The Northern Ireland Phytophthora infestans population 1998–2002 characterized by genotypic and phenotypic markers

Abstract

A total of 204 isolates of Phytophthora infestans from Northern Ireland, almost all from commercial potato crops, were collected over 5 years (1998–2002). Phenotypic diversity was assessed using mating type and metalaxyl resistance; genotypic diversity was assessed using two allozyme loci (glucose‐6‐phosphate isomerase, Gpi, and peptidase, Pep), mitochondrial DNA haplotype and the multilocus RFLP probe RG57. All isolates were A1 mating type and Gpi 100/100. The majority were Pep 100/100, but four Pep 83/100 and six Pep 96/100 isolates were identified. Three mtDNA haplotypes were detected; haplotype IIa was the most common, but each year up to 2001 its frequency declined, with a concomitant increase in Ia isolates. Three isolates had the rare haplotype IIb, but this was only detected in 1998. Metalaxyl resistance and mtDNA haplotype were markedly associated: most haplotype Ia isolates were metalaxyl‐resistant, whereas haplotype IIa was more commonly associated with metalaxyl sensitivity. Analysis of a subsample of 91 isolates revealed nine RG57 genotypes, three associated exclusively with haplotype IIa and six exclusively with haplotype Ia. The most common RG57 genotype (51% of isolates) comprised both metalaxyl‐resistant and ‐sensitive haplotype IIa isolates. However, of haplotype Ia isolates, all metalaxyl‐resistant phenotypes belonged to one of four RG57 types, one of which was the second most frequent overall (29% of isolates), while all metalaxyl‐sensitive isolates belonged to one of two other types. The P. infestans population in Northern Ireland appears to consist of a limited number of clones related to, but differentiated from, the populations in mainland Britain and elsewhere in Europe.