CucCAP scientists find high prevalence of gummy stem blight resistance to common fungicides

watermelon graphicThe gummy stem blight fungus, Stagonosporopsis citrulli, is now resistant to fungicides in four chemical classes. The newest resistance is to fungicides in FRAC Code 3. Most isolates (94% of 251) collected from watermelon in South Carolina in 2019 and 2021 were moderately resistant to tebuconazole, a fungicide widely used on watermelon because it is available in low-cost generic products. Highly resistant isolates were cross-resistant to five other fungicides in the same chemical class as tebuconazole: tetraconazole, flutriafol, difenoconazole, prothioconazole, and the newly registered mefentrifluconazole. Currently, only 4% of the isolates are highly resistant, so growers are encouraged to limit use of tebuconazole to one early season application to avoid shifting the population from moderately resistant to resistant.

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