Cause and Control of Rice Diseases - 84
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Research on the etiology and control of rice diseases has been conducted
with industry support since 1969. Although California is free of some of the
serious diseases that are troublesome in other rice growing regions, there
are diseases that can reduce yields significantly. Much of the concern in the early 1970s was on seedling diseases which destroyed many seedlings and often resulted in a need to replant. These were shown to be caused by the water molds Achlya and Pythium. Over 30 different fungicides were tested for their control. By 1972 captan and difolatan were shown to provide effective control. In 1976, another fungicide, kocide SD was added and registered for use on rice. Three methods of treatment were compared. Treating the dry seed by the slurry or spray methods was better than treating after soaking the seeds or adding the fungicide to the soaking water. The dry seed treatment did not damage the germination ability of seeds even when done four months before planting. Although studies show that seedling rates can be decreased when treated seed is used, growers continue to sow at about the same rate to ensure getting good stands. Stem rot control
Other diseases of rice have been much more difficult to control. The most serious one in California is stem rot, a fungus disease that invades the sheath and stem of the rice plant. Infected stems either die or produce small panicles and shrunken kernels. Infected plants also are more prone to lodging. A 1974 survey led to an estimate that stem rot causes 5 to 8 percent losses in yield. Research for a chemical control of stem rot disease showed that Duter gave good control, but Duter has never been approved for use on rice. Other promising chemicals are being studied. Studies of the life cycle of Sclerotium oryzae, the organism causing stem rot disease, show that it overwinters as sclerotia in loose straw or standing stubble. When rice fields are flooded in the spring, the sclerotia float to the surface and invade the rice seedlings as they emerge from the water. Efforts to find cultural practices that reduce the amount of inoculum (sclerotia) carried over from year to year show that:
The sclerotia can survive several years. Therefore, alternate cropping of rice is no assurance of avoiding the disease, although alternate cropping greatly reduces the number of surviving sclerotia. Temperatures over 52°F favor the development of sclerotia in overwintering straw and stubble. Therefore, it is probable that fields not burned until spring have more disease than those burned in the fall. However, if spring burns are thorough, they are nearly as effective as fall burns in limiting inoculum. The advent of the new short-statured rice varieties appears to be correlated with an increase in stem rot disease, although some of the new varieties are more tolerant to stem rot than the varieties they are replacing. A possible reason for this correlation may be that field burning is not as thorough as with the older tall varieties and, therefore, favors the carryover of sclerotia. The search for genetic resistance to stem rot was not encouraging after screening more than 5,000 varieties and strains of cultivated rice. Some of the weedy, or wild species, of the rice genus Oryzae were promising sources of genetic resistance. Successful crosses were made between the cultivated and the more resistant wild species. Those that appear to carry the resistance have been selected and turned over to the rice breeders for incorporation into the breeding program. Sheath blightAggregate sheath spot (sheath blight) is becoming an increasing concern in California. By 1980, it was recognized as being widely distributed throughout the rice growing areas. The life cycle of the organism and disease symptoms are somewhat similar to those of stem rot. Genetic resistance from wild species is being sought in a program similar to that used for stem rot. Kernel smutKernel smut of rice was identified for the first time in California in 1982. Kernel smut results from infection of florets. The grain is replaced by a black sooty mass of smut spores. Studies are under way to determine the nature and extent of this disease in California. Other diseases of minor importance at present are Pythium crown rot and Helminthium leaf spot. Currently, they are not a threat to California rice production, but they are being carefully watched and studied.
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