NorPhyt - Studies on the Nordic Blight population

A total of 743 single-lesion isolates of Phytophthora infestans were collected in summer 2003 from Denmark, Finland, Norway and Sweden. Most of the isolates were tested for mating type, and subsets were tested for sensitivity to fungicides and virulence. The ge- netic variation was examined from 200 isolates by SSR microsatellites. Aggressiveness properties, infection efficacy, latency period, lesion growth and spore production was stud- ied from 25 isolates from each four Nordic country. Approximately 60 percent of the isolates were A1 mating type in each country. Both mating types were present in 40 % of the fields where more than one isolate was tested indicat- ing strong potential for sexual reproduction. The proportion of metalaxyl resistant isolates dropped to under 15 % from the 60 % observed in the early 1990s in Norway and Finland. A possible reason for the reduction is lowered selection pressure due to decreased use of metalaxyl. Propamocarb-HCl sensitivity remained unchanged in the Nordic countries when compared to the situation in 1997 2000 in Finland. Four isolates collected from Finland and Sweden were able to sporulate in the presence of this fungicide at a concentration of 1000 mg L-1. In Norway and Finland the frequencies of virulence factors and pathotypes remained nearly unchanged since the 1990s, but the mean number of virulence factors per isolate increased from 5.6 to 6.3. In Denmark and Sweden virulence factors 2 and especially 6 were more common than in Norway and Finland. In addition, in the Swedish population the frequencies of pathotypes were quite even while in other countries pathotype 1.3.4.7.10.11 was clearly the most prevalent one. Genetic variation in the Nordic blight population as indicated by 9 SSR markers was con- siderable. 192 isolates out of 200 gave results for seven or more markers. 181 different genotypes were found. Two genotypes occurred twice, two genotypes occurred three times and one genotype was present in six isolates. The differences in aggressiveness between four countries were generally small or inconsistent between the two test cultivars Bintje and Matilda. On the contrary the variation within the countries was substantial. An esti- mated mean length of latent period varied between 80 and 140-180 hours, sporulation ca-pacity between 0 and 400-1300 sporangia mm-2 and lesion growth rate nearly zero to 5-8 mm day-1. The frequency of infectious sporangia remained below 1 % in all labs except in Norway preventing making a clear conclusion. High variation in results between the test laboratories emphasizes the need of caution when comparing results obtained by different research groups.