Competitive Parasitic Fitness of Mefenoxam-Sensitive and -Resistant Isolates of Phytophthora erythroseptica under Fungicide Selection Pressure

A 2-year field and laboratory experiment was initiated to study the competitive parasitic fitness of mefenoxam-resistant (50% effective concentration [EC(50)] > 100 mu g ml(-1)) and mefenoxam-sensitive (EC(50) = 0.07 mu g ml(-1)) isolates of Phytophthora erythroseptica with equal aggressiveness. The competitive ability of the mefenoxam-resistant and -sensitive isolates was tested under no selection pressure (nonfungicide treated) as well as under the influence of mefenoxam and non-mefenoxam (phosphorous acid) fungicides. P erythroseptica isolates were combined in four ratios of mefenoxam-resistant (R) to mefenoxam-susceptible (S) (0R:0S, 1R:1S, 3R:1S, and 1R:3S) and subsequently infested into the soil at the time of planting. In-furrow mefenoxam applications were applied to the soil immediately following infestation with P erythroseptica. Phosphorous acid was applied at tuber initiation and 14 days after tuber initiation. Noninfested, non-fungicide-treated plots served as controls. P erythroseptica isolates recovered from field-infected pink rot tubers at harvest and 3 to 4 weeks after harvest were tested for mefenoxam sensitivity in vitro. In vivo studies were performed by challenge inoculating a zoospore suspension in the four ratios described above onto potato tubers harvested from nontreated, phosphorous acid-treated, or mefenoxam-treated field plots. These field plots were not infested with P. erythroseptica at planting. Results from both field and in vivo studies demonstrate that mefenoxam-resistant isolates of P erythroseptica are as fit as sensitive isolates in the absence of selection pressure or in the presence of a phosphorous acid fungicide treatment. Under mefenoxam selection pressure, mefenoxam-resistant P erythroseptica isolates were more parasitically fit than -sensitive isolates. These studies suggest the lack of an apparent fitness penalty in mefenoxam-resistant P erythroseptica populations under field conditions and that these isolates could be stable in most agroecological systems. Based on these results, mefenoxam-based fungicides are no longer recommended for the management of pink rot once mefenoxam-resistant P erythroseptica populations are detected in a specific field.