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Agricultural crops :: Cereals :: Rice

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
                    

Brown Elongated Lesions on Sheaths near Water Level

Mode of Spread and Survival

  • 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.

Management:

  • 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).

Source of information:

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

Acknowledgements:

  • IRRI, Phillippines

 

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