Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/2/2002
Publication Date: N/A
Interpretive Summary: Variability in virulence and fungicide resistance of fungal plant pathogens contributes to failure in established disease management procedures. Recently, variation in sensitivity to fungicides has been especially troublesome, and for decades diversity in specific virulence has severely limited the usefulness of cultivars with specific resistances. Phytophthora infestans (the fungus that causes potato late blight) fits this scenario. Resistance to metalaxyl limits the usefulness of the fungicide in various locations in the world, and the occurrence of many specific virulence factors in populations of P. infestans eliminated the use of genes for specific resistance as an important disease management strategy. On the premise that knowledge of the population genetics of fungal plant pathogens may contribute to development of more durable disease management strategies, we initiated a study for P. infestans from Uruguay. Results show a predominate presence of rare virulence genes in complex races. These races appear to contain unnecessary genes including ones that have never been of importance to commercial cultivars. This information will greatly benefit plant pathlogists and breeders attempting to track the spread of this pathogen in potato seed and make control recommendations to farmers, seed importers/exporters and agriculture officials.
Technical Abstract: Isolates of Phytophthora infestans were collected from several potato growing regions in Uruguay in 1998 and 1999 from late blighted plants and subsequently characterized. In total, 25 representative isolates were characterized in terms of allozymes (glucose phosphate isomerase [Gpi] and peptidase [Pep], mating type, mitochondrial haplotype, pathotype, and metalaxyl resistance. Allozyme analyses revealed the Uruguayan isolates were monomorphic and homozygous at the loci coding for allozymes of glucose-6-phosphate and peptidase (Gpi 160/100, Pep 100/100). All isolates were A2 mating type and mitochrondrial DNA haplotype IIa. Metalaxyl-resistant isolates accounted for 93% of the total samples. Most of the isolates displayed broad-spectrum virulence and 5 of the isolates carried virulence genes to 9 of the 10 R-genes tested. Virulence phenotypes in this country were extremely complex even without the selection pressure of R-genes in commercial cultivars. By analyzing these isolates for genetic and phenotypic diversity, it could be concluded that the P. infestans populations in Uruguay are representative of the new pathogen populations that are currently predominant in many countries.