Characterization of Phytophthora capsici Associated with Roots of Weeds on Florida Vegetable Farms

Weeds were sampled in commercial vegetable fields in Palm Beach County, FL in August 2001, December 2001, and March 2002 for the presence of Phytophthora capsici. Fields sampled had a recent history of this plant pathogen. P. capsici was successfully isolated from the roots of six of 42 Carolina geranium (Geranium carolinianum) plants, four of 28 American black nightshade (Solanum americanum) plants, and two of 130 common purslane (Portulaca oleracea) plants. All but one of the 12 isolates were of the A1 mating type. All 12 isolates were resistant to mefenoxam, although at different levels. All but one isolate were strongly pathogenic on pepper seedlings. When two or three isolates recovered from each weed were inoculated onto the roots of their weed host of origin, P. capsici was recovered from the roots. Isolates of P. capsici were tested on four other solanaceous weeds of importance or potential importance to agricultural fields in Florida: Solanum nigrum, S. ptycanthum, S. carolinense, and S. capsicoides. Recovery of P. capsici from roots varied with weed species and isolate tested. P. capsici caused disease mortality on S. nigrum, and no reisolation of P. capsici was possible with S. capsicoides. This is the first report of S. americanum and G. carolinianum being alternative hosts for P. capsici under field conditions. This study also validated P. oleracea as an alternative host. In Florida, and perhaps elsewhere, weeds may contribute to pathogen survival in the absence of a host crop or when propagules may not readily survive in soil or plant debris.