Previous and current research studies have identified multiple genotypes, or lineages, of Phytophthora infestans. Genotypes are identified based on RFLP and SSR alleles and isolates may vary in sensitivity to mefenoxam. When the genotype of a sample from a given outbreak is known, it can help guide fungicide management decisions.
For the past several years, we have been tracking changes in the current populations of P. infestans in the US based on samples submitted through USABlight. This country-wide study has revealed a dramatic shift in the distribution of genotypes in recent years. US-23, a mefenoxam sensitive lineage that attacks both potato and tomato hosts, has been almost exclusively the only lineage detected, especially in the eastern United States since 2013. Detections of other lineages have been rare. Displacement of the US-22 lineage that spread on tomato transplants in 2009 has occurred. This displacement phenomenon is reminiscent of a similar occurrence in the mid 1990s, when the mefenoxam-sensitive lineage US-1 was replaced by mefenoxam-resistant lineages.
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