How does Phytophthora capsici survive in squash fields in Southeastern Florida during the off-season?

Randy C. Ploetz, Jody L. Haynes


Phytophthora capsici causes destructive diseases of several vegetable and fruit crops in Florida. Studies were conducted to investigate how the pathogen survives between seasons for summer squash, Cucurbita pepo, in southeastern Florida. Seventy-one isolates of Phytophthora were recovered from plants in four different commercial fields and examined for mating type and sensitivity to mefenoxam (Ridomil Gold). The A1 and A2 mating types of P. capsici were found in each field. Although ratios varied in three of the fields in which one mating type was two or more times more prevalent than the other, mating types occurred in an approximate 1:1 ratio (totals of 32 and 34, respectively) when all fields were considered. In five instances both mating types were recovered from the same squash plant. Six additional isolates were either P. nicotianae or an unknown species. Sensitivity to mefenoxam varied considerably among individual isolates of P. capsici, with ED50s ranging from 5 mg mefenoxam mV to more than 60 |Lig ml1. It is unlikely that the least sensitive isolates would be controlled with this fungicide under field conditions. Thus, the reduced effectiveness of mefenoxam against phytophthora blight that had been noted recently by local producers is probably due to the decreased sensitivity of local populations of this pathogen to this fungicide. Fifteen weed species were examined as possible alternative hosts for the pathogen. Only one, Portulaca oleracea (common purslane), was colonized by P. capsici; this is apparently the first report of its susceptibility to this pathogen. Although alternative weed hosts may not play an important overseasoning role for P. capsici in south eastern Florida, the potential for its long-term survival as oospores clearly exists due to the prevalence of both mating types in the fields that were studied.

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Proc. Fla. State Hort. Soc.     ISSN 0886-7283