Diseases California's arid climate provides an unfavorable environment for foliar diseases when compared to humid tropical and subtropical rice areas. Seedling diseases, stem rot, and aggregate sheath spot are the most important rice diseases in California. Kernel smut is occasionally present, but is considered a minor disease. Seedling diseases are caused by the water mold fungi, Pythium spp. and Achyla klebsiana, and can reduce seedling survival and result in poor rice stands. These diseases are especially severe when rice emergence is slowed by cool air and water temperatures during stand establishment. Vigorous seed, moderate water depths, and later planting dates minimize seedling diseases. Increased seeding rates can help offset seedling disease losses.
Stem rot (Sclerotium oryzae) and aggregate sheath spot (Rhizoctonia oryzae-sativae) are the most serious rice diseases in California (fig. 23). These fungi overwinter as small resting structures, the sclerotia, in infected crop residue or free in the soil. Stem rot sclerotia are small and black; aggregate sheath spot sclerotia are rectangular to globose, and brown. In spring and early summer, the sclerotia float to the water surface and infect the rice plant at the water line. Stem rot symptoms first appear during tillering as small black lesions on leaf sheaths at the water line. As the disease progresses, infected sheaths die and the infection may penetrate the culm.
Aggregate sheath spot lesions first appear on lower leaf sheaths at the water line and are circular to elliptical with gray-green to straw-colored centers surrounded by distinct brown margins (fig. 24). Secondary infections progress up the stem and may spread to the flag leaf and panicle. Both diseases are managed by minimizing the carryover of inoculum to the next rice crop. Burning rice residue, removing infected residues from the field, or rotation out of rice for two to three years are the most effective control alternatives. Excessive N fertilizer or dense stands caused by seeding rates that are too high can increase the severity of these diseases. All currently grown public varieties of rice are susceptible to some degree. Rice blast (Pyricularia grisea) is a new disease in California, but is considered to be one of the worst rice diseases worldwide. Under the right environmental conditions this disease is known to have multiple disease cycles in a single season, resulting in very high inoculum levels that can be highly damaging to the crop. The fungus can produce lesions on most of the shoot, including the leaves, leaf collar, stem, nodes, panicle and grains, but not the leaf sheath. Successful control of blast requires an integrated management program including the use of resistant varieties, cultural practices and chemical control.
More Information on Rice Blast. | ||||||
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