Oospores of Phytophthora infestans in soil provide an important new source of primary inoculum in Finland

There have been numerous indications since the 1990s that oospore-derived primary infections play an increasing role in the epidemiology of potato late blight. The aim of this study was to verify that oospore-derived epidemics actually occur in Finland. For this purpose, 20 suspected foci of oospore-derived potato late blight were scouted in 2000-2002. All of these were located in fields in which late blight had been observed in at least one of the four previous years. Primary symptoms in these foci occurred always on the lowest leaves near or touching the ground. Leaflets typically showing numerous primary infections or lesions were in direct contact with the soil. In the former, oospores were observed after incubation. Soil samples from two experimental fields, marked by severe epidemics in most years during the last decade, caused infections in a bioassay. Both mating types were on every occasion present in groups of single lesion isolates collected from foci and the bioassay. Oospores survived over the winter, as shown by soil samples taken during the spring that infected potato leaflets in the bioassay. The results presented indicate that oospore-derived epidemics occur in Finland. This paper also discusses the role of oospores in causing blight epidemics in Finland compared to more southern countries.