Ecological study of crabgrass blast disease in the snowy Shonai district of Yamagata Prefecture.

Abstract

Crabgrass blast, caused by Pyricularia grisea, is widespread in the Shonai district of Yamagata Prefecture and occurs every year in early July. Because the primary inoculum in snowy areas is not always known, here we looked for sources of primary inoculum in areas of the Shonai district that receive heavy snowfall. First, we tested for overwintering of the pathogen in crabgrass. Although ~0.1% of crabgrass seeds collected in the fall carried P. grisea, none of the seed collected after snowmelt the next spring carried the fungus. Nor did any nursery blast develop on seedlings grown from these seeds. The pathogen was reisolated from infected seeds and leaf blades after overwintering under low temperatures and dry conditions, but not from seeds and infected leaf blades and ear branches that had been covered with snow during the winter. On infected leaf blades, P. grisea survived for ~2 months in the field under snow, and for about 3 months without snow. These results suggest that P. grisea does not survive successfully on crabgrass plants under snow. Next, the timing of spore dispersal and the conditions required for infection were examined. Spores were dispersed in early July, but not in June in Shonai. For infection, 3–6 h of wetness and a density of 10 spores/ml were needed. Pot2 genotypes of P. grisea, found during early, middle and late periods of epidemics were analyzed for any changes in the genotypes in the populations. at multiple fixed points. As the disease progressed, the genotypic constitution of the populations diversified. Because the fungus is not successful in overwintering in Shonai, dispersed spores serve as the primary inoculum in a single event to initiate crabgrass blast in Shonai, and secondary infections are initiated from newly produced spores.