Rice In Sri Lanka - Rice Diseases
Fungi, Thanatephorus cucumeris (Rhizoctonia solani)
Plant parts and life stages affected
Tillering or flowering stage
Sheath blight is perhaps the second most important fungal disease of rice in Sri Lanka.
Infection occurs through infected plants or sclerotia which survive in soil for a long time (depending on the temperature and moisture levels). In addition, infected straw, stubble, weeds could be the other sources of primary inoculum.
Spots or lesions initially develop near the water level (in flooded fields) or soil (in upland fields) and on the leaf sheath.
Spots or lesions might measure 1-3 cm long, may be oval or ellipsoidal initially but may get enlarged and irregular . mainly on the leaf blade. Lesions are white in the center, banded with green, brown, and orange coloration.
At advanced stages, when the flag leaf is infected, panicle exertion is affected.
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.
Conditions that favour the disease
High temperature (up to 400C)
High humidity (>90%)
Excessive use of nitrogen fertilizer
Foggy and dark climatic conditions
High densities of plants in the field, and varieties with higher number of tillers
Short aged early maturing varieties specially at higher plant densities
Within the crop season
If the disease spread fast, following fungicides could be applied
- Hexconazole 50G/L EC – dissolve 32 ml in 16 l of water (8-10 tanks per acre)
- Propiconazole 250 G/L EC – dissolve 16 ml in 16 l of water (8-10 tanks per acre)
- Thiophanate methyl 70% WP – dissolve 16 g in 16 l of water (8-10 tanks per acre)
- Pencicuron 25% WP – dissolve 32 g in 16 l of water (8-10 tanks per acre)
- Tebuconazole 250g/l EC – dissolve 10 ml in 16 l of water (8-10 tanks per acre)
If the crop is infected, following management options should be applied for the next season
- Use of certified seed paddy free from the disease
- Deep ploughing to bury infested plant residues into the soil
- Use of recommended seed rate i. e. 2 bushels per acre (direct sowing).
- Maintaining an average level plant population in the field
- Addition of burnt paddy husk (253 kg per acre) to the soil during land preparation.
- Abstain addition of straw infected with disease
- There are no genetically resistant varieties to this disease. Varieties with clean plant base (less/no unproductive tillers) seem to escape the disease.