Cucurbit Downy Mildew Forecast

Jake Jones, Extension Agriculture Agent, Kent County;

Cucurbit downy mildew epidemics are an annual occurrence in the Mid-Atlantic and Eastern US. The disease spreads northward from Florida up the Atlantic coast as weather patterns and, more importantly, host availability allow. A disease reporting and forecasting website at is a valuable resource for cucurbit growers and consultants. According to the website, southern Delaware and southern Maryland currently have a high risk of cucurbit downy mildew, which will continue through the weekend with the cloudy, rainy forecast. Cucurbit downy mildew first reports have historically occurred around the 4th of July but since 2015, have been trending earlier in Delaware and Maryland (Table 1). This is an expensive trend, since weekly preventative fungicides sprays are needed to manage the disease. Effective fungicides are listed in the Mid-Atlantic Commercial Vegetable Production Recommendations: Fungicide bioassays from the University of Maryland have shown Orondis, Zing, Bravo, Ranman, and Omega to be some of the most efficacious fungicides tested on cucumber seedlings (2016-2019). Fungicide rotation and the use of resistant varieties can help maintain the efficacy of fungicides and reduce cucurbit downy mildew severity.

Table 1: Cucurbit Downy Mildew First Reports in Maryland

2015 2016 2017 2018 2019
Jul 8 Jun 29 Jun 23 Jun 15 Jun 26


All cucurbit crops are susceptible to cucurbit downy mildew with symptoms usually occurring first in cucumbers. In cucumber, early symptoms are angular, water soaked lesions delineated by the leaf veins (Figure 1). As the symptoms progress, they become chlorotic and necrotic, with signs of the disease apparent on the underside of the lesions where heavy sporulation can occur (Figure 2).

Water soaked lesions, a symptom of cucurbit downy mildew

Figure 1. Water soaked lesions, a symptom of cucurbit downy mildew.

Dark sporulation of cucurbit downy mildew, a diagnostic sign of the disease

Figure 2. Dark sporulation of cucurbit downy mildew, a diagnostic sign of the disease.