Frequency chart of isolates genotyped from USABlight from 2009 to 2023. There is a general trend towards fewer samples over time as well as a homogenization towards US-23.

Frequency chart of genotypes identified from USABlight samples from 2009 to 2023. Note: The three tomato samples identified as US-25 in 2023 were all identified from counties in central NY.

The USABlight team has been monitoring late blight populations using SSR genotyping since 2011.  Genotypes before that time were identified based on RFLP and allozyme genotype alleles. Isolates may vary in sensitivity to mefenoxam.  The genotype of a sample from a given outbreak can help guide fungicide management decisions since some lineages are resistant to mefenoxam (US-8, US-11), while others are sensitive.

For the past several years, we have been tracking genetic changes in the current populations of P. infestans in the US using samples submitted through USABlight.  This country-wide study has revealed a dramatic shift in the genotypes in recent years.  Displacement of the US-22 lineage that spread on tomato transplants in 2009 has occurred.  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.  The US-8 and US-11 lineages are mostly found on the west coast of the US and are mefenoxam resistant.

Click here for info on recent genotype characteristics.