Emmalea Ernest, Scientist – Vegetable & Fruit Crops; emmalea@udel.edu
Since 2004, cucurbit downy mildew (CDM), caused by Pseudoperonospora cubensis, been a significant threat to cucumber, cantaloupe, and other cucurbit crops in the Mid-Atlantic region. Once CDM is present in our region, preventative fungicide sprays will be needed to protect susceptible crops. The first occurrence of downy mildew on Delmarva often occurs in late June or early July and the first CDM strains to arrive are usually those that impact cucumber and cantaloupe. Downy mildew was reported on cucumber in Atlantic County, New Jersey on June 13, but as of June 22, 2023 I am not aware of any reports on Delmarva. In past years there have been forecasts available to predict the arrival of CDM, however this work was not funded for 2023 and forecasting is suspended for this year. You can still monitor disease occurrence reports that are submitted to the cdm.imppipe.org website and see where CDM has been reported (Fig. 1).
Figure 1. As of June 22, 2023, cucurbit downy mildew has been reported in New Jersey, South Carolina and Georgia.
Recommended fungicides for cucurbit downy mildew are listed in the Mid-Atlantic Commercial Vegetable Production Recommendations.
On cucumber, early disease symptoms are most apparent in the morning before the dew has dried. Scout shaded or wet areas, as these are often the first places that disease develops. Angular, water-soaked lesions are the earliest symptom of the disease. As the disease progresses, the lesions turn yellow and then necrotic (Fig. 2). In areas with lesions, dark colored spores are visible on the underside of the leaf (Fig. 3).
Figure 2. Chlorotic angular lesions on the upper leaf surface are a symptom of CDM in cucumber.
Figure 3. Dark colored CDM spores are visible on the lower leaf surface.