Sensitivity of , f. sp. , and to Succinate Dehydrogenase Inhibitors Used for Control of Watermelon Diseases.

Watermelon is affected by diseases such as Fusarium wilt, gummy stem blight, and root-knot nematode (RKN). Succinate dehydrogenase inhibitors (SDHIs) with potential fungicide and nematicide activity provide the opportunity to control multiple diseases with one compound. In this study, we aimed to determine the sensitivity of race 4 (MI4), f. sp. (FON), and (SCIT) to existing SDHIs: benzovindiflupyr, fluopyram, cyclobutrifluram, and pydiflumetofen. All SDHIs had fungicidal activity against 19 SCIT isolates in mycelial growth assays, but isolates were most sensitive to pydiflumetofen (median EC = 0.41 μg/ml). Most of the 50 FON isolates tested were sensitive to cyclobutrifluram for mycelial growth (median EC = 4.04 μg/ml) and conidial germination (median EC = 0.2 μg/ml) assays but were not sensitive to fluopyram. MI4 was most sensitive to cyclobutrifluram for egg hatch (mean EC = 0.0019 μg/ml) and J2 motility (mean EC = 1.16 μg/ml) assays but was not sensitive to pydiflumetofen. Significant positive correlations between the sensitivity of SCIT (mycelial growth) and FON (mycelial growth and conidial germination) for cyclobutrifluram and benzovindiflupyr (SCIT = 0.88; FON = 0.7; < 0.0001) and cyclobutrifluram and pydiflumetofen (SCIT = 0.83; FON = 0.67 and 0.77; < 0.0001) indicate a potential for cross-resistance between these SDHIs for these fungal pathogens. Overall, results suggest that cyclobutrifluram may be used for managing RKN, whereas it should be used judiciously for Fusarium wilt of watermelon and gummy stem blight due to the existence of insensitive isolates to the fungicide.