POPULATION GENETIC-STRUCTURE OF PHYTOPHTHORA-INFESTANS IN THE NETHERLANDS

Isolates of Phytophthora infestans were collected from six different regions in the Netherlands in September-October 1989 and subsequently characterized. Regions contained one to four sampling sites and yielded 186 isolates. Additionally, 19 isolates from an ongoing metalaxyl-resistance monitoring project were characterized. In total, 205 isolates were characterized in terms of allozymes (glucose phosphate isomerase [Gpi-1] and peptidase [Pep-1]), mating type, and metalaxyl resistance. The analysis revealed 17 different genotypes. Samples from some sites were highly heterogeneous, whereas samples from other sites appeared homogeneous. Three genotypes each were detected in five of the six regions and together accounted for 61% of all isolates. Metalaxyl-resistant isolates accounted for 35% of the total sample and 45% of the samples from commercial fields. Chi-square contingency analysis indicated significant differences in genotype frequencies among subpopulations from different regions of the country, between A1 and A2 individuals, and between potato and tomato isolates. In most locations the frequency of allozyme alleles differed significantly from frequencies expected according to Hardy-Weinberg equilibrium. The results were consistent with asexual reproduction, although the occurrence of a low level of sexual reproduction cannot be excluded.