Sheath blight is characterized by large oval spots on the leaf sheaths (Fig. 2) and irregular spots on leaf blades (Fig. 3). Infections usually begin during the late tillering-joint elongation stages of growth. Lesions about 0.5 - 1 cm in width and 1 - 3 cm in length are formed a little above the waterline on infected culms. Fungus mycelium grows up the leaf sheath, forms infection structures, infects and causes new lesions. The infection can spread to leaf blades. The lower leaf sheaths and blades are affected during the jointing stages of growth. After the panicle emerges from the boot, the disease progresses rapidly to the flag leaf on susceptible varieties. With very susceptible varieties, the fungus will spread into the culm from early sheath infections. Infected culms are weakened, and the tillers may lodge or collapse.

The disease is caused by Rhizoctonia solani. The teleomorphic /sexual stage of the fungus is thanatophorus sasaki.

The hyphae of this fungus are hyaline when young and turns into dark brown when matures .the lateral branches arise at right angle from the main branches and having constriction at the point of origin. The fungus has three types of hyphae

(i) Runner hyphae
(ii) Short, swollen much branched hyphae that produce infection peg.
(iii) Hyphae that produce sclerotia.

Rhizoctonia solani consist of many morphologically and pathologically distinct strains based on their anastomosis behavior (Anastomosis group). Different strains of R. soloni cause disease in different crops. Sheath blight of rice is caused by Anastomosis group -1 (AG-1) . the fungus produce sclerotic which are light brown to dark brown colour. The Teleomorphic stage produce basidia which are barrel shaped, 10-15 x 7-9 micron in size and the basidiospore measures about 8-11x 5-6.2 micron in size.

The fungus can spread in the field by growing from tiller to tiller on an infected plant or across the surface of the water to adjacent plants. The fungus also grows across touching plant parts, for example from leaf to leaf, causing infections on nearby plants. Infected plants are usually found in a circular pattern in the field because the fungus does not produce spores and must grow from plant to plant. The lesions have grayish-white or light green centers with a brown or reddish-brown margin. As lesions coalesce on the sheath, the blades turn yellow-orange and eventually die. As areas in the field with dead tillers and plants increase, they may coalesce with other affected areas to cause large areas of lodged, dead and dying plants. Damage is usually most common where wind-blown, floating debris accumulates in the corners of cuts when seed-beds are prepared in the water.

The fungus mainly survives as sclerotia or mycelium in destructive plant debris left in the field after harvest. The fungus is reported to be seed borne and its survival in weed hosts such as Echinocla colona, cynodon dactylon ,sateria glauca, E. crusguli,penicum ripens ,C.irrea etc has also reported. The fungus also survive in maize and finger millets. The mycelia and sclerotia which survives in diseased plant debris come in to soil surface due to the various cultural operations and then come in contact with the rice seedling and starts infection.

Seath blight become very severe and destructive when rice is grown under high temperature and high humidity condition . Close planting along with high dose of nitrogenous fertilizer become more favourable humidity for infection is 100 per cent and minimum is about 85-88%. Maximum sclerotia develops at temperature of about 300 C. The optimum temperature for infection varies from 30-320 C. infection can also occur at temperature range of 23-350C.

Sheath Blight

Disease severity can be reduced by integrating several management practices. Dense stands and excessive use of fertilizer both tend to increase the damage caused by this disease. Broadcast seeding tends to increase stand and canopy density. Rotation with soybeans or continuous rice increases the amount of inoculum in field soils.