Sensitivity of Colletotrichum orbiculare Isolates in Virginia Watermelon to Thiophanate-methyl, Pyraclostrobin, and Prothioconazole

Fungicide resistance development is a major concern for growers due to the limited modes of action available and the limited number of applications that can be used in the field. Determining sensitivity to fungicides and assessing risk of resistance is vital to the development of future chemistries necessary to inhibit or control pathogens. One common simple in vitro method of measuring sensitivity to fungicides is measuring radial growth of a pathogen exposed to multiple doses of fungicides in fungicide-amended agar to determine EC50 which is the concentration of fungicide that provides 50% inhibition of the isolate as compared to a non-fungicide-amended control. Colletotrichum orbiculare (syn. C. lagenaria) is the causal organism for anthracnose of cucurbits. Thiophanate-methyl and pyraclostrobin are common fungicides used to manage anthracnose in cucurbit crops. Prothioconazole, is labeled for use in cucurbits but not specifically recommended for management of anthracnose; however, control of anthracnose with fungicides containing this active ingredient has been observed in the field. A mycelial growth assay was conducted using fourteen C. orbiculare isolates collected from watermelon (Citrullus lanatus) in Southampton County, Virginia. Isolates were incubated on fungicide-amended PDA at fungicide concentrations 0 – 100 µg/mL of each fungicide, and the diameter of fungal growth on fungicide-amended and non-amended media was measured and compared to determine percentage of growth reduction. There was little variation in the fungicide sensitivity profiles of the fourteen isolates examined. Overall they were highly sensitive to pyraclostrobin (EC50 <0.1 µg/mL), and insensitive to thiophanate-methyl (EC50 > 1 µg/mL) and prothioconazole (EC50 > 100 µg/mL).