This important disease of rice is very widespread in Arkansas, easily found in 50-66% of rice fields randomly surveyed in 1993 and 1994 and at damaging levels in most long term rice fields growing highly susceptible semidwarf long grain rice varieties. Actual yield loss to sheath blight appears to have lessened somewhat the past decade with the development of more tolerant varieties and improved cultural practices employed by many farmers, however, yield losses of 5-15% are still fairly common in many fields. Losses of 50% and greater, reported in the 1980's on Lebonnet and other very susceptible varieties, are now only rarely observed and are confined to smaller areas of fields. Before 1970, sheath blight was only a curiosity in Arkansas, but with the introduction of shorter and more susceptible varieties during the 1970's -1980's, the increased use of nitrogen fertilizer for higher grain yields on these varieties, alternate year rice rotations, and reduced tillage practices, sheath blight rapidly became the most widely established rice disease in the state. Sheath blight is more consistent but much less dramatic than blast, causing routine problems and losses in many fields each year. Weather can affect the severity of the disease with hot, humid years like 1991, 1993, and 1995 resulting in greater damage state-wide than years with cooler summers like 1992, 1994, and 1996.
Symptoms - Sheath blight is usually not noticed until late tillering to midseason (panicle differentiation). Early symptoms include oval sheath spots (lesions) at or just above the water line, often at the junction of the leaf and sheath. Early lesions are pale green to off-white with a narrow purple-brown or brown border, usually 2" or less wide and 1-2" long on most varieties. Lesions may join as the disease moves up the plant. Both sheaths and leaves are commonly attacked and killed as the disease grows upward. Other sheath spot diseases sometimes confused with sheath blight include black sheath rot (Gaeumannomyces graminis var graminis), stem rot (Sclerotium oryzae), aggregate sheath spot (Rhizoctonia oryzae-sativae), and bordered sheath spot (Rhizoctonia oryzae) (Fig.1). The first two diseases have black or gray-black lesions on the leaf sheaths. Aggregate sheath spot lesions are very similar to sheath blight lesions but have a narrow, brown vertical line in the center of the lesion, do not tend to overlap, and the disease rarely attacks the leaf blades. Bordered sheath spot lesions also tend to be separate and distinct, be more brown in color, have wide dark brown borders, and usually only appear on the sheaths (Fig.1).
