Submitted to: Plant Pathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/27/2005
Publication Date: 6/1/2006
Citation: Cooke, L.R., Carlisle, D., Danaghy, C., Perez, F.G., Deahl, K.L. 2006. The Northern Ireland Phytophthora infestans population 1998-2002 characterized by genotypic and phenotypic markers. Plant Pathology. 55(3):320-330. Interpretive Summary: The pseudofungus, Phytophthora infestans, causes the late blight disease of potato and is more commonly known for its role in causing the disease of potato resulting in the Irish Potato Famine of 1846- 1850. This blight of tomato and potato has become an increasingly important problem to agriculture in the world in the past decade as more aggressive, fungicide resistant, and specialized strains of the pathogen have appeared. This work was conducted to elucidate mechanisms leading to genetic changes that had occurred in natural populations of P. infestans in Northern Ireland. Genetic variation was examined by evaluating 204 strains of P. infestans that were collected during a five year period from 1998-2002. We found unique DNA sequences revealing differences that exist in these populations. These sequences provide a mechanism for monitoring changes that occurred either because of sexual recombination or through introduction of genetically different strains on potato stock. Genetic variation and mechanisms by which the late blight pathogen can overcome host resistance are critical for developing strategies to improve the durability of resistance. The appearance of exotic strains has stimulated comparisons among them and between the old and new strains. These comparisons have provided insight into disease management for pathologists and have emphasized that population structures may differ from one location to another.
Technical Abstract: Two hundred and four isolates of Phytophthora'infestans from Northern Ireland, almost all from commercial potato crops, were collected over the 5 years 1998-2002. Each isolate was obtained from a single crop and/or cultivar. Genotypic and phenotypic diversity was assessed using mating type, metalaxyl resistance, allozyme analyses (glucose-6-phosphate isomerase and peptidase), mitochondrial DNA (mtDNA) haplotype and the multilocus RFLP probe RG57. Single zoospore cultures derived from 20 isolates all proved identical to their parents. All isolates were of the A1 mating type. Allozyme analyses revealed that all were monomorphic and homozygous at the locus coding for allozymes of glucose-6-phosphate isomerase (Gpi 100/100) and the vast majority were similarly monomorphic and homozygous at the peptidase locus (Pep 100/100). However, four isolates were Pep 83/100 and six were Pep 96/100. Three mtDNA haplotypes were detected; haplotype IIa was the most common, but in each successive year up to 2001, its frequency declined with a concomitant increase in Ia isolates and also in the proportion of isolates containing phenylamide-resistant strains. Three isolates from 1998 had the rare haplotype IIb, but this was not detected in other years. There was a marked association between mtDNA haplotype and phenylamide resistance: haplotype Ia was associated with metalaxyl resistance whereas haplotype IIa was more commonly associated with sensitivity to metalaxyl. Analysis of a sub-sample of 91 isolates revealed the presence of nine RG57 genotypes, three associated exclusively with mtDNA haplotype IIa and six exclusively with haplotype Ia. The commonest RG57 genotype (50% of isolates) comprised both metalaxyl-resistant and ñssensitive haplotype IIa isolates. However, of haplotype Ia isolates, all metalaxyl-resistant ones belonged to one of four groups, of which one was the second most frequent overall (30% of isolates), while all metalaxyl-sensitive isolates belonged to one of two other groups. The Pep 96/100 and the Pep 83/100 isolates were all haplotype Ia. The four Pep 96/100 isolates which were included in the RG57 sub-sample all belonged to a single RG57 genotype, which contained no other isolates. Three of the Pep 83/100 isolates were analysed using RG57 and proved to be the sole examples of two different genotypes.