Potato late blight (PLB) is caused by the oomycete Phytophthora infestans, and is considered the most important disease affecting potato worldwide and is also a major pathogen of tomatoes. The appearance of a new clonal lineage was suggested that rapidly spread through the Eastern US, was introduced to the region affecting both crops. Isolates of Phytophthora infestans were characterized as the newgenotype US-22 collected from potatoes and tomatoes from 2008-10 in Michigan show a drastic change on the population. This new genotype has displaced the US-8 clonal lineage in Michigan at least in potato crops. This study focused on the analysis of P. infestans isolates obtained during 2008 to 2010 inMichigan, and compared them to reference isolates to characterize phenotypic and genotypic variation. Characterization included mating type, Gpi allelozymes, virulence, resistance to mefenoxam, mitochondrial DNA haplotype (mtDNA) and genotyping based on simple sequence repeats (SSR). Most of the isolates were mating type A2, 100/122 Gpi profile and Ia mtDNAhaplotype. These characteristics were attributed to a new clonal lineage US-22, recently found in the US. Resistance to mefenoxam (EC50) ranged from <0.1 – 91 μg/mL,where most of the isolates were classified as intermediate. Race composition and tuber pathogenicity were also variable among isolates, but those obtained from tomato were less pathogenic in tubers than those obtained from potato. To further characterize the population, SSR were used and revealed population structure changes over time that the epidemics occurred. Despite thedifferent virulence races and the broad spectrum of tuber blight pathogenicity observed, the genotypic diversity observed was low. Since tubers are an essential component of epidemics and due to the change in the Michigan P. infestans populations, the responses of six cultivars of potato to different genotypes of the potato late blight pathogen on tuber late blight development were evaluated. Different genotypes of P. infestans,including different isolates of the clonal lineage US-22 were included in the study. Tuber blight measured as RARI (%) showed that US-8 was more aggressive than the recent introduced US-22 in general, but US-22 isolates obtained from potato were more virulent than isolates obtained from tomato. The periderm responses to infection tended to be low, but still US-8 was more likely to infect than US-22. The cultivar Jacqueline Lee showed a consistent tuber blight resistance. Although, US-22 was not highly virulent, it can still infect tuber and it should be considered in breeding programs. It does not exclude the possibility of prevalence and increased fitness over time of this genotype, since it has been recently establish. This study provided useful information about the risk that implies the appearance of clonal lineage US-22, a new genotype in the region affecting breeders and growers. The continuous tracking of changes within P. infestans population could yield evidence of genetic and phenotypic changes shifting due to introduction of new genotypes to the region and the impact that it can have on growers, breeding programs and monitoring of the pathogen itself.
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