Late Blight Epidemics in the Columbia Basin

Late blight was not originally expected to be a serious threat to potato in the semi arid environment of the Columbia Basin. However, the disease has occurred every year at various severities since 1990. Migration of Phytophthora infestans into the Columbia Basin in the 1990’s is well documented. The US-1 strain predominated in 1992 and several unique isolates were discovered in 1993, which were likely the result of genetic recombination. The recombinants were ephemeral and were not found in 1994. The US-8 stain was first observed in 1994 and came to predominate in 1995 and in subsequent years. Epidemics in the Columbia Basin have been traced to infected seed tubers, refuse tubers and volunteers. Late blight spreads in fields by foci with foci enlarging in size, producing daughter foci, and coalescing. The process continued as favored by the environment. Sporangia of P. infestans have the capability of surviving in water for extended periods of time after detachment from sporangiophores on potato tissue. Late blight has been successfully forecasted and managed regionally in the Columbia Basin. Early season rain is an effective indicator of late blight outbreaks because moisture is important for the build-up of inoculum in fields during the early stage of epidemics. Early in epidemics, moisture promotes transmission of P. infestans from infected seed tubers to emerged shoots in fields. Method of fungicide application affects fungicide distribution and cost. The alternate use of air application and chemigation provides good protection at a reduced cost compared to only air or ground application methods. Application of phosphorous acid to tubers after harvest and prior to storage can result in a reduction in post-harvest infection by P. infestans.