Fungicide Resistance in the Early and Late Leaf Spot Pathogens of Peanut and Their Management

Abstract

The aim of this project was to improve management recommendations for leaf spot diseases of peanuts in Alabama. Early leaf spot (caused by Passalora arachidicola) and late leaf spot (caused by Nothopassalora personata) are the two most widespread and damaging foliar diseases of peanuts in the southeastern United States, which can cause yield losses of up to 70%. Leaf spot diseases are managed through a combination of fungicide applications, cultivar selection, and various production practices. A leaf spot fungicide spray program includes one or more single-site fungicides such as demethylation inhibitors, succinate dehydrogenase inhibitors, and quinone outside inhibitors in rotation with the multi-site fungicide chlorothalonil. Repeated use of single-site fungicides can lead to resistance development in the target pathogens. Therefore, peanut leaf spot pathogen populations in Alabama were assessed for resistance development risk against six single-site fungicides and chlorothalonil. Study results indicated varying levels of resistance risk to the single-site fungicides, but penthiopyrad and pydiflumetofen posed the highest risk for resistance in the leaf spot pathogens. To lower the risk of fungicide resistance, producers alternate, or tank mix single-site fungicides with different modes of action or incorporate the multi-site fungicide chlorothalonil in spray programs. Currently, chlorothalonil is the only multi-site fungicide used to manage leaf spots in peanuts, which was recently banned for use in agriculture by the European Union in 2020. Since this could potentially impact US peanut production and exports, it is critical to identify potential alternatives to chlorothalonil. In this study, research results demonstrated that copper sulfate, dodine, and sulfur alone or in combination with single-site fungicides can serve as potential alternatives to chlorothalonil under field conditions in southeast Alabama. To potentially reduce fungicide inputs, the response of fourteen selected commercial peanut cultivars to leaf spot diseases as influenced by low- and high-input fungicide programs was also evaluated. The low input fungicide program included seven applications of chlorothalonil and the high input fungicide program comprised combinations of fluxapyroxad, pyraclostrobin, mefentrifluconazole, flutolanil, bixafen, flutriafol, tebuconazole, and/or chlorothalonil. Both the fungicide programs provided adequate levels of leaf spot control and significantly increased yields. Leaf spot severity was higher for TUFRunnerTM ‘297’, TUFRunnerTM ‘511’, Georgia-16HO, and Georgia-20VHO, but was lower for AU-NPL 17, Georgia-12Y, Georgia-14N, Georgia-19HP, and TifNV-High O/L. Study results indicated that tolerant cultivars combined with effective fungicide programs can reduce fungicide inputs and minimize yield losses incited by leaf spot diseases.