Didymella bryoniae, which causes gummy stem blight (GSB) of watermelon, has a history of developing resistance to fungicides, most recently the succinate-dehydrogenase-inhibiting (SDHI) fungicide boscalid. To facilitate fungicide resistance monitoring, baseline sensitivity distributions were established for demethylation-inhibiting (DMI) fungicides tebuconazole and difenoconazole and the SDHI fungicide penthiopyrad, and reestablished for the SDHI fungicide boscalid. In all, 71 isolates with no known prior exposure to SDHIs or DMIs were used to determine the effective concentration at which mycelial growth was inhibited by 50% (EC50). EC50 values for boscalid, penthiopyrad, tebuconazole, and difenoconazole were 0.018 to 0.064, 0.015 to 0.057, 0.062 to 0.385, and 0.018 to 0.048 μg/ml, with median values of 0.032, 0.026, 0.118, and 0.031 μg/ml, respectively. Significant positive correlations between the sensitivity to penthiopyrad and boscalid (P < 0.0001, r = 0.75) and between tebuconazole and difenoconazole (P < 0.0001, r = 0.59) indicate a potential for cross-resistance between chemically related fungicides. In 2009, 103 isolates from fungicide-treated watermelon fields were tested for sensitivity to boscalid and penthiopyrad using a discriminatory concentration of 3.0 μg/ml. Of the isolates tested, 82 were insensitive and 14 were sensitive to both fungicides. Because of the significant potential for cross-resistance between closely related fungicides, growers will be advised not to use both SDHIs or both DMIs successively in the same fungicide spray program.
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