The first symptoms are generally observed in the field after the mid- tillering stage. Initially, the disease appears as a small, blackish, irregular lesion on the outer leaf sheath near the water line. The lesion enlarges as the disease progresses with the fungus penetrating into the inner leaf sheaths.
Eventually, the fungus penetrates and rots the culm while the leaf sheath is partially or entirely rotten. Infection of the culm may result in lodging, unfilled panicles, chalky grains, and in severe cases, death of the tiller. Brownish-black lesions appear and finally one or two internodes of the stem rot and collapse.
Upon opening the infected stem, dark grayish mycelium may be found within the hollow stem and numerous tiny, black sclerotia are embedded all over the diseased leaf sheath tissues. Sclerotia and mycelium of the fungus are generally present inside the infected culms. The presence of sclerotia is usually a positive and easy way of diagnosing the disease. Attack on the stems increases in intensity as the plants approach maturity and reaches its peak during harvest time. Weakened stalks break during this stage and plants lodge making harvest difficult. Plants infected early produce lower yields.
The fungus survives between crops as sclerotia in crop debris or in soil. After flooding, sclerotia float to the surface of the paddy water where they infect rice sheaths at the water line. Sclerotia are produced abundantly on diseased tissue as the disease progresses and the rice approaches maturity. The fungus also produces conidia and ascospores on infected plants that may serve as an additional source of inoculum.
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