Downy mildew was confirmed on cucumber from South-Central Kentucky on August 19, 2019. Cucurbit growers located between Interstates 75 and 65 should strongly consider making a fungicide application before the next rainfall to reduce the potential damage from downy mildew. Now that downy mildew has arrived, the disease should be managed in crops until 2 weeks prior to final harvest.
As the most economically important disease of cucurbits in Kentucky, it is not a matter of if growers and homeowners will experience downy mildew, it is a matter of when. All cucurbits, such as cucumber, cantaloupe, summer and winter squashes, pumpkin, and watermelon are susceptible to downy mildew. This foliar disease occurs severely in field plantings, but can also affect greenhouse plants. Since the cucurbit downy mildew pathogen does not overwinter in Kentucky, each summer it must be blown in from overwintering locations. Prevention, early identification, and management will help reduce yield losses.
Here are the symptoms to look for, preventative tactics, and brief suggestions on how to treat crops once disease is confirmed.
Cause and Disease Development
Cucurbit downy mildew is caused by the water mold (fungus-like organism/Oomycete) pathogen, Pseudoperonospora cubensis. This pathogen requires living plant tissue to complete its life cycle and is not known to survive Kentucky winters. The pathogen is maintained in warmer overwintering locations and spores pathogen propagules are blown into Kentucky on wind currents each year. The time of arrival can vary greatly from year to year.
The movement of the pathogen is monitored on the Cucurbit Downy Mildew Forecasting page of the IPM pipe website. This tool provides valuable information to growers regarding the spread and risk of cucurbit downy mildew. Cucurbit growers can sign up for free e-mail or text alerts to keep them updated on the proximity of downy mildew to their individual farm. After receiving an alert, growers can investigate their farm’s forecasted risk for the pathogen’s arrival. Fungicides can be applied based on these alerts.
Downy mildew is favored by rainy, humid conditions coming from directions with confirmed reports of the disease. Cucurbit downy mildew can spread extremely rapidly, resulting in defoliation and complete plant death in a matter of days during rainy conditions.
Cucurbit downy mildew is strictly a foliar disease. The first symptoms are pale or bright yellow spots on the upper surfaces of leaves (Figure 1). These symptoms may first be observed near the crown of the plant where leaves stay wet for extended durations. Over time, spots become irregular or “blocky” in appearance and develop into necrotic lesions (Figure 2). On the undersides of leaves, lesions may appear slightly water-soaked. When humidity is high, gray to purple spore production may be observed (Figure 3). The pathogen does not infect cucurbit fruit.
- Utilize disease tolerant cultivars (primarily cucumber).
- Monitor disease spread and risk through the Cucurbit Downy Mildew Forecasting page of the IPM pipe website.
- Avoid overhead watering to reduce leaf wetness.
- Plant in sunny areas with good airflow.
- Use recommended plant spacing to facilitate air movement and leaf drying.
- Remove and destroy heavily infected plants (gardens only).
Protectant fungicides, such as chlorothalonil, mancozeb, and copper, should be applied when crops are at low risk based on the forecasting website. At higher risk levels, systemic fungicides are recommended. A number of systemic fungicide options are listed in ID-36; among these the Orondis products, Elumin, and Ranman are highly recommended. Other systemic fungicides such as Gavel, Tanos, and Zampro are also effective. Other fungicides are labeled for use against cucurbit downy mildew, but their efficacy has varied depending on the production year. For management of resistance to fungicides, it is recommended to always tank-mix a systemic downy mildew fungicide with a compatible protectant fungicide.
Growers can identify their individual risk levels by using the Cucurbit Downy Mildew Forecasting page and/or contacting their local extension agent for further assistance.
As always, all label recommendations must be followed when applying chemicals to crops. In particular, pay close attention to pre-harvest intervals.
- IPM Scouting Guide for Common Problems of Cucurbit Crops in Kentucky (ID-91)
- Cucurbit Downy Mildew Forecasting (IPM pipe website)
- Sustainable Disease Management of Cucurbit Crops in the Home Garden (PPFS-VG-19)
- Home Vegetable Gardening (ID-128)
- Vegetable Production Guide for Commercial Growers (ID-36)
By: Kim Leonberger, Extension Associate and Emily Pfeufer, Extension Plant Pathologist