Abstract

Phytophthora blight, caused by Phytophthora capsici, is a destructive disease of cucurbits and peppers in Illinois and worldwide. Fungicide use is a significant component of management strategies of P. capsici in cucurbits and peppers in the United States. Studies were conducted in the laboratory and field to evaluate efficacy of selected fungicides for control of P. capsici in pumpkin in Illinois. In the laboratory, sensitivity of five P. capsici isolates from Illinois was evaluated to the fungicides cyazofamid, dimethomorph, and mandipropamid. Development of colonies and production of sporangia of the isolates were assessed at concentrations of 0.0, 0.1, 0.5, 1.0, 5.0, and 10.0μg/mL of cyazofamid, 0.0, 0.1, 0.5, 1.0, 2.5, and 5.0μg/mL of dimethomorph; and 0.0, 0.05, 0.1, 0.5, 1.0, and 5.0μg/mL of mandipropamid in V8 juice agar in Petri plates. In 2014, 2015, and 2017, effectiveness of the fungicides cyazofamid, dimethomorph, and mandipropamid for control of P. capsici in pumpkin was investigated.

In 2014, incidence of fruit rot caused by P. capsici was 31% in control plots and 5, 10, and 2% in the plots treated with cyazofamid, dimethomorph, and mandipropamid, respectively. In 2015, incidence of fruit rot caused by P. capsici was 69% in control plots and 42, 21, and 26% in the plots sprayed with cyazofamid, dimethomorph, and mandipropamid, respectively. Similarly in 2017, incidence of fruit rot caused by P. capsici was 48% in control plots and 7, 27, and 13% in the plots sprayed with cyazofamid, dimethomorph, and mandipropamid, respectively. In all field trials, percent vines infected with P. capsiciin treated plots with cyazofamid, dimethomorph, and mandipropamid was significantly lower than that of control plots.

Keywords: Phytophthora capsici; Oomycete; Soilborne pathogen; Fungicide resistance; Fruit rot; Pumpkin; Cucurbit