Late Blight of Potatoes and Prediction of Epidemics in Arid Central Washington State. G. D. Easton, Plant Pathologist, Department of Plant Pathology, Washington State University, Irrigated Agriculture Research and Extension Center, Prosser 99350. Plant Dis. 66:452-455. Accepted for publication 19 August 1981. Copyright 1982 American Phytopathological Society. DOI: 10.1094/PD-66-452.

A late blight epidemic occurred in arid central Washington state in 1975 on an estimated 800 ha of potatoes under center pivot sprinkler irrigation. Infected seed used to plant the 1973 and 1974 crops apparently provided the primary inoculum. In the spring of 1975, many volunteer plants from overwintering tubers were pushed aside during harvest. Some of these tubers apparently had been infected and had not frozen during the mild winter of 1974–1975, thus providing inoculum for the epidemic. Rains in August, heavy irrigation, and dense foliage on potatoes relatively free of Verticillium wilt, which ordinarily reduces foliage, created an ecoclimate favorable for blight. In 1976 and 1977, only a trace of blight recurred on about 80 ha, and blight disappeared after the severe winter of 1978–1979. Under conditions in central Washington, blight forecasting is not now possible. Occasional epidemics can be expected after mild winters on the cultivar Russet Burbank planted in new soils initially free of Verticillium wilt.

Keyword(s): epidemiology, Phytophthora infestans.