Population Genetics and Fungicide Resistance of Botrytis cinerea on Vitis and Prunus spp. in California

Botrytis cinerea, the causal agent of gray mold, has high genetic diversity and a broad host range. In Vitis sp. and Prunus spp., B. cinerea causes pre- and postharvest diseases, and fungicides are routinely applied to prevent yield loss. In total, 535 isolates of B. cinerea collected from Vitis sp. and Prunus spp. in 2012, 2016, and 2017 were genotyped using 18 microsatellite markers and the transposable elements (TEs) Boty and Flipper. Only nine of the polymorphic markers and the two TEs were considered informative and retained for the final analyses. Of the 532 isolates, 297 were tested for resistance to seven fungicides representing six Fungicide Resistance Action Committee classes. After clone correction, 295 multilocus genotype groups were retained across the 3 years in 326 individuals, and four genetic subpopulations were detected. High levels of clonality were observed across the dataset. Significant pairwise differentiation was detected among years, locations, and TE composition. However, most of the diversity observed was within a subpopulation and not among subpopulations. No genetic differentiation was detected among resistant and sensitive isolates for individual fungicide classes. When resistance to the total number of fungicides was compared, regardless of the fungicide class, significant differentiation was detected among isolates that are resistant to two fungicide classes and those resistant to three or four fungicide groups. Fungicide resistance frequencies were stable for most chemistries evaluated with the exception of fluopyram, which increased from 2012 to 2016/2017.