Prevalence of Mefenoxam Resistance Among Phytophthora erythroseptica Pethybridge Isolates in Minnesota and North Dakota

In an 8 year survey of Minnesota potato fields, a total of 1,772 isolates of Phytophthora erythroseptica were collected from potato tubers showing symptoms of pink rot. All isolates were evaluated for sensitivity to the systemic fungicide mefenoxam and categorized as mefenoxam-sensitive, -intermediate or -resistant. A significant yearly increase was observed in the frequency of mefenoxam-resistant P. erythroseptica isolates recovered and in the frequency of fields from which mefenoxam-resistant isolates originated. Additionally, isolates with intermediate levels of mefenoxam resistance were recovered in each year of the survey. In a similar survey, greater than 80% of P. erythroseptica isolates recovered were mefenoxam-sensitive among 293 total isolates collected over 3 years in North Dakota, and no P. erythroseptica isolates with intermediate resistance to mefenoxam were recovered. Results reported here confirm the presence of mefenoxam resistance among isolates of P. erythroseptica in Minnesota and North Dakota but at varying levels among years of the survey in both states. The presence of P. erythroseptica isolates with intermediate levels of resistance to mefenoxam collected in Minnesota support earlier studies which indicate that isolates displaying intermediate responses to mefenoxam are indicative of a transitional flux from a mefenoxam-sensitive to a mefenoxam-resistant population.