Sheath Blight Tips

Causal Agent

• The fungal disease is caused by the Rhizoctonia solani and Thanatephorus cucumeris fungi. Other host plants of the organisms include barley, maize, wheat, soyabean, cotton, carrots, citrus, lettuce and tomato. Sheath blight is second to rice blast in terms of serious rice disorders, cites the International Rice Research Institute.

Symptoms

• The disease starts with the appearance of oval, small, greenish-gray, water soaked spots near the water line. As spots get larger, they develop grayish white centers with brown margins and join to create, irregularly shaped patches. Entire affected leaves start to die. Fungus spreads rapidly to other plants. Less-affected plants only display spots on the lower foliage and grains may remain unaffected. In severe cases, the entire plant dies with no grain production.

Favorable Conditions

• Sheath blight fungi frequently infect new plants from the spores leftover in the soil from last planting. Spores are also transported to healthy plants on floating water from infected sites. Fungal growth is favored by high air humidity and warm, 82 to 89 degree F weather. Rice that is fertilized with excessive nitrogen becomes highly prone to infection. Many higher-yielding, improved rice varieties have low resistance to disease.

Control

• Keeping rice fields sanitized is among the primary preventative strategies against sheath blight. Pay special attention to weed control as the fungi survive in many weeds commonly found in rice fields. Remove all previous crop residues and stubbles prior to new planting. Avoid planting very close together as this encourages the fungi to spread horizontally through the field. Do not use excessive nitrogen. Recommended fungicides include the use of iprodione and benomyl and the antibiotics validamycin and polyoxin.