Sensitivity of Didymella bryoniae to DMI and SDHI fungicides and the relationship between fungicide sensitivity and control of gummy stem blight in watermelon
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Baseline isolates of Didymella bryoniae were tested for sensitivity to penthiopyrad, tebuconazole and difenoconazole using mycelia growth assay. Based on the sensitivity distribution, a discriminatory concentration of 3.0 µg/ml was selected for future monitoring of shifts in sensitivity. Cross-resistance between boscalid and penthiopyrad was identified in field isolates of D. bryoniae and a potential for cross-resistance between DMIs is observed based on baseline sensitivity profile. Field experiments were conducted to establish a relationship between frequency of resistance and fungicide efficacy in managing gummy stem blight (GSB). Tebuconazole and chlorothalonil were proved to be most effective in managing GSB. A consistent negative association was observed between the frequency of resistance and disease control in the field. Isolation of D. bryoniae resistant to thiophanate-methyl from commercial seedlots helped explain the unpredictable development of fungicide resistance in watermelon fields. Results from this study will help manage GSB with efficient use of fungicides.