Phenotypic and genotypic diversity in Phytophthora infestans on potato in Great Britain, 1995–98

Abstract

A total of 2691 single‐lesion isolates of Phytophthora infestans was established from samples of late‐blight disease from 354 commercial and garden/allotment sites in Scotland, England and Wales over four growing seasons, 1995–98. The A2 mating type was rare (3·0% of isolates) and was detected at only 34 sites. In vitro tests of sensitivity to the phenylamide fungicide metalaxyl showed that 316 sites yielded isolates with some insensitivity (resistant and/or intermediate); these were more often commercial sites than garden/allotment sites. Over the four seasons, the frequency of isolates with intermediate fungicide sensitivity increased, while the frequency of resistant phenotypes decreased. Resistant isolates were always of A1 mating type. A subset of 1459 isolates from 326 sites was analysed for molecular diversity. Mitochondrial DNA (mtDNA) haplotype Ia predominated (91·0% of isolates); haplotype IIa was present at 54 sites and both haplotypes at 33 sites. The multilocus RFLP probe RG57 detected 30 fingerprints. Four fingerprints were particularly common (RF002, RF006, RF039 and RF040) and 10 were unique to a single site in a single year. The three commonest fingerprints (RF039 > RF002 > RF006) were of A1 mating type and the fourth (RF040) was A2. RF002 isolates were resistant to the phenylamide metalaxyl and were more common in Scotland than in England and Wales. Small sample sizes limited the usefulness of estimates of diversity. Although approximately half of all sites appeared to be colonized by RF039 genotypes, some sites (both commercial and garden/allotment) showed a higher diversity, having both common and unique genotypes. The genotypic diversity within isolates collected from commercial sites and those from garden/allotment sites were similar. The contributions of sexual reproduction and alternatives to sex to the generation of variation are discussed.