Variation in populations of Phytophthora infestans in Finland and Norway: mating type, metalaxyl resistance and virulence phenotype

Phytophthora infestans was isolated from potato leaves and tubers collected from different parts of Finland in 1990-96 and Norway in 1993-96. Isolates were assessed for mating type, resistance to metalaxyl and virulence pheno type. In Finland 15% of 200 isolates tested and in Norway 25% of 642 tested were A2 mating type. Differences in the A1/A2 ratio were evident among reg ions and A2 was not found in northern areas. In Finland the frequency of me talaxyl-resistant isolates, among 1834 tested, decreased from 59% in 1990-9 5 to 2% in 1996. In Norway 59% of 491 isolates tested were resistant to met alaxyl in 1996. Among 269 Finnish and 105 Norwegian isolates, all known vir ulence genes were found in both countries and race 1.3.4.7.10.11 was the mo st common (resistance gene R9 was not included in the differential set). Oo spores were observed in potato leaves from three locations in the southern part of Norway. The implications of the 'new' populations in the Nordic cou ntries are discussed.