Characterization of Phytophthora infestans isolates collected from potato in Flanders, Belgium

A collection of 49 isolates of Phytophthora infestans, collected from blighted potato foliage in 1998 in Flanders, were analysed for mating type, response to metalaxyl (48 isolates), allozymes of glucose-6-phosphate isomerase (Gpi), peptidase (Pep) and restriction fragment length polymorphism (RFLP) with probe RG57 (39 isolates). Forty-one A1 and eight A2 isolates were detected. Resistance to metalaxyl was frequently observed: 30resi stant, four intermediate and 14 sensitive isolates were found. Gpi genotypes 100/100 and 90/100 occurred in 30 and nine isolates, respectively. The dominating banding pattern for Pep was 100/100 (32 isolates), but genotypes 96/96, 96/100 and 83/96, never previously reported in continental western Europe, were also identified. Thirteen RG57 fingerprints were observed. On the basis of combined traits, 15 multilocus genotypes were revealed among 39 isolates. Genotypic diversity measured by the normalized Shannon diversity index was 0.56. The results indicate that, besides asexual variation and/or migration, sexual events may have played a role in the establishment of the isolates studied.