Late blight has been occurring on tomatoes more commonly in the northeastern region of the USA since 2009 compared to previous years. Many growers and gardeners experienced late blight developing on their plants for the first time. New genotypes of Phytophthora infestans (US-22 and US-23) were associated with most occurrences. They are more aggressive on tomato than US-8, the dominant genotype occurring in the region before 2009. US-8 mostly occurred in major potato production areas. While the genotypes occurring in the northeastern USA are of opposite mating type, only one genotype typically has been found associated with discrete outbreaks. Oospores do not appear to be an initial source of inoculum. US-22 and US-23 are sensitive to mefenoxam, a highly effective fungicide to which US-8 is resistant. Pathogen genotype now can be determined within one day, which has enabled growers to confidently use mefenoxam to effectively manage late blight in conventionally produced crops. There has been extensive effort to educate gardeners about late blight and the impact this disease can have when left unmanaged in a garden. This has resulted in at least one case of prompt response stopping an outbreak. A national late blight program (USABlight.org) was launched in 2010. It includes a national monitoring program and a decision support system for managing late blight. The first tomato cultivars with the Ph-2 and/or Ph-3 resistance genes became commercially available in 2011. Growers, especially organic growers, have started to use these cultivars as part of the disease management program.
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