Widespread Resistance to Tebuconazole and Cross-Resistance to Other DMI Fungicides in Stagonosporopsis citrulli Isolated from Watermelon in South Carolina

Abstract

Tebuconazole, a demethylation-inhibitor (DMI) fungicide, is widely used on watermelon and muskmelon because it is inexpensive and has been effective against Stagonosporopsis citrulli, the primary causal agent of gummy stem blight in the southeastern United States. Most isolates (94% of 251) collected from watermelon in South Carolina in 2019 and 2021 were moderately resistant to tebuconazole at 3.0 mg/liter in vitro. Ninety isolates were identified as S. citrulli, and no isolates of S. caricae were found in this study. On watermelon and muskmelon seedlings treated with the field rate of tebuconazole, sensitive, moderately resistant, and highly resistant isolates were controlled 99, 74, and 45%, respectively. In vitro, tebuconazole-sensitive isolates were moderately resistant to tetraconazole and flutriafol but sensitive to difenoconazole and prothioconazole, while highly resistant isolates were highly resistant to tetraconazole and flutriafol and moderately resistant to difenoconazole and prothioconazole. On watermelon seedlings treated with field rates of five DMI fungicides in the greenhouse, severity of gummy stem blight did not differ significantly from the nontreated control when seedlings were inoculated with a highly resistant isolate, while severity was lower with all DMIs on seedlings inoculated with a sensitive isolate, although severity was greater with tetraconazole than with the other four DMIs. In the field, tetraconazole rotated with mancozeb did not reduce severity of gummy stem blight caused by a tebuconazole-sensitive isolate when compared to the nontreated control, while the other four DMIs did. With a highly resistant isolate, all DMIs rotated with mancozeb reduced severity of gummy stem blight compared to the nontreated control, but severity with tetraconazole and tebuconazole was greater than with mancozeb alone, and severity with flutriafol, difenoconazole, prothioconazole, and difenoconazole plus cyprodinil did not differ from mancozeb applied alone. Results from in vitro, greenhouse, and field experiments with the five DMI fungicides were highly correlated with each other. Thus, determining relative colony diameters with a discriminatory dose of 3 mg/liter of tebuconazole is an effective way to identify isolates of S. citrulli highly resistant to tebuconazole.