LAUSR

Samples from potato fields with late blight-like symptoms were collected from eastern North Carolina in 2017 and the causal agent was identified as Phytophthora nicotianae. We have identified P. nicotianae in potato and tomato from North Carolina, Virginia, Maryland, Pennsylvania, and New York. Ninety-two field samples were collected from 46 fields and characterized for mefenoxam sensitivity, mating type, and SSR genotype using microsatellites. Thirty two percent of isolates were the A1 mating type, while 53% were A2 mating type. In six cases, both A1 and A2 mating type were detected in the same field in the same year. All isolates tested were sensitive to mefenoxam. Two genetic groups were discerned based on STRUCTURE analysis: one included samples from North Carolina and Maryland, and one included samples from all five states. The data suggest two different sources of inoculum from the field sites sampled. Multiple haplotypes within a field and the detection of both mating types in close proximity suggests that P. nicotianae may be reproducing sexually in North Carolina. There was a decrease in the average number of days with weather suitable for late blight, from 2012-2016 to 2017-2021 in all of the NC counties where P. nicotianae was reported. Phytophthora nicotianae is more thermotolerant than P. infestans and grows at higher temperatures (25-35°C) than P. infestans (18-22°C). Late blight outbreaks have decreased in recent years and first reports of disease are later, suggesting that the thermotolerant P. nicotianae may cause more disease as temperatures rise due to climate change.