Impact of Phytosanitary Quality of Seed Potato and Temporal Epidemic Progress on the Phenotypic Diversity of Phytophthora infestans Populations

Altogether 365 isolates of Phytophthora infestans were sampled from potatoes propagated from seed potatoes of high (multiplied for two years in open field after meristem phase) and low (commercial certified seed multiplied for several consecutive years in open field) phytosanitary quality at different phases of epidemic progress during the growing seasons of 2001–2007 from field plots at two experimental institutes in Estonia, North-East Europe. High or low phytosanitary quality of seed potatoes had no effect on mating type ratio or response to metalaxyl in populations of P. infestans isolated from these two different groups of potato material. In contrast, the incidence of certain virulence factors, as well as the diversity of pathotypes, was very high in populations collected from potatoes propagated from low-quality seed in comparison to those from high-quality seed. The incidence of A2 mating type and fully metalaxyl sensitive strains was statistically significantly higher at the epidemic outbreak than later during epidemic progress. The incidence of most virulence factors and overall pathotype diversity were not affected by the temporal progress of the epidemic. Rare virulence factors 5 and 9 were more frequent at the outbreak of the epidemic and declined in the population during the course of epidemic.