UC Davis

Erysiphe necator is a primary pathogen on wine and table grapes that requires several fungicide applications for its effective control. However, there are indications that the pathogen is developing resistance to many of the fungicide classes used in its control in the US and other parts of the world. During the 2017-2020 growing seasons, 711 samples of E. necator were collected from wine and table grape vineyards of 18 counties of California. Collected samples were then analyzed for the presence of polymorphisms in the sdhB, sdhC and sdhD genes of E. necator, as mutations in these genes are known to mediate increased tolerance to the succinate dehydrogenase inhibitor (SDHI) class of fungicides. A large number of polymorphisms were discovered, six of which triggered the amino acid substitutions B-p.H242R/L, B-p.I244V, C-p.A83V, C-p.G169S and D-p.I71F that are known to confer SDHI resistance in other fungi. Among the 18 counties that were sampled, Napa and Sonoma were the two counties from which most samples with polymorphisms associated with SDHI resistance were obtained. During the 2017-2020 growing seasons, the B-p.H242R and B-p.I244V substitutions increased in frequency dramatically, whereas the other three polymorphisms were still present in low frequencies in the fungal population. Overall the data indicate that isolates of E. necator with SDHI-target-site mutations are already present in this state, which calls for a more cautious use of the SDHI fungicides.