Instead of spores, the rice sheath blight fungus produces sclerotia, usually measuring 1 to 3 μm in diameter which are relatively spherical. Sclerotia are formed on or near the spots and can be easily detached from the plant. Under natural conditions, sclerotia usually occur singly but may sometimes coalesce to form larger masses. They are whitish when young and turn brown or dark brown when older.
Sclerotia on the soil or floating on the water are assumed to be the sole source of inoculum in most temperate countries. In a subsequent cropping season, they float to the surface of the paddy water during soil puddling, leveling, and other farm operations. However, sclerotial densities are much lower in the tropics than those reported in temperate and subtropical areas.
Mycelium in colonized crop residues are relatively persistent and are recognized as another source of primary inoculum. It may be a more important source of primary inoculum than sclerotia in the tropics because climatic conditions are more favorable for its survival.
The pathogen is soilborne but it can also infect seeds. Under a stereobinocular microscope, infected seeds on a blotter show dark brownish spreading mycelia.
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