Sensitivity of Phytophthora capsici on Vegetable Crops in Georgia to Mandipropamid, Dimethomorph, and Cyazofamid

Phytophthora blight, caused by Phytophthora capsici, is a serious disease in vegetable production, and selective use of fungicides continues to be a significant component of disease management programs. The effect of three chemical compounds-mandipropamid, dimethomorph, and cyazofamid-on asexual stages of P. capsici collected from bell pepper and cucurbits in Georgia was assessed in this study. Forty isolates of P. capsici were determined to be sensitive to mandipropamid and dimethomorph based on mycelial growth, zoospore germination, and sporangial production. Concentrations that were 50% effective (EC50 values) of mandipropamid that inhibited mycelial growth, zoospore germination, and sporangial production of the isolates averaged 0.03, 5.70, and 0.02 mu g/ml, respectively. EC50 values of dimethomorph in inhibiting mycelial growth, zoospore germination, and sporangial production averaged 0.24, 0.10, and 0.46 mu g/ml, respectively. The majority of isolates were either resistant or intermediately sensitive to cyazofamid at 500 mu g/ml or lower concentrations based on mycelial growth or sporangial production, although all the isolates were sensitive to this compound based on zoospore germination, with an average EC50 of 0.04 mu g/ml. The results indicated that P. capsici populations in Georgia have not developed resistance to mandipropamid and dimethomorph whereas, for the majority of the isolates, certain asexual stages were resistant to cyazofamid.