Sheath Blight (fungus - Rhizoctonia solani): Sheath blight is a major disease of the rice plant. Initial symptoms usually develop as lesions on sheaths of lower leaves near the water line when plants are in the late tillering or early internode elongation stage of growth (approximately 10 - 15 days after flooding). These lesions usually develop just below the leaf collar as oval-to-elliptical, green-gray, water-soaked spots about 1/4 inch wide and 1/2 to 1 1/4 inch long.
With age, the lesions expand and the center of the lesions may become bleached with an irregular tan-to-brown border. When humidity exceeds 95 percent and temperatures are in the range of 85-90 degrees F (30-32°C) , infection spreads rapidly by means of runner hyphae to upper plant parts, including leaf blades, causing extensive, tan, irregularly shaped lesions with brown borders. Disease development progresses very rapidly in the early heading and grain filling growth stages during periods of frequent rainfall and overcast skies. Plants heavily infected at these stages produce poorly filled grain, particularly in the lower portion of the panicle. Additional losses result from increased lodging or reduced ratoon production due to infection of the culm and reduced carbohydrate reserves.
As plants senesce from maturity, lesions will dry and become grayishwhite to tan with brownish borders. Sclerotia, initially white but turning dark brown at maturity, are produced superficially on or near the lesions. Sclerotia are loosely attached and easily dislodge from the plant. Sclerotia are the primary means for fungus survival between crops. They survive long periods in the soil and will float to the surface of flooded rice fields in the subsequent rice crop, infect rice plants at the waterline and continue the disease cycle. Sclerotia can survive from one to several years in the soil. They can also attack several weed hosts and cause infection.
New varieties and changing cultural practices often combine many of the factors that favor disease development. In recent years, the wide acceptance of susceptible varieties, because of their high yielding potential, has contributed greatly to the rapid increase in sheath blight. In addition, the new varieties respond to heavy nitrogen applications in order to achieve their high yielding potential. Heavy applications of nitrogen predispose susceptible plants to attack by the sheath blight organism. Rotation with susceptible crops, such as soybean (see Soybean Aerial Blight) can also increase the severity of sheath blight in succeeding rice crops.
Disease incidence may be reduced by planting less susceptible varieties. Excessive seeding rates and high nitrogen applications should be avoided in fields with a history of the disease. Grass and weeds should be controlled. Long-term rotations may reduce the incidence of sheath blight, but soybeans, sorghum, and many weeds are susceptible to Rhizoctonia solani. Excessive seeding rates and thick plant populations favor sheath blight development. In some cases, foliar fungicides may be economical for reducing sheath blight losses.