Sensitivity of populations of Phytophthora capsici from South Carolina to mefenoxam, dimethomorph, zoxamide, and cymoxanil

In summer and fall 2003, Phytophthora blight and crown rot, caused by Phytophthora capsici, was found in three fields each of summer squash and pepper on three farms in two counties in South Carolina. Although this disease had been confirmed previously in the state, five of these outbreaks were in fields thought to be free of P. capsici. The objectives of this study were to determine whether isolates of P. capsici in South Carolina were sensitive to mefenoxam and to determine baseline sensitivities to dimethomorph, zoxamide, and cymoxanil, fungicides recently registered to control Phytophthora blight. Of 120 isolates tested for sensitivity to mefenoxam at 100 mg/liter, 8 isolates were resistant (relative colony diameter [RCD] > 90% of nonamended control), 60 isolates were sensitive (RCD < 30%), and 52 isolates were intermediately sensitive. Only sensitive isolates were found in two fields in which no mefenoxam-containing fungicides had ever been used. Intermediately sensitive or resistant isolates were found in the four fields in which mefenoxam had been applied previously. In all, 15 to 61 isolates were tested for sensitivity to dimethomorph, zoxamide, and cymoxanil. The concentrations at which RCD, percent cyst germination, and relative zoospore production were reduced to 50% (EC50 values) for mycelial growth were 0.19 +/- 0.02 ( standard deviation) mg/liter for dimethomorph, 0.50 +/- 0.50 mg/liter for zoxamide, and mostly > 50 mg/liter for cymoxanil. EC50 values for zoospore cyst germination were 0.07 +/- 0.02 mg/liter for dimethomorph and > 50 mg/liter for cymoxanil. EC50 values for zoospore production were 0.63 +/- 0.42 mg/liter for dimethomorph, 0.47 +/- 0.51 mg/liter for zoxamide, and < 50 mg/liter for cymoxanil. Sensitivity values obtained in this South Carolina study can be used as a comparative baseline to monitor shifts in sensitivity to the fungicides mefenoxam, dimethomorph, zoxamide, and cymoxanil in populations of P. capsici.