Rapid Detection of Fungicide Resistance Phenotypes Among Populations of Nothopassalora personata in South Carolina Peanut Fields

We evaluated the occurrence of fungicide resistance phenotypes in populations of Nothopassalora personata, the causal agent of late leaf spot (LLS), sampled from South Carolina peanut fields in 2018 using a modified detached leaf assay (DLA). Spore suspensions obtained from each of 72 groups of LLS-symptomatic leaves collected from nine counties were used in the DLA to examine phenotypic resistance to four site-specific fungicides (azoxystrobin, benzovindiflupyr, prothioconazole, and thiophanate-methyl) and one multisite contact fungicide (chlorothalonil). Lesion development was measured as a binary event in which presence of a lesion indicated control failure (phenotypic resistance) and absence of a lesion indicated successful control (sensitivity). Phenotypic resistance probabilities of N. personata samples to each site-specific fungicide were compared against the control fungicide (chlorothalonil). Variation in phenotypic resistance probabilities to the four single-site fungicides was observed from most counties (6 of 9), with frequencies of phenotypic resistance having differed among counties. The DLA facilitated rapid evaluation of N. personata phenotypic resistance to azoxystrobin, benzovindiflupyr, prothioconazole, and thiophanatemethyl. This assay has potential to be used as an alternative method for routine monitoring of phenotypic resistance, identification of populations to test for the presence of resistance genes, and improving reliability of current fungicide recommendations.