Occurrence, Distribution, Epidemiology, Cultivar Reaction, and Management of Rice Blast Disease in California

Rice blast, caused by Pyricularia grisea, was first found in California in 1996. Disease surveys have shown the blast disease is spreading at a moderate rate in California rice fields. Although no effective major resistance genes are known to occur in widely grown commercial California cultivars, there appear to be differences among the cultivars with respect to field susceptibility to the pathogen. P. grisea was recovered from rice crop residue and commercial seedlots which are suggested as 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. Spore trapping determined that the majority of P. grisea conidia are generally not released until approximately 6:00 A.M. and would not have sufficient time for infection before leaf wetness periods end. Azoxystrobin showed positive results with respect to reduction of neck blast incidence and yield increases in small-plot and large-scale fungicide trials.