P-111 - Fungicide Resistance Among Genetically Distinct Populations of the Potato Black Dot Pathogen, Colletotrichum coccodes

Colletotrichum coccodes is the pathogen responsible for Black Dot and Tuber Blemish on potato. Fungicide-resistant populations of this pathogen in the Midwestern United States are not well documented. We evaluated the genetic variation within C. coccodes collected in the region and compared fungicide resistance/sensitivity among genetically distinct groups. Pure cultures of Colletotrichum-like fungi were isolated from potatoes exhibiting symptoms of Black Dot and Tuber Blemish. Isolates (n=40) were sequenced for the GAPDH gene and compared with publicly available sequences of type cultures in Maximum likelihood (ML) and bootstrap support (BS) phylogenetic analyses. We confirmed all our isolates were C. coccodes and identified four genetically distinct within-species groups, potentially representing different Vegetative Compatibility Groups (VCGs). Representative isolates of each group were tested in vitro against different concentrations of fungicides in amended media. One population was resistant to a multi-site fungicide containing active ingredients prothioconazole, trifloxystrobin, and fluopyram. Another population showed resistance to a single-site fungicide containing flutriafol. Both populations exhibited resistance at high concentrations despite multiple active ingredients and modes of action. This study represents the first attempt to document single- and multi-site fungicide resistance among genetically distinct C. coccodes populations in the Midwest. Further research may reveal genetic differences responsible for fungicide resistance in these groups.