Sheath Blight

Causal organism: Rhizoctonia solani (Sexual stage: Thanetophorus cucumeris)

Symptoms:

• The fungus affects the crop from tillering to heading stage.
• Initial symptoms are noticed on leaf sheaths near water level.
• On the leaf sheath oval or elliptical or irregular greenish grey spots are formed.
• As the spots enlarge, the centre becomes greyish white with an irregular blackish brown or purple brown border.
• Lesions on the upper parts of plants extend rapidly coalesing with each other to cover entire tillers from the water line to the flag leaf.
• The presence of several large lesions on a leaf sheath usually causes death of the whole leaf,
  
• In severe cases all the leaves of a plant may be blighted in this way.
• The infection extends to the inner sheaths resulting in death of the entire plant.
• Older plants are highly susceptible.
• Plants heavily infected in the early heading and grain filling growth stages produce poorly filled grain, especially in the lower part of the panicle..

Pathogen

• The fungus produces septate mycelium which are hyaline when young, yellowish brown when old.
• It produces large number of spherical brown sclerotia.

Favourable conditions:

• Presence of sclerotia or infection bodies floating on the water
• Presence of the sclerotia in the soil
• Relative humidity from 96 to 100%
• Temperature from 28-32 °c
• High levels of nitrogen fertilizer
• High seeding rate or closing plant spacing
• Frequent rain

• The pathogen can survive as sclerotia or mycelium in dry soil for about 20 months but for 5-8 months in moist soil.
• Sclerotia spread through irrigation water.
• The fungus has a wide host range.

• Apply organic amendments viz., neem cake @ 60Kg/ac or FYM 5 tonnes/ha.
• Avoid flow of irrigation water from infected fields to healthy fields. .
• Deep ploughing in summer and burning of stubbles.
• Spray Carbendazim 100 g /ac. Soil application of B. subtilis @ of 1 kg/ac after 30 days of transplanting.
  (The product should be mixed with 20 kg of FYM/Sand and applied).
  Foliar spray at 0.2% at boot leaf stage and 10 days later.(400 g/ac).

• AICRIP rice, TNAU, Coimbatore Rice diseases- online resource, IRRI, Phillippines.

• IRRI, Phillippines