Rice Sheath Rot
Early symptoms are oblong to irregular spots on the leaves (0.5 to 1.5 mm) that enclose the panicles. Spots are characterized by gray centers and brown margins and are often combining to form rotting and then discoloration of the leaf sheaths. In severe infections, the young panicles may not emerge. Affected leaf sheaths may have abundant whitish powdery fungal growth visible on the outer surface. Grains of emerged panicles become discolored and sterile. Unemerged panicles produce florets that turn red-brown to dark brown. Infection is most damaging when it occurs at late booting stages and can cause severe damage.
Sheath rot is principally a seed-borne disease. The disease is caused principally by the fungus Sarocladium oryzae but also in some cases by Sacroladium attenuatum. Its incidence increases with increasing planting density and in plants that provide entry points for the fungus, in the form of injuries and wounds caused by insects at panicle initiation stage. The application of potassium, calcium sulphate or zinc fertilizers at the tillering stage strengthen the stem and leaf tissues and thus avoid extensive damage. It is also associated with plants weakened by virus infections.Hot (20-28°C) and humid (wet) weather favour the growth of the disease.
Bacteria such Rhizobacteria of Pseudomonas fluorescens isolated from citrus and rice are toxic to the rice sheath rot fungus, resulting in lesser incidence and more yield. Bipolaris zeicola is another potential antagonist of sheath rot that can completely inhibited mycelial growth of S. oryzae. Antifungal activity of extracts from leaves and flowers of Tagetes erecta also inhibit S. oryzae mycelium by 100%.
Always consider an integrated approach with preventive measures together with biological treatments if available. In case of severe infestation, the application of fungicides such as benomyl, carbendazim, mancozeb, copper oxychloride or propiconazole have been shown to reduce the incidence of the disease. Application of a seed treatment fungicide like carbendazim, edifenphos, or mancozeb at rates of 3g/kg of grain before sowing is also effective.