Occurrencce and nature of rice blast disease caused by Pyricularia grisea in California

Rice blast disease, caused by the fungus Pyricularia grisea, was first identified in California in 1996. P. grisea was isolated from symptomatic rice panicle neck nodes and shown to be pathogenic on rice in the greenhouse. Rice blast disease was first observed in Glenn and Colusa counties in 1996 and Sutter County in 1997. The disease appears to be spreading at a moderate rate in California rice fields. Severity of rice blast was highest in 1996, less in 1997, and least in 1998.;Pathotype testing and DNA fingerprinting of California P. grisea isolates revealed a remarkably homogeneous population with only one international race (IG-1) and one MGR586 fingerprint group (lineage) detected in extensive isolate collections from 1996, 1997, and 1998. Homogeneity of the California P. grisea population suggests that the introduction of a single clonal lineage of the pathogen was responsible for the occurrence of rice blast in California.;P. grisea was recovered from rice crop residue and commercial seedlots, but not from weed species in and around rice fields. Crop residue and seed are possible sources of initial P. grisea inoculum in California rice fields.;Examination of weather data indicates that environmental conditions in California rice producing areas are permissive for rice blast but generally not optimal for epidemic development. Weather data from California rice fields in 1998 indicated that long leaf wetness and high relative humidity periods required for sporulation and infection were common. However, peak spore release generally occurred around 6 a.m. which would not allow sufficient time for efficient infection before the leaf wetness periods end over the majority of the growing season.;Concentrations of sodium hypochlorite in seed soaks that did not adversely affect germination and seedling development were found to be ineffective in eliminating P. grisea from infested rice seed. Quadris fungicide shows promise for control of neck blast, resulting in improved yield and milling quality, and reduced percent infested seed. Excess nitrogen over that required for maximum yield resulted in an increase of severity of the blast disease. Cultivar M-201 is more susceptible to blast than other California cultivars.