Mefenoxam resistance in Phytophthora erythroseptica Pethybridge: Prevalence, inheritance, and its effect on parasitic fitness

The prevalence of mefenoxam resistance among isolates of Phytophthora erythroseptica was studied for 4 years in Minnesota and for 3 years in North Dakota. The prevalence of mefenoxam resistance appears to be increasing in Minnesota. Logistic regression analyses (Wald chi2 = 60.6; P < 0.001) showed an overall significant yearly increase in the recovery of mefenoxam-resistant P. erythroseptica isolates in Minnesota during the survey period. The prevalence of P. erythroseptica with intermediately-resistant isolates from Minnesota is indicative of a transitional flux from a mefenoxam-sensitive population to a resistant one. In contrast, the P. erythroseptica population in ND is largely mefenoxam-sensitive with complete lack of intermediately-resistant isolates.;Eight hundred single-zoospore isolates of P. erythroseptica, produced from each of eight parental isolates with diverse sensitivity (2 -resistant, 4 -intermediately resistant and 2 -sensitive isolates) to mefenoxam were evaluated for their inheritance of mefenoxam resistance. The progeny of mefenoxam-sensitive (EC50 < 1 mug/ml), the mefenoxam-resistant (EC50 > 100 mug/ml), and the intermediate (EC50 values range from 1 to 99 mug/ml) parental isolates had the same phenotype as the parents and there was no major shift towards increased insensitivity. Results of the present inheritance studies of mefenoxam resistance using single-zoospore progeny do not support previous reports of the quantitative nature of inheritance.;Parasitic fitness studies with mixed populations of mefenoxam-resistant (R) and mefenoxam-sensitive (S) isolates of P. erythroseptica in the combinations of 1R:IS, 3R:1S and 1R:3S indicate that the competitive abilities of both mefenoxam-sensitive and -resistant isolates of P. erythroseptica were similar in the absence of fungicide pressure and under the selection pressure of a non-mefenoxam fungicide (phosphorous acid) (P > 0.05). However, with selection pressure from mefenoxam, -resistant isolates out-competed -sensitive isolates in all combinations ( P < 0.05) compared to other whole plot treatments. Field and in vivo generated data suggest that there was no fitness penalty observed in the mefenoxam-resistant isolates in the presence or absence of selection pressure of mefenoxam and also in the presence of the non-mefenoxam fungicide. The knowledge provided in the current research is important in the development of effective pink rot disease management strategies.