by Katie Navarra
Each year Cucurbit Downy Mildew (CDM) causes severe yield loss in northeast pumpkin, squash, melon and cucumber crops. Understanding the origin of the pathogen and the conditions that encourage its spread can help farmers better prepare for and contain the spread of CDM in their fields.
The webinar, Managing Cucurbit Downy Mildew in Organic Systems in the Northeast, featured Christine Smart from Cornell University’s School of Integrative Plant Science. During the hour-long webinar, Smart offers methods for controlling CDM. She discusses cultural controls and the results from studies on the effectiveness of commercial products that have been approved for use in organic systems. The webinar was organized by members of the NIFA-OREI funded Eastern Sustainable Cucurbit Project and was hosted by eOrganic.
In the article that follows, learn more about the pathogen’s lifecycle and how you can prepare to combat the pathogen when it arrives in your field.
What is Cucurbit Downy Mildew
Cucurbit Downy Mildew is not a soil borne pathogen and cannot survive northeast winter conditions. CDM is closely related to late blight in potato and tomato crops, but is actually quite different from powdery mildew. It is blown in from other geographic regions and thrives in wet conditions.
The pathogen first appears in Florida in March and migrates north through Georgia and the Carolinas by June. It moves into Virginia and New Jersey by July and finally arrives in New York in August. In recent years, the pathogen has started showing up in the Great Lakes region much earlier.
“It’s showing up much earlier in large part because of the commercial greenhouses near Lake Erie and Lake Ontario,” Smart said. “In 2016, the first spotting was around July 20 or 25.”
The pathogen thrives in wet conditions and can spread quickly. In the morning when humidity is high, the spores rapidly reproduce and as humidity drops in the afternoon the spores break off and drift to other plants. If any water is present on the leaf of a plant where the pathogen lands, the spores are released and attack the plant. During drought seasons, like the one experienced in 2016, the pathogen spreads more slowly.
“Within one week to 10 days the field can go from healthy to diseased,” she explained.
CDM only attacks the plant leaves and prefers younger, healthy leaves, unlike other pathogens that prefer older, lower leaves. Yellow spots on the upper leaves of the plant are the first visible indication of CDM’s presence in a field. Over time those lesions turn brown and papery.
“When scouting in the field, look across the field and look for leaves with yellow spots. You’ll quickly be able to spot it,” she said.
Although CDM doesn’t attack the fruit of the plant directly, by killing the plant leaves it exposes the fruit to sun scorch.
Control options
If it’s inevitable that Cucurbit Downy Mildew will arrive in the northeast, what can farmers do to reduce the impact it has on their crops?
“Change your cultural practices and/or harvest crops early,” Smart said.
Cucumbers are most susceptible to CDM and have a shorter growing window than other cucurbits. Smart has observed some farmer’s having success with planting and harvesting cucumber crops earlier in the season to avoid loss due to CDM.
“This is not as effective with pumpkins, melons and squash, which need a longer season, but has been with cucumbers,” she said.
Another alternative is to use high tunnels to protect plants from spores blowing in the wind. Although high tunnels are used to keep the spores out of the crop, Smart has observed other farmers using row covers to contain spores and reduce their spread to other plants.
“It won’t keep every spore off plants, but if you get them on early enough you can help reduce the spread,” she said.
Smart also recommends using resistant seed. Breeders across the country are working towards developing disease resistant seed that is commercially available.
Regular scouting and attention to weather data and local forecasts can help estimate when CDM may appear in your fields. Smart recommends the Cucurbit Downy Mildew Forecasting website (, which can send you regular text or email alerts with updates on CDM locations.
“Early detection is the key. You don’t need to spray in June if the spore doesn’t arrive until August,” Smart explained.
Once CDM is detected in your area, be prepared to act. Smart says organic producers currently have seven different products to choose from. The application cost of such products range from $12 per acre to $108 per acre. Products featuring copper as the active ingredient are the cheapest, but not necessarily as effective. Studies have found that rhamnolipid biosurfactant, the active ingredient found in Zonix, provides the most control, but is the most expensive.
What’s next?
Researchers are working to develop detection equipment to identify CDM. Even though a model developed by USDA (Oregon State) is used with success to detect hop downy mildew, it is not quite as effective at capturing CDM spores.
“The system has promise, but there is still a lot of work to do,” Smart said.
Ideally, detection should be used in combination with an Integrated Pest Management (IPM) approach in conjunction with forecast and weather data.
“Just because spores are in the area doesn’t mean CDM will infect the plant. You will not see disease without rain as evidenced by the drought in 2016,” she concluded.
To listen to the webinar in its entirety, visit .