Written by Emma Wallace and Dr. Lina Quesada-Ocampo

Downy mildew of cucurbits, a critical disease on cucumbers, has been reported in North Carolina. It was observed on cucumber in Sampson county. The field had 5% disease incidence, with the infected plant/leaf having 5% disease severity.

Cucurbit downy mildew is a foliar disease caused by a fungal-like organism called Pseudoperonospora cubensis. This disease attacks all commercial cucurbits (cucumber, cantaloupe, squash, watermelon, pumpkin, etc.), but is most severe on cucumbers. The chlorotic lesions and greyish-purple “downy” growth on the underside of infected leaves are all-too familiar to anyone who has planted cucurbits in the last ten years. In 2005 the pathogen overcame resistance in host plants and became seriously damaging to cucumber crops in many states. Although many research groups and companies are working on developing resistant lines and effective chemical control, downy mildew continues to be problematic for cucurbit growers annually.

Each year, this foliar pathogen of cucurbits is blown up the East Coast of the United States through wind currents. It is believed to survive the winter year-round in Florida and then travel north field by field as the spring and summer progress and susceptible hosts are planted. Reports of Pseudoperonospora cubensis, the cucurbit downy mildew pathogen, occurred in southern Florida in late January of this year, and there have been increasingly more reports coming from northern Florida, Georgia, and South Carolina through May and early June.

There are several practices that can help manage cucurbit downy mildew. P. cubensis is a water mold (oomycete) which thrives in moist conditions. Minimizing leaf wetness by using drip irrigation, watering early in the day, and allowing adequate spacing between plants are good approaches. Planting early in the season can also help avoid the high disease levels usually found in the fall in North Carolina. At present, resistant varieties are not available and fungicide applications are required to control the disease.

If cucurbit growers have not started preventative sprays, they should do so immediately. The Cucurbit Downy Mildew factsheet, previous posts about fungicide efficacy trials in North Carolina, and The Southeastern US Vegetable Crop Handbook (page 181-182) provide recommendations for chemical control options. Growers should use intensive spray programs (every 5-7 days) once disease if found in their fields if weather conditions are conducive to disease (wet and cool weather). Pseudoperonospora cubensis, the causal agent of cucurbit downy mildew, can generate fungicide resistance very quickly. It is critical that growers alternate products in their fungicide programs to protect the few chemistries we have that are still highly effective in controlling downy mildew.

North Carolina State University maintains the CDM IPM pipe, an online tool that uses disease outbreak reports and meteorological data to forecast when the disease will arrive in a particular area. Reports of the disease are made to the system by researchers and county agents and then the system evaluates weather patterns to determine which regions are at risk for the disease. This information helps growers optimize preventative chemical applications. If you think you have downy mildew in your field, please contact your local Extension agent and send photos and/or physical samples to the Plant Disease and Insect Clinic. Reporting the occurrence of cucurbit downy mildew to the CDM IPM Pipe helps us protect our state cucurbit industry by providing them with timely disease management information.

Resources

View factsheets about cucurbit downy mildew and other vegetable diseases.

Learn more about diagnosing cucurbit downy mildew on different cucurbit hosts (cucumber, squash, cantaloupe, pumpkin, watermelon).

Visit the CDM IPM Pipe.

Learn more about breeding cucurbits for downy mildew resistance at NC State.