the environmental conditions. Evapotranspiration increases with increase leaf area. Variation in rice crop ET during its growth is shown in fig. 1. Evapotranspiration is low at early stages of crop growth and achieve maximum towards heading. Hence the frequency of irrigation should increase accordingly towards flowering to meet the increasing demand for water. Experiment conducted at Agriculture Research Station, Mahailluppallama showed that the total ET in the dry zone during in Yala season in higher than during Maha season (Table 1).

Table 1.Total Evapotranspiration (mm) form a 4 1/2 and 3 month rice crop during Yala and Maha seasons at Mahailluppallama

Method of estimation

Evapotranspiration per season, mm

4 1/2 month

3 1/2 month

Yala

Maha

Yala

Maha

Experimentally determined ET

830

455

-

-

*Calculated ET

770

520

465

665

  • Calculated using modified Penman method using long term average climatic values.
Seepage and percolation losses
Rates of seepage and percolation, when compared with ET which is relatively stable in a given period within a given agro-ecological region with uniform climate, vary very much from place to place. Seepage and percolation rates are mainly govern by the profile characteristics and topography and are much greater in sandy than clay soils. It increases with increase depth of standing water. The rate of S & P is about 6 mm/day in well drained and 3 mm/day in poorly drained soils. In general RBE soils have greater S & P compared to LHG. Further dry land preparation increases S & P rates due to increase porosity, hence puddling decrease S & P by clogging the pores and forming a hardpan below the plough layer. Poorly constructed bunds and crab holes increase seepage.


Total water requirement for lowland rice

Total water requirement for lowland rice increases with the age of rice crop and could be summarized as follows. Water requirement (WR) per season = Sum of daily ET + Sum of daily S & P As suggested earlier S & P rates are highly variable between locations thus WR varies accordingly. Experiments conducted under controlled situations at Agriculture Research Station, Mahailluppallama suggest the following total WR for the Maha season (Table 2).

Table 2. Total water requirement for a rice crop at ARS, Mahailluppallama during Maha season

Soil type

Age of the crop

3 month

4 month

RBE moderately drained

1057

1232

LHG

948

1128


Irrigation requirement and frequency

Water loss through ET, S & P should be supplemented by either natural means such as rain, and seepage from adjoining plots or through irrigation. If an average of 5 mm of water is lost per day by ET, and about 3 - 6 mm/day by seepage and percolation from poorly drained and well drained soils respectively, a total of 8 to 11 mm of water is lost per day from a low land rice field. If irrigation water could be supplied to a depth of about 7.5 cm per issue, irrigation frequency should be maintained at 7 to 10 days interval. When initial water height in the field is lower, frequent irrigation is needed. However, in this system of irrigation field will be kept without standing water towards later days after irrigation. If soil moisture level drops below field capacity, subsequent formation of soil cracks increase

Irrigation systems in Sri Lanka

Water for rice culture in Sri Lanka is received through rainfall or through irrigation. In areas where rain fall distribution during the season is satisfactory to meet the crop water requirement of rice culture, crop is raised completely as rainfed crop. In this case crop depends on direct rainfall to the field and seepage and run off from surrounding areas. There is no properly constructed system of channels for directing of distribution of water. Dykes are constructed to retain water in the field and they are maintained well to prevent water leakages. In areas where rainfall is not assured to supply water requirement of the crop, supplementary irrigation is provided through distributory channel systems from tanks and anicuts. These irrigation networks essentially designed for rice culture are divided into two main categories based on command area namely (1) Minor irrigation system (2) Major irrigation system by the Irrigation Department. The minor irrigation systems are the systems where command and area is less than 80 ha. Both tank and anicut systems are included. The major irrigation systems are with command areas greater than 80 ha. They also include both tanks (reservoir) and anicut systems. Minor irrigation systems These systems come within the justification of Agrarian services department. Since the command area is comparatively smaller and distributory channelled lengths are shorter, better regulation can be expected. The water availability in these systems depends on the catchment area rainfall tank capacity and the size of command area. Major irrigation systems came within the authority of either the Irrigation Department of Mahaweli Authority of Sri Lanka. The tanks and streams which are used for anicut systems depend on their own catchments for water in many systems. However, some tanks are benefitted by water diverted to them from other catchment through trans-basin channels. The water supply under these reservoirs is more assured than the tanks which depend on their own catchments. The distributor channeled systems in these systems are better equipped with control structures than in the minor irrigation schemes. Hence somewhat controlled water management practices have been introduced into these systems. Water is issued mostly on a pre scheduled rotation in major tank systems.

Problems related to water management

Salinity Development in Paddy fields Wrong water management practices cause salinity built up in paddy fields, Observations show that lack of surface drainage is the main cause of salinity development in Sri Lankan paddy fields. Seepage and runoff water which collects in depressions in inland scape evaporate, leaving salts dissolved in them causing salinity built up. Collection of water in these depressions or low lying areas can be due to purposeful blocking of drainage ways or by mere negligence by the farmers. Improvement of drainage will correct the problem. Iron toxicity Iron toxicity is a problem largely found in the rice soils of intermediate zone and up and low country Wet Zone soils. The problem is commonly observed in flat valleys and its occurrence is mainly confined to those positions in the flat valley where interflow streamlines from adjacent landscape emerge within the valley. Interception of this interflow is a water management practice that can alleviate the problem. This can be achieved by digging drains at the boundary between paddy land and adjacent highland.

Water management in relation to other practices

Water management in relation to weed control
It is not an exaggeration that total success of rice weed control is a function of better water management. Abundance, composition and temporal distribution of weeds in rice fields are controlled by the depth and duration of water availability. Most of the weed seeds are highly sensitive to soil moisture and standing water. Usually, optimum soil moisture regime for weed seed germination is below the saturated conditions. Increasing soil moisture above saturated levels progressively reduces the seed germination and maintenance of standing water for one to two inches can arrest more than 90% of the potential weed emergence. On the other hand, periodic wetting and drying of rice soil provides an ideal soil moisture condition for a prolific weed growth. Therefore, maintaining standing water right from the inception of crop establishment is and effective method to reduce weed growth in rice. In transplanted rice where seedlings are fairly tall, an effective level of standing water can be maintained right from the planting. In fact, post planting weed competition could be completely eliminated in transplanted rice through management of water. In broadcast rice, however, standing water can only be maintained in rice when the seedlings are at least 7/8 days old. Water management in relation to plant disease control Moisture on foliage or standing water in the field is very important condition for fungal and bacterial disease occurrence and development. Fungal spore germination requires a moisture film on the plant surface. High relative humidity is essential in maintaining this leaf wetness that often occurred through condensation. Since a normal paddy cultivation provides above conditions it is very difficult to use water management methods for disease control. However, prevention of rice field submerges by stormy rain water could prevent out-break of bacterial blight, bacterial leaf streak and sheath blight epidemics. On the other hand upland dry soil condition completed with cool weather condition favour occurrence and development of blast disease.

Mitigation options

Field water requirement for a rice crop depends mainly on the growth duration of the crop and its growing environment. It is calculated that about 30-40% of the total water supplied to an irrigated crop is often supplied before the establishment of the rice crop and the amount is dependent on the soil drainage class, weed density and time taken for land preparation. Time taken for land preparation could be minimised to about 2 weeks using total killing herbicides (e.g. Paraquat) which also would help to reduce one tillage operation and conserve irrigation water. Dry sowing could be an alternative for the well drain or sandy soils where water use is very high. Mulching straw after seeding could conserve moisture which facilitate early and uniform germination and suppress weeds to a certain extent. However, poor plastering of bunds and "not puddling" the field would increase subsequent water use due to rapid percolation and lateral seepage. The potential of existing rainfall for growing rice in underutilized. Timely cultivation with maximum utilization of rain water has a tremendous potential for increased rice production. It will also maximize irrigation water use efficiency. Initial land preparation with the onset of rains when soil is moist could not only conserve irrigation water but also help to plough deep into the soil and facilitate growing a longer duration rice crop without exposing to terminal drought. Selection of an age class to suit the available water would increase the field irrigation water use efficiency.

In general lowering the age decreases the water requirement for paddy but at the expense of yield. Cost of land preparation and other agronomic practices would be the same or higher except a small decrease in use of fertilizers and pesticides with short age varieties. However, short growing season demand better weed control and optimum timing which could increase cost of production. Very short duration (75 days) varieties (Bg 750) could be used in drought prone areas to avoid terminal drought but potential yield of such varieties are rather low (about 70 bu/ac). These varieties could be used as an escape mechanism. Similarly Kakulan type varieties with good weed competitive ability (e.g. 62-355) could also be used in areas with short growing season. Scientists have been unsuccessful so far in developing varieties for drought avoidance or tolerance due to its complexity and difficulty in combining those desirable traits. New techniques in breeding could be a solution to these problems. One reason farmers keep rice fields continuously flooded is to keep down weeds, which complete less well with rice under such conditions and also as an insurance against moisture stress. Minimizing percolation and seepage losses by proper land preparation and plastering of bunds could keep standing water in the field for a long time which help in both conserving irrigation water and keep weed pressure low.

Rice does not require standing water to maximize yields. Maintaining saturated condition could save up to 40% of water in clay loam soils (IRRI, 1995) without yield reduction, however weed control should be made through chemical, mechanical and manual means. Failure to maintain saturated condition (drying) could increase soil cracking which could increase percolation through soil cracks. Weed control by chemicals would eventually be an alternative with scarce water and labour, however risk of development of weeds resistant to herbicides, human health and environment hazards and cost could increase with the increase usage of herbicides. New frontier research is ongoing in many parts of the world to kill weeds by infecting their own natural pathogens. Suppress growth using allelopathic activity against weeds. New plant types to smother weeds.
Insect Pest Management

MAJOR INSECT PESTS AND MITE PESTS OF RICE

On the basis of the extent and severity of the damage, the following insects are considered as major pests of rice in Sri Lanka (RRDI, 1996).


Rice Thrips (RTH): Stenchaetothrips biformis (Bagnall) (Thysanoptera: Thripidae)

A pest of young rice seedlings. Adult and nymphs suck the cell sap from the leaf tissues. Damaged leaves roll inwards along the margins, feeding cause leaf drying resulting poor crop growth. The damage is severe under water stress conditions. Late planted crops are more prone for damage. Short duration traditional rice varieties like Dahanala, Kaluheenati, Kalubalawee are resistant to thrips. Higher trichome density on leaf surface found to be responsible for thrips resistance in rice. Effective control methods available:

  1. Submerge infested crops intermittently for 1-2 days.
  2. Drag a wet cloth on the seedlings
  3. Apply recommended insecticides if difficult to control
  4. For endemic areas use a recommended seed-dressing formulation

Brown Plant Hopper (BPH): Nilapavata lugens (Stal) (Homoptera: Delphacidae)

Heavy infestations produce symptoms of hopper burn. Leaves dry and turn brown after insect feeding, and patches of burned plants are often lodged. It is a vector of grassy stunt and ragged stunt virus diseases. The rice plant is most sensitive to attack at late vegetative and reproductive stages.

The economic threshold for BPH at booting stage is 2 per hill and at heading 5 per hill. Since spiders are considered major predator of BPH, the economic threshold levels are adjusted according to the number of spiders present. Number of effective predators and parasites are known.

Ptb 33, a variety with a high level of resistance to BPH, is extensively used in the breeding program. A number of varieties with moderate level of resistance to BPH have been developed: Bg 379-2, Bg 300, Bg 403, Bg 304, Bg 357, Bg 358, Bg 360.

Effective control methods available:

  1. Cultivate resistant varieties.
  2. Draining the paddy field to reduce moisture help prevent BPH multiplication.
  3. Indiscriminate use of insecticides during vegetative stage known to cause BPH outbreaks. Use insecticides only when and where needed during vegetative stage especially for the control of leaf eating caterpillars.
  4. Monitor crop regularly for signs of early BPH infestations.
  5. Select a safer insecticide if required.

Safer and effective insecticides are available for use during epidemics

Yellow Stem Borer (YSB): Scirpophaga incertulas (Lepidoptera: Pyralidae)

The caterpillars bore into the rice stem and hollow out the stem completely. Attacked young plant shows dead heart and older plants show white heads. Often plants break where the stem is hollowed out causing lodging.

Serious out breaks of YSB are very rare. Resistant varieties are not available. Effective insecticides are available for YSB control.

Rice Leaffolders (RLF): Cnaphalocrocis medinalis; Marasmia spp. (Lepidoptera: Pyralidae)

The caterpillars infest the leaves and feed on the mesophyll. They fasten the edges of a leaf together and live inside the rolled leaf. Feeding reduces productive leaf area that affects plant growth. Cloudy and humid weather, shady locations and high N-fertilizer favor pest build up.

Control measures available

  1. Establish crop at recommended plant spacing
  2. Use recommended dose of N-fertilizer
  3. Monitor crop regularly. ELT
  4. ETL 25% of leaves showing > 50% damage
  5. Use safer IGR for control

Rice Gall Midge (RGM) Orseolia oryzae (Diptera: Cecidomyiidae)

coleoptern predatory beetle have been identified.
Severity of damage is related to the crop growth stage of attack. The larvae move down between the leaf sheaths until they reach the apical bud or one of the lateral buds. They lacerate the tissues of the bud and feed until pupation. The feeding cause formation of a gall called a "Silver" or "Onion" shoot. Galls terminate the tiller development and hence affect rice yield.

Gall midge damage is high in wet humid weather. As such gall midge infestation is high in dry and intermediate zones in maha and in the wet zone during yala.

Resistance available in varieties like Ptb 18/ Ptb 21 and Eswarakora has been used to develop a number of improved varieties with resistance to gall midge. Since biotype development on resistant varieties is common breeding for resistance is difficult. The gall midge biotype detected after 1986 is termed as biotype II. Varieties resistant to biotype II are: Bg 304, Bg 357, Bg 359, Bg 360.

Control methods:

  1. Granular insecticides are recommended for gall midge control. Since farmers use granules after observing damage symptoms it is difficult to obtain a good control with granules.
  2. Cultivation of resistant varieties in endemic areas is the most economical method.

Paddy bug (PB) Leptocorisa oratorius (Hemiptera: Alydidae)

Sucks the developing grains causing empty or partially filled grains. Both nymphs and adults damage the grains. Damage estimated to reduce 3-5% rice yield in the country.

Paddy bug can feed and reproduce only on rice. Adults and mature nymphs can feed and survive on alternate weed hosts.

A number of predators and egg parasitoids have been identified. Gryon nixoni is the most common egg parasitoid found in Sri Lanka.

Rice sheath mite(Panicle rice mite): Steneotarsonemus spinki smiley (Acari : Tarsonemidae)

Sheath mite can be considered as a serious pest in rice crop causing 5% - 90% yield losses.It is microscopic in size.Sheath mites live in the space between leaf sheaths and feed on the adaxial surface of leaf sheath and developing kernels.High infestation at booting stage of the crop resulted erected by the presence of chocolate brown colour lesions on leaf sheaths.Sheath mite damage the crop indirectly by rectoring pathogenic fungi such as sheath rot causing sarocladium oryzae.

Effective control methods available

  1. Destroy infected stubbles by plugging just after harvesting or by burning.
  2. Sun drying of seed paddy before soaking.
  3. Weed management
  4. Monitor the crop from early booting stage for chocolate color lesions on leaf sheaths.
  5. Single application of recommended insecticides if the signs have been observed.
  6. Crop rotation with leguminous crops such a meeng
  7. Follow the fied for 2-3 seasons if the problem was not controlled with the above measures.


Insecticide recommendations for rice pest control

Chemical Groups/ generic name/ formulation and recommended pest

Organophosporous

Diazinon 5% GR

RWM, RGM, RSB

Diazinon 500 g/l EC

RWM, TH, RB, RFC

Carbamate

Carbosulfan 200 g/l EC

TH, RSB, RB

Fenobucarb 500 g/l EC

BPH, RFC

Neonicotinoids

Imidaclorprid 200 g/l SL

BPH

Imidacloprid 70 WS

TH

Acetamiprid 20 SP

BPH

Thiacloprid

TH

Phenyl pyrazole

Fipronil 50g/l SC

TH, BPH, RLF

Fipronil 0.3 G

RGM, RSB

Others IGR/ Molt accelerating compounds/ chitin inhibitors

Buprofezin 10% WP

BPH

Nuvaluron

BPH

Tebufenozid

RLF

Methoxyfenozide

RLF

Chlorfluzuron

RLF

Botanicals

Azadirachtin 10g/l

RLF

Acaricide

Fenpyroximate 50g/l EC

Etoxazole 10 % SC

Hexithiazox 50g/l EC


RWM - Rice whorl maggot
MC - Mole cricket
CW - Case warm
RSC - Rice swarming caterpillars
RGM - Rice gall midge
Th - Thrips
RLF - Rice leaf folder
RSB - Rice Stem borer
BPH - Brown plant hopper
RB - Rice bug
RFC - Rice field crab

Pests of Stored Paddy and Rice

Four important species of pests of stored paddy and rice are found in Sri Lanka.

Grain Moth- Sitotroge cerealella
Infestation of grain moth starts in the field and may reach serious levels in the store. The damage is done by larvae which are elongated, dirty white about 8 mm long. The pupa is dark brown. The adult is a small, straw colored moth about 7 mm long; the wings are 15 mm across when open. Infestation of grain moth could be minimized by sun drying of seeds to minimize moisture content down to 8.0% and pack in polythene or paper bags.

Grain weevil- Sitophilus granarius
Two species of stored product pests belongs to genus Sitophilus are found in rice in Sri Lanka. They are Sitophilus granerius and S. oryzae. Infestation of these two species starts in the field. Eggs laid on rice seeds, hatch into tiny grubs which feed the grain. Mature larvae are legless and dirty white about 4 mm long. Pupation takes place in the grain. Adult beetles are small brown weevils. They are about 3.5-4.0 mm long with rostrum. This pest could be controlled by spraying the store with pirimiphos-methyl at the rate of 27 ml per 9 liters of water. Spray gunny bags with the same insecticide and sundry them before use.

Red flour beetle: Tribolium cestaneum
Red flour beetle is a secondary pest and their damage is extensive in previously holed or broken grains. Both larvae and adults damage the seeds.
The larvae are yellowish white. The head is pale brown. They are about 6 mm long when fully grown. Pupae are yellowish white at the early stages and become brown in colour later. The adult is flat and reddish brown in colour. This pest could be controlled by following sanitary measures and spraying pririmiphos methyl to the stores and gunny bag before use.

Disease Management
RICE DISEAES

Numerous disease of rice, caused by fungi, bacteria, viruses and nematode have been recorded in literature. Some diseases occur where ever rice in grown. Some are of both regional and international important, others occur in local areas. Some diseases reach epidemic proportion and causes serious crop losses which others causes only negligible crop losses. This articles deals with only rice diseases of national importance which may causes considerable crop losses,
How to diagnose rice diseases accurately in the field

Accurate diagnosis and timely solving of field problems in rice crop is a vital component of crop management which assures optimum use of inputs for enhanced productivity leading to increased profits. Field problems in rice cultivation could broadly be divided into 3 major categories viz. insect pests, weeds, and plant diseases. Identification and management of problems related to insect pests and weeds are being presented in separate articles. This article will focus on accurate diagnosis of rice disease related problems in the field and approaches for their appropriate management.

During the last 30 years major changes have occurred in the varietal composition of and cultural practices for rice in Sri Lanka. Prior to 1970, hundreds of tall traditional cultivars were planted into 0.72 M ha of rice extent in Sri Lanka. However, beginning 1970, new improved varieties (NIV) of rice were introduced and at present 80% of rice extent is cultivated to about 10 NIVs. This indicates the reduced genetic variability of rice crop. New improved varieties are characterized by early maturity, photoperiod insensitivity, short stature, high tillering and dark- green leaves.

Introduction of NIVs compelled farmers to use improved cultural practices such as better water & weed control, application of higher rates of fertilizer, and establishment of higher plant populations per unit area. The development of major irrigation facilities such as Mahaweli, and availability of early maturing, photoperiod - insensitive varieties have enabled the farmers in Sri Lanka to grow successive rice crops throughout the year in large extents. Reduced genetic variability (comprising few NIVs), improved cultural practices and continuous cropping with rice helped to increase the rice production in the country, however, the same factors increased the genetic vulnerability of the rice crop to diseases and insect pests.

Each season the rice crops in farmer fields are affected by many plant disease problems. Effects of plant diseases on rice crop productivity often varies depending on inherent capacity of the variety to withstand the disease condition in question, environmental factors, stage of crop growth, level of soil fertility management and indirect and harmful effects of agrochemicals such as herbicides and other pesticides. Plant disease problems can be grouped into two major areas; plant disorders and plant diseases for the convenience of discussion. A plant disorder is a state of disruption of the normal healthy status of the plant or plant parts caused by external factors such as soil problems (iron toxicity, saline soil, acid sulfate soil, deficiencies of nitrogen, phosphorous, potassium, and zinc), environmental stresses (water stresses, cold temperature) or other physical effects (wind damage, insect damage). Symptoms of plant disorders cannot be transferred from an affected plant to a healthy plant. A plant disease on the other hand is defined as an impairment of the normal physiological functioning of a plant or plant part caused by disease causing agents such as fungi, bacteria, viruses or nematodes. Plant diseases can be spread from an infected plant into a healthy plant.

The symptoms of many rice plant disorders, diseases and insect pest attacks have been very clearly described and recorded individually. However, in field situations these diseases, disorders or pest attacks do not always occur in isolation, but rather as mixed occurrences. Therefore, identification of the primary cause of the field problem in question is of vital importance in providing an effective and efficient management/control measures. To establish an accurate diagnosis of a field problem, adoption of three logical steps as indicated below are very helpful. They are; (a) investigation and collection of previous and current season management practices (field history), (b) observation of field symptoms for any pattern of occurrence and (c) close examination of affected individual plants on whole plant basis in comparison to healthy plants from the same fields. These steps would provide some very useful clues as to what factor/s may be involved with the present field problem leading to accurate diagnosis of causal factors.

Field History
Field history is a crucial consideration in making a proper diagnosis. We learn from the past, so it becomes imperative to know last year’s crop and variety, rate and kind of fertilizers used, pesticide applied, and tillage programs. Rice crops planted on fields with vegetable crops in the previous season are liable to get more diseases such as blast, sheath blight and bacterial leaf blight due to luxurious plant growth caused by higher level of residual plant nutrients. Other factors that could play a role in our present diagnosis are last year’s growing conditions and occurrence and distribution of similar field problems or unrelated symptoms, if any. Are the affected plants confined to a specific area of ill drained or well drained portion of the field? These historical information would suggest if the field problem concerned is a new occurrence or a reappearance of seasonal occurrence. Then the next step is to focus on this year’s management procedures and the present field problem to see if common crop and soil fertility management practices have been attended to.

Field Symptoms and Patterns
Distribution of affected plants in a field is studied to understand if the problem is developing a pattern. Are the disorders associated with particular parts of the field such as headlands, well drained or low-lying areas, or border rows? Border plants showing normal healthy appearance, while, center plants showing yellowing and poor growth could be due to lack of sufficient plant nutrients. The crop growth stage may provide possible causes of the present problem as some pest and diseases are crop growth stage specific. Are single plants affected, groups of plants, or are large areas of the field showing uniform symptoms? The problem may be related to environmental conditions such as low or high temperature, water availability, related to field practices such as herbicide or pesticide application, biological, or a combination of above factors. A stunted plant may be an indication of poor root growth as a result of compacted shallow plough pan, root rot, root injury from nematode feeding, or a nutrient deficiency. Discoloration, missing plants, or a stunting of plants in pockets could indicate a high population of nematodes, root rots, or specific soil related problem. Once the field has been sufficiently scouted and analyzed, the affected plants should be examined on whole plant basis in relation to healthy plants from same fields so that useful clues may be found.


Plant Symptoms

Symptoms of affected plants hold the key to an accurate diagnosis of the field problem in question. Symptoms and signs of already recorded rice diseases have been very well described and recorded, and therefore one should use such available literature in relation to present field problem in question. Careful examination should include looking for discoloration, abnormal growth, or wilting of the leaves, storm and insect injuries, lesions, galls or any abnormality on the stem that may result in a disruption in the flow of water or nutrients. Leaf symptoms are often a reflection of root abnormalities; therefore, plants should be carefully dug up and the soil removed from the roots. Without close observation of root system, herbicide injury could be confused with a root rot. Both potentially cause browning of leaves or seedling death. Roots should appear off-white with elongated, fibrous, lateral rootlets. Root diseases may be observed either as a brown discoloration of the root or lesions. Nematode feeding and herbicide injury may be confused and normally require a soil analysis to determine the cause of a stunted or stubby root system. An example is dinitro aniline carryover and nematode injury to corn. The symptoms of both causes are short, stubby roots. A rotted root system may be caused directly by a root rotting pathogen or indirectly as secondary rot of dead tissue initially killed by excessive moisture or phytotoxicity effects of residual herbicides. In plant disease diagnosis and recognition, it is best to take a holistic approach in your investigations. Look at all aspects of a fields history, observe the entire field for patterns, and finally examine the entire plant. Making a diagnosis from insufficient observations is a disservice to a client and yourself. Utilize your past training, the literature, and the opinions of others to their fullest. Alternatives exist for providing a proper diagnosis. Some plant diseases have symptoms that are relatively unique; thus a diagnosis can be based only on the symptom. In other instances, symptoms are not a clear reflection of a specific disease and a well-equipped laboratory becomes important in identifying the causal agent. Plant or soil samples can be removed from the affected field and submitted to a trained professional. These samples should include information on field history, field symptoms, and plant symptoms as you see them. The care taken in providing the needed information, removing the affected plant, and submitting a properly packaged sample will be repaid with a reliable diagnosis.
COMMON RICE DISEASES
Rice diseases of economic importance that need careful attention for increased productivity in farmers fields are as follows.

Disease Causal organism:
1. Rice blast - Magnaporthe grisea
2. Rice sheath blight - Rhizoctonia solani
3. Brown spot - Cochiobolus miyabeanus
4. False smut - Ustilaginoidia virens
5. Grain spotting and pecky rice - many fungal species and bacteria
6. Leaf scald - Gerlachia oryzae
7. Narrow Brown Leaf spot - Cercospora janseana
8. Sheath rot - Sarocladium orysae
9. Root knot - Meloidagane spp.
10 Bacterial blight - Xanthomonan campestris pv. oryzae
11.Bacterial leaf streak - Xanthominan campestris PV oryzicola

Rice varietal improvement program always focus on breeding for disease resistance wherever possible. Therefore, newly developing varieties are screened for rice blast and bacterial diseases. In addition, special attention is made to select disease free or tolerant pedigree lines in the process of varietal development. Introduction of new germ plasm for disease resistance are necessary in order to improve the durability of disease resistance of newly developing varieties.

In order to manage rice diseases in the farmer fields, integrated disease management program has to be adopted. This program should emphasize, use of quality seed paddy, proper control of weeds and balance application of major plant nutrients.

Rice Blast (Magnaporthe grisia)


Symptoms

It may infect young seedlings, leaves, panicles and other aerial parts of the adult plant. It is also known as leaf blast, node blast, panicle blast, or neck rot. Leaf spots are of spindle-shaped with brown or reddish-brown margins, ashy centers, and pointed ends. Fully developed lesions normally measures 1.0-1.5 cm in length and 0.3-0.5cm in breadth. Their characteristics vary with the age, susceptibility level of the cultivar and environmental factors. When nodes are infected, they become black and rotted. Infection of panicle base causes rotten neck or neck rot and causes the panicle to fall off. In severe infection, secondary branches and grains are also affected.

Disease development

Found in both upland and lowland environments blast occurs most often in upland environments in Sri Lanka. Water deficiency predisposes the crop to severe infection in all environments. The upland ecosystem with high night humidity accompanied by low night temperature presents a favorable environmental for development of blast. Rice grown in irrigated flooded condition are at a less risk for diseases development.

Airborne conidia, which are present year round in the atmosphere, are the most potent source of infection. Conidia may be seed borne or they may come from straw, stubble or numerous graminae weeds.

Disease management
Use of resistant varieties is the first important step in successful disease management program. Application of high amount of nitrogenous fertilizers induces a heavy incidence of blast in disease susceptible varieties irrespective of the supply of phosphorus or potassium. Adjustment of planting time to avoid blast favorable weather condition may not be practicable. Suitable fungicide can be used at the onset of disease under disease favorable conditions.
Sheath blight (Rhisoctoni solani)

Sheath blight is caused by the fungi, Rhisoctonia solani. Sheath blight is perhaps the second most important fungal disease of rice in Sri Lanka.

Symptoms
Symptom become apparent at filtering or flowering stage. Spots or lesions first develop near the water level (in flooded fields) or soil (in upland fields) and spots initially appear on the leaf sheath. Spots may be oral or ellipsoidal and measure 1-3 cm long. Lesions on the leaf blade are usually irregular and banded with green, brown, and orange coloration. Lesions are greenish white in the center with brown margins. At advanced stages, when the flag leaf is infected the panicle exertion is affected. Leaves with extended lesions. Eventfully die.

Asexual over-wintering structures known as sclerotia are formed on leaf sheath surface. They are usually 4-5 mm in diameter, white when young, turn brown or purplish brown at maturity and fall off easily on to soil surface and remain for years.

Disease development

Sheath blight incidence is higher in drained rice fields than in upland fields. Infection normally occurs through sclerotia which survive in soil for a long time, depending on the temperature and moisture levels. Infected straw, stubble, weeds are other source of primary inoculum. High humidity (>90%) and temperature (up to 400C) high tillering, short statured early maturing varieties specially at higher plant densities and heavy use of nitrogenous fertilizers are two other factors attributing to increased incidence of sheath blight.

Disease management

There are no genetically resistant varieties to this disease. However, varieties with clean plant base ie, varieties with less or no unproductive tillers escape disease. The disease can be controlled through,
  • Cultural practices such as green maturing with Sesbania aculeate, deep
  • ploughing to bury infested plant residues into the soil.
  • Use of recommended seed rate ie. 2 bushels per acre.(direct sowing)
  • Weeds free or less fields.
  • Avoid excessive use of nitrogenous fertilizer

Brown Spot

Brown spot, caused by the fungus Cochiobolus miyabeanus, is another rice diseases found in some parts of Sri Lanka. The disease was also called Helminthosporium leaf spot. When C. miyabeanus attacks the plants at emergence, the resulting seedling blight causes sparse or inadequate stands and weekend plants. Leaf spots are present on young rice; however, the disease is more prevalent as the plants approach maturity and the leaves begin to senesce. Yield losses from leaf infection or leaf spots are probably not serious. When the fungus attacks the panicle, including the grain, economic losses occur. Heavy leaf spotting is an indication of some unfavorable growth factor, usually a soil problem.

The pathogen also attacks the coleoptiles, leaves, leaf sheath, branches of the panicle, glumes and grains. The fungus causes brown, circular to oval spots on the coleoptile leaves of the seedlings. It may cause seedling blight. Leaf spots are found throughout the season. On young leaves, the spots are smaller than those on upper leaves. The spots may vary in size and shape from minute dark spots to large oval to circular spots. The smaller spots are dark brown to reddish-brown. The larger spots have a dark brown margin and a light, reddish-brown or gray center. The spots on the leaf sheath and hulls are similar to those on the leaves. The fungus attacks the glumes and causes a general black discoloration. The fungus also attacks the immature florets, resulting in no grain development or kernels that are lightweight or chalky.

Brown spot is an indicator of unfavorable growth conditions. These unfavorable growth conditions include insufficient nitrogen, inability of the plants to use nitrogen because of injury to root system by root rot, or other unfavorable soil conditions. As the plants approach maturity, brown spot becomes more prevalent, and the spots are larger on senescing leaves. > Damage from brown spot can be reduced by maintaining good growing conditions for rice by proper fertilization, crop rotation, land leveling, proper soil preparation and water management. Seed-protectant fungicides reduce the severity of seedling blight caused by this seed borne fungus. Some varieties are less susceptible than others

Flash Smut,
False smut, caused by the fungus Ustilaginoidea virens, is a minor disease in Sri Lanka. The disease is characterized by large orange to brown-green fruiting structures on one or more grains of the mature panicle. When the orange covering ruptures, a mass of greenish-black spores is exposed. The grain is replaced by one or more sclerotia. All varieties appear to have a high level of resistance and disease control measures are not required.
Grain Spotting and Pecky Rice
Many fungi infect developing grain and cause spots and discoloration on the hulls or kernels. Damage by the rice stinkbug, Oebalus pugnax F., also causes discoloration of the kernel. Kernels discolored by fungal infections or insect damage are commonly called pecky rice . This is a complex disorder in rice that involves many fungi, the white-tip nematode and insect damage. High winds at the early heading stage may cause similar symptoms. Proper insect control and disease management will reduce this problem.

Leaf Scald disease, caused by Gerlachia oryzae, is common and sometimes severe in major rice growing districts in Sri Lanka. The disease affects leaves, panicles and seedlings. The pathogen is seed borne and survives between crops on infected seeds. The disease usually occurs on maturing leaves. Lesions start on leaf tips or from the edges of leaf blades. The lesions have a chevron pattern of light (tan) and darker reddish-brown areas. The leading edge of the lesion usually is yellow to gold in color. Fields appear yellow or gold. Lesions from the edges of leaf blades have an indistinct, mottled pattern. Affected leaves dry and turn straw-colored.
Panicle infestations cause a uniform light to dark, reddish-brown discoloration of entire florets or hulls of developing grain. The disease can cause sterility or abortion of developing kernels.

Control measures are not recommended, but foliar fungicides used to manage other diseases have activity against this disease.
Narrow Brown Leaf Spot
Narrow brown leaf spot, caused by the fungus Cercospora janseana, varies in severity from year to year and is more severe as rice plants approach maturity. Leaf spotting may become very severe on the more susceptible varieties and causes severe leaf necrosis. Some premature ripening, yield reduction and lodging occur.

Symptoms include short, linear, brown lesions most commonly found on leaf blades. Symptoms also occur on leaf sheaths, pedicels and glumes. Leaf lesions are 2-10 mm long and about 1 mm wide. They tend to be narrower, shorter and darker brown on resistant varieties and wider and lighter brown with gray necrotic centers on susceptible varieties. On upper leaf sheaths, symptoms are very similar to those found on the leaf. On lower sheaths, the symptom is similar to a "net blotch" or Cercospora sheath spot in which cell walls are brown and intracellular areas are tan to yellow.
The primary factors affecting disease development are
(1) susceptibility of varieties to one or more prevalent pathogenic races,
(2) prevalence of pathogenic races on leading varieties, and
(3) growth stage. While rice plants are susceptible at all stages of growth, the plants are more susceptible from panicle emergence to maturity.
Plant breeders have found differences in susceptibility among rice varieties, but resistance is an unreliable control method as new races develop readily. Some fungicides used to reduce other diseases also may have activity against narrow brown leaf spot. Low nitrogen favors development of this disease.

Root Knot

Species of the nematode Meloidogyne cause root knot. The disease symptoms include enlargement of the roots and the formation of galls or knots. The swollen female nematode can be found in the center of this tissue. Plants are dwarfed, yellow, and lack vigor. The disease has been reprted from Hambantota area since past 15 years and it was detected in other parts of the country such as Kurunegala, Pollonnaruwa and even in Ampara during Yala 2000. The nematode becomes inactive after prolonged flooding.
Deep ploughing and application of organic matter help reduce the disease condition.

Sheath Rot

This disease is caused by the fungal pathogen Sarocladium oryzae. Symptoms are most severe on the uppermost leaf sheaths that enclose the young panicle during the boot stage. Lesions are oblong or irregular oval spots with gray or light-brown centers and a dark reddish-brown, diffuse margin, or the lesions may form an irregular target pattern. The lesion is usually expressed as a reddish-brown discoloration of the flag-leaf sheath. Early or severe infections affect the panicle so that it only partially emerges. The unemerged portion of the panicle rots, turning florets red-brown to dark brown. Grains from damaged panicles are discolored reddish-brown to dark brown and may not fill. A powdery white growth consisting of spores and hyphae of the pathogen may be observed on the inside of affected sheaths. Insect or mite damage to the boot or leaf sheaths increases the damage from this disease.

This disease affects most rice varieties. The disease is usually minor, affecting scattered tillers in a field. Occasionally, larger areas of a field may have significant damage. Control measures are not recommended. Fungicidal sprays used in a general disease management program reduce damage.
Weed Management
Weeds in Rice

Weed flora varies from place to place due to type of rice culture, soil type hydrology, tillage, cultural practices and irrigation pattern etc.
  • Grasses are dominant in irrigated systems
  • Sedges are dominant in rainfed system
  • Weed problems are more severe in irrigated rice fields due to rotation at pattern of water issues
  • Nearly 134 weed species identified from rice field in Sri Lanka belongs to 32 taxonomic families
More than 70 species - grasses
More than 50 species - sedges
More than 20 species - broad leaves
However 10-20 species may be considered as economically important in rice culture Grasses are considered as the major weed group in rice fields in Sri Lanka.
Major Rice Weeds in Paddy Fields

Grasses

Sedges

Broadleaves

Ferns

Echinochloa colonum

Cyperus iria

Commelina diffusa

Marsilea quadrifolia

Echinochloa crusgalli

Cyperus difformis

Eclipta alba

Salvinia molesta

Panicum repens

Cyperus rotundus

Eichnornia crassipes

Ischaemum rugosum

Fimbristylis miliacea

Monochoria vaginalis

Isachne globosa

Linderina spp.

Murdannia nudiflora

Fimbristylis dichotoma

Scirpus supinus

Sphenoclea zeylanica

Paspalum distichum

SpLudwigia perennis
Haeranthus africanus


Echinochola crusgalli is rated as the most troublesome weed in both well and poorly drained soils. Ischeamum rugosum and leptochloa chinensis are becoming important in rice fields due to continuous application of herbicides.
Critical period for weed control
Weed competition do not occur during the entire cropping period. Control of weeds in the critical period of competition is important.

Usually it commences around 2 weeks of seeding and may continue up to 5-8 weeks. Hence early weeding is important to reduce yield losses.

Control of Weeds using IWP Methods

Weeds are most efficiently and economically controlled by the simultaneous application of a variety of practices. These practices including preventive, cultural, manual, mechanical, biological and chemical. Integrated weed control practices (IWP) combines these different practices. If any single control method is used for a long time, weed species resistant to that method may build up and eventually the control measure will fail. So, the objective of IWP is to create conditions unfavorable to weeds while maintaining suitable condition for crop.

Preventive Control Measures
  • Use clean seeds
  • Keep seed bed weed free
  • Keep leaves, bunds and irrigation canal clean
  • Keep tools and machinery clean
  • Keep livestock out of field
  • Prevent weeds from seedlings
  • Prevent vegetative reproduction in seed bed
Cultural Control Measures
  • Proper land preparation
  • Cultivar selection
  • Crop establishment
  • Water management
  • Control of fertilizer application

Mechanical Control Measures

Use rotary weeder

Manual Weed Control
Hand weeding


Biological Control Method
Weed utilization in rice fields, Using mulching effect and Allopathic effect
Chemical Weed Control Measures

Recommended Herbicides for Rice Weeds

Category of Herbi- cide

Generic Name/

**Trade Name

Dialution (product per 16l of water)

Rate of application (l/ha)

Time of

Spraying (days after estab- lishment)

Mode of Action *

Pre-plant Herbicides

Not available at the moment in Sri Lanka

One-shot herbicides (14)

Pretilachlor 300g/l EC

**Sofit 30EC

64-80ml

1.6l

0-3

Inhibiton of Cell Division

Oxyfluorfen 240g/l EC

**Goal 2XL, Galigan

4ml

0.5l

0.3l

3-5 2-3

Protoporphyringen Oxidase Inhibitor

Pyrazosulfuron-ethyl 10% WP

**Sirius

8.96-11.2g

225g

3-7

Acetolactate Synthase Inhibitor

Pretilachlor 300g/l+Pyribenzoxim 20g/l EC

**Solito 320EC

48-64ml

1.25l

6-10

Inhibiton of Cell Division+ Acetolactate Synthase Inhibitor Inhibitor Inhibitor Inhibitor Inhibitor Inhibitor Inhibitor Inhibitor Inhibitor

Propyrisulfuron 10% SC

** Sumo

20ml

0.5l

6-14

Acetolactate Synthase Inhibitor

Bispyribac sodium 15g/l+Thiobencarb 900g/l OD

**Solo

60.8-76.8ml

1.5l

7-14

Acetolactate Synthase Inhibitor

Azim sulfuron 50% WG

**Galiver

2.4-3.04g

60g

7-15

Acetolactate Synthase Inhibitor

Pyribenzoxim 50g/l EC

**Pyanchor 5% EC

20-25.6ml

0.5l

7-18

Acetolactate Synthase Inhibitor

Bispyribac sodium 100g/l

**Nominee

12-16ml

300ml

8-14

Acetolactate Synthase Inhibitor

Bispyribac sodium 40g/l SC+Metamifop 100g/l SC

**Kiseki

25-32ml

625ml

8-14

Acetolactate Synthase Inhibitor+ Acetyl CoA Carboxylase Inhibitor

Flucetosulfuron 10% w/w WG

** ??

8g

200g

8-14

Acetolactate Synthase Inhibitor

Bispyribac sodium 20% WP

**Kensolo

8.96-11.2g

225g

10-18

Acetolactate Synthase Inhibitor

Penoxulam 240g/lSC

**Granite

4-5.12ml

100ml

10-18

Acetolactate Synthase Inhibitor

Fenoxaprop-p-ethyl 69g/l+Ethoxsulfuron 20g/l OD

**Tiller Gold

20-25.6ml

0.5l

14-21

Acetyl CoA Carboxylase Inhibitor

Grass Killers (04)

Cyhalofop-butyl 100g/l EC

**Clincher 10EC

80-102.4ml

2.0l

7-15

Acetyl CoA Carboxylase Inhibitor

Metamifop 10% EC

** Matari

52.8ml

1250ml

7-21

Acetyl CoA Carboxylase Inhibitor

Quinclorac 250g/l SC

** Facet

32-40ml

800ml

8-15

Unknown MoA

Fenoxaprop-p-ethyl 75g/l EW

**Whip Super 7.5EW

14.4-17.6ml

350ml

16-25

Acetyl CoA Carboxylase Inhibitor

Sedges and broad leaves killers (08)

Cyclosulfamuron 10% WP

**Invest

10.08-12.48g

250g

12-25

Acetolactate Synthase Inhibitor

Ethoxysulfuron 15% WG

**Sunrice

3.2-14g

80g

14-21

Acetolactate Synthase Inhibitor

Carfentrazone-ethyl 240g/l EC

**Affinity 24EC

4.8-6.08ml

120ml

14-21

Protoporphyringen Oxidase Inhibitor

Bensulfuron methyl 8.25%+Metsulfuron methyl1.75%

10.08-12.48g

250g

12-25

Acetolactate Synthase Inhibitor

Flucetosulfuron 10%WG

**Fluto

10.08-12.48g

250g

12-18

Acetolactate Synthase Inhibitor

Orthosulfamuron 50% WG

**Pivot

6.08-7.52g

150g

15

Acetolactate Synthase Inhibitor

MCPA 400g/l SL

**M40

112-140.08ml

2.8l

21-28

Synthetic Auxin (Growth Regulator)

MCPA 600g/l SL

**M60

72-89.6ml

1.8l

21-28

Synthetic Auxin (Growth Regulator)


(Source: Department of Agriculture, Pest Management Recommendations, 2015)

Application of a same herbicide or hericides having similar mode of action, continously in a same field may cause resistance

development of weeds against herbicides. Therefore, rotation of herbicides having different mode of actions to be practiced in order to manage resistance development problem.