Mefenoxam Sensitivity in US-8 and US-23 Phytophthora infestans from Wisconsin

Abstract

The contemporary dominant clonal lineage of heterothallic Phytophthora infestans in Wisconsin, US-23, is classified as sensitive to the systemic fungicide mefenoxam and is of the A1 mating type. With the sporadic appearance of clonal lineage US-8, classified as resistant to mefenoxam and of the A2 mating type, there is a need for ongoing monitoring and characterization. Isolates of P. infestans collected from Wisconsin during the 2017 and 2018 growing seasons were tested for sensitivity to mefenoxam with discriminatory dose of 100 ppm. In 2017, both US-23 and US-8 were isolated. On average, isolates of US-23 were significantly more sensitive to mefenoxam than were US-8 isolates (P = 8e-04). There were significant differences in the sensitivity levels among the US-8 isolates (P = 2.02e-06), with a single isolate testing sensitive at 100 ppm of mefenoxam based on the one-way ANOVA. There were significant differences in the sensitivity levels among US-23 isolates (P = 3.75e-09), with two isolates showing resistance. In 2018 only US-23 was found, and isolates were tested for mefenoxam response at 0, 0.1, 1, 10, and 100 ppm. At 0.1 ppm, isolates showed significantly different levels of sensitivity (P = 2.1e-09), and a single isolate showed complete resistance. Isolates from both clonal lineages and years that exhibited moderate levels of resistance had greater variability among replicates. The phenotype of this multigenic trait comes through in the variability seen in isolates that are showing more resistance. Continued screening of P. infestans for mefenoxam sensitivity will help track the development and mechanism of resistance, as well as aid in development of best management approaches.