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

Abstract

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 mefenoxamsensitive, -intermediate or -resistant. A significant yearly increase was observed in the frequency of mefenoxamresistant 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 mefenoxamsensitive 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.Resumen En inspecciones por ocho años en campos de papa de Minnesota, se colectaron un total de 1,772 aislamientos de Phytophthora erythroseptica de tubérculos con síntomas de pudrición rosada. Todos los aislamientos se evaluaron para sensibilidad al fungicida sistémico mefenoxam y se categorizaron como sensitivos al mefenoxam, intermedios o resistentes. Se observó un aumento significativo anual en la frecuencia de los aislamientos de P. erythroseptica resistentes recuperados y en la frecuencia de los campos de los cuales se originaron los resistentes al mefenoxam. Adicionalmente, se recuperaron, en cada año del estudio, aislamientos con niveles intermedios de resistencia a mefenoxam. En una inspección similar, se recuperaron más del 80% de aislamientos de P. erythroseptica sensibles, entre un total de 293 colectados durante tres años en Dakota del Norte, y no se recuperaron aislamientos de P. erythroseptica con resistencia intermedia a mefenoxam. Los resultados que aquí se reportan confirman la presencia de resistencia a mefenoxam entre los aislamientos de P. erythroseptica en Minnesota y Dakota del Norte, pero a niveles variables entre los años del estudio en ambos Estados. La presencia de aislamientos de P. erythroseptica con niveles intermedios de resistencia a mefenoxam colectados en Minnesota respalda estudios previos que indican que los aislamientos que muestran respuestas intermedias a mefenoxam indican un flujo transicional de una población sensible a una resistente a mefenoxam.

Identification of sequence mutations in Phytophthora cactorum genome associated with mefenoxam resistance and development of a molecular assay for the mutant detection in strawberry (F. × ananassa)

“…Several Phytophthora spp. resistant to mefenoxam have been reported in multiple hosts, such as P. capsici on cucurbits and peppers, P. infestans on tomato, and P. erythroseptica on potatoes 16 – 21 . In some cases, fitness penalties, such as slow growth, are associated with mefenoxam-resistant isolates 14 , 15 ; however, stable resistance has been reported in P. insfestans without fitness disadvantages 22 .…”