An evaluation of fungicide efficacy and cultivar susceptibility against watermelon anthracnose in Virginia

Watermelon anthracnose, caused by Colletotrichum orbiculare, is a significant threat to Southeastern watermelon production, with growers reporting reduced effectiveness of race 1-resistant cultivars and with quinone-outside inhibitor (QoI) fungicides. Field trials in 2024 and 2025 in Painter, VA evaluated the susceptibility of ten commercially available seedless cultivars and the efficacy of 11-12 fungicides representing multiple modes of action. All cultivars, including differentials, developed severe disease (> 90% final severity), and the susceptibility of differential cultivars indicated that the local isolate used for inoculation was likely not race 1. Fungicide efficacy varied by chemistry. Mancozeb and chlorothalonil were consistently among the treatments with the greatest reduction in final disease severity and standardized area under the disease progress curve (sAUDPC). Demethylation inhibitor (DMI; FRAC 3) triazoles fungicides flutriafol, mefentrifluconazole, and tebuconazole were either ineffective (2024) or not as effective as most other fungicide (2025) at reducing disease compared to the untreated control. However, prothioconazole, a DMI triazolinthione, provided a similar level of disease control as chlorothalonil in both years. QoI and methyl benzimidazole carbamate fungicide, applied alone or in premixtures, reduced disease severity compared to the nontreated check. Postharvest evaluations in 2025 revealed high fruit incidence in several treatments, while mancozeb, chlorothalonil, thiophanate methyl, and pyraclostrobin maintained low (<5%) postharvest disease levels. These results demonstrate the limited effectiveness of current commercial cultivars and identify several effective fungicide options for Virginia growers. Ongoing evaluation of regional C. orbiculare race structure and fungicide sensitivity will be essential for improving sustainable anthracnose management.