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Biological Significance of Mefenoxam Resistance in Phytophthora erythroseptica and Its Implications for the Management of Pink Rot of Potato

Tubers from plants treated with in-furrow and foliar applications of mefenoxam were inoculated with eight isolates of Phytophthora erythroseptica having varying levels of sensitivity to the fungicide. Two isolates with effective concentration causing 50% reduction of mycelial growth (EC₅₀) values of 0.02 and 0.04 μg ml^-1 were categorized as being mefenoxam sensitive. Isolates with EC₅₀ values >1.0 μg ml^-1 were designated as insensitive to mefenoxam and were grouped two each into low intermediate (EC₅₀ = 1.1 and 5.3 μg ml^-1), high intermediate (EC₅₀ = 26 and 74 μg ml^-1), and resistant (EC₅₀ ≥ 100 μg ml^-1). The biological significance of these isolates was examined by quantifying disease control. P. erythroseptica isolates classified in the resistant group infected a significantly greater proportion of untreated tubers than isolates in any other group. Mefenoxam reduced infection frequency of sensitive isolates by as much as 37%. Mefenoxam did not provide disease control of any isolate possessing insensitivity to the fungicide, with the greatest decrease in control observed with the low intermediate group. Aggressiveness indices, representing tuber infection frequency and depth of penetration, were calculated for untreated and mefenoxam-treated tubers. According to these indices, both isolates classified in the resistant group and high intermediate isolate 252--4 were more aggressive than sensitive isolates in the absence of mefenoxam pressure, and significantly so in the presence of mefenoxam. These results suggest that pink rot may become more severe in fields known to contain P. erythroseptica populations with mefenoxam EC₅₀ values >1.0 μg ml^-1 if the fungicide is applied. These factors should be considered when developing strategies to manage pink rot and mefenoxam-resistant populations of P. erythroseptica.