Effects of Sowing Dates on the Progress of Disease Caused by Purple Stain of Soybean

A survey of soybean fields in Hiroshima Prefecture revealed that the incidence of purple stain caused by Cercospora kikuchii in soybean (Glycine max (L.) Merr.) seeds has been significantly promoted by sowing seeds in the middle of rainy season (early-sowing) . The progress of disease on leaves of plants from early-sown seeds was more marked during the rainy reason, especially when the disease has already established before the onset of the rainy season. This promotion of disease development was recognized not only on the leaves but also on seeds and lasted until harvest time. This effect of early sowing of seeds has been attributed to the longer exposure of plants to the rainy season. Late-sowing, which is done in early July, had afforded the plants some time to escape the rainy season and hence, enabled them to be less prone to the disease. In addition, seed maturation period in early-sowing field was longer than that in late-sowing, and thus, exposing the seeds to a longer infectious period. These results indicate that late sowing of seeds can be used as a cultural practice for the control of this disease.