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SPECIAL SESSION: The Promise And Limitations of SDHI fungicides - Panel Discussion

The use of SDHI fungicides in premixtures; successes and concerns
Guido Schnabel - Clemson University.

SDHI fungicides have largely been used in form of FRAC 7/11 premixtures for disease control in specialty crops. The argument being that the two active ingredients help each other delay resistance development and that the efficacy spectrum is broadened. Using two case studies we show that for some crops the combination has proven to be an effective, long-term strategy for control of the target disease giving only occasional rise to outbreaks of secondary pathogens due to resistance development. The other case study discusses how the combination led to dual resistance in the target pathogen and perhaps accelerated selection of resistance in non-target organisms. Specifically, for peach growers in the southeastern United States the rotation of FRAC 3 and FRAC 7/11 combinations applied preharvest has resulted in excellent pre- and postharvest brown rot control over the last decade despite traditionally high disease pressure. A few peach growers, however, did experience outbreaks of secondary diseases, including Alternaria and Anthracnose fruit rots, associated with yield loss and fungicide resistance. For strawberry growers the application of FRAC 7/11 combinations for gray mold control has become a risky endeavor due to widespread resistance to FRAC 11 fungicides and location-specific resistance to FRAC 7 fungicides. Thus, the ‘internal’ resistance management strategy argument for the use of the mixture for gray mold control is no longer applicable. The continued use of these mixtures for primarily gray control may has likely contributed to the selection of FRAC 11-resistant Colletotrichum acutatum associated with recent outbreaks in strawberry anthracnose disease.