It starts as small, yellowish spots, often indistinguishable from other common diseases in vegetables. When temperatures and humidity are right, spores begin to develop, creating soft gray areas on the undersides of leaves. Thriving in wet conditions, it spreads quickly, producing thousands of spores that can move hundreds of miles on the wind. In cold climates, it can’t survive the winter, but come spring, it blows in from warmer areas, ready to infect fields as soon as conditions allow.1
Downy mildew is a parasitic water mold disease that can devastate fruit and vegetable crops. It attacks a wide range of produce many of us consume every day, like tomatoes, cucumbers, melons, grapes, greens, herbs and squash. When it infests the leaves of fruiting vegetables, like cucumbers and melons, the leaves turn brown and die, decreasing the green surface area that drives fruit production. The result can be smaller fruits and lower sugar content, affecting yield, flavor and quality. In leafy vegetables, the impact can be even more severe because marketplace quality standards are so high. In these vegetables, downy mildew grows right on the portions of the plant that are harvested and consumed. Even a few spots can render a head of cabbage or bunch of kale unmarketable.2 Downy mildew infestation can mean less abundant harvests and, ultimately, fewer choices and higher prices at the grocery store.
Using a combination of controls
To control water-borne diseases like downy mildew, farmers have to use multiple methods in an approach known as Integrated Pest Management (IPM). IPM helps farmers make decisions around managing all kinds of pests—including weeds, insects and diseases—in ways that reduce the risks of resistance development and minimize environmental impact. To effectively manage downy mildew, farmers need to use a variety of practices and technologies, including:
Today, there are more crop varieties available with resistance to downy mildew, particularly for cucumber and cantaloupe. This can be an excellent first line of defense, but not all crops have resistant varietal options, and even resistant varieties may still have some disease susceptibility.
There are a number of things farmers can do in the field to help reduce incidence and severity of downy mildew. For example, by adjusting plant spacing and irrigation, farmers can create conditions less favorable for downy mildew to thrive. If downy mildew has appeared in a field in the past, farmers also need to manage crop residues carefully, particularly in areas where the climate allows the disease to overwinter. This helps limit the sources of pathogen spores that may re-infect crops the following season.
Downy mildew loves wet weather and temperatures of 15-23 degrees Celsius. Depending on the region and crop, farmers may be able to time their planting and harvest to avoid times of year when weather creates the greatest downy mildew pressures.1 In leafy vegetables, downy mildew seems to have a greater impact on older leaves. In this case, it may be possible to harvest plants before newer leaves are infected and remove older, damaged leaves so the plant is still marketable.2
In damp weather, farmers can get out and scout fields, regularly looking for early signs of downy mildew. This can help inform whether another intervention, such as a fungicide, is needed. Support from digital solutions can help farmers and consultants make even better decisions about when and how to treat crops.
Along with other management techniques, fungicides are also often necessary to control downy mildew. In the decades before fungicides were readily available, downy mildew would decimate crops.3 Today, this disease is showing resistance to several commonly used fungicides, which means farmers need more options to treat their crops. Farmers look for products that are not only effective, but help reduce risks of resistance and have a favorable environmental profile. Zorvec™ active is one fungicide active ingredient that fits these criteria. Zorvec is highly effective against downy mildew at a very low use rate—the amount of active ingredient needed to control the disease. It also works in a completely new way against downy mildew. To help ensure this remains a durable solution for protecting crops, Zorvec is always used in combination with another fungicide mode of action and should never be applied more than the label specifies.
Cucurbit downy mildew4 (Pseudoperonospora cubensis)
Untreated Control Zorvec
Zorvec helps protect leaves of cucurbit plants against downy mildew. When leaves stay green and healthy, the plant can direct more nutrients to fruit development to maximize yield, taste and quality.
Defending crops against difficult diseases requires every tool farmers have at their disposal. With downy mildew—which can spread quickly, travel far and ruin harvests—farmers need to use a diverse set of tactics including selecting resistant varieties, management and scouting practices and using fungicides when necessary. Options like Zorvec help farmers manage downy mildew responsibly and keep it from ruining the lovely lettuce, cooling cucumber and crisp kale that make our salads (and more) tasty and nutritious.
1 Martha Sudermann, “Downy Mildew,” Cornell Vegetables (Cornell University College of Agriculture and Life Sciences), accessed December 2, 2022, https://www.vegetables.cornell.edu/crops/cucurbits/downy-mildew/.
2 Anthony P. Keinath, “Why Disease Control Is More Challenging on Leafy Vegetables,” Growing Produce, July 6, 2022, https://www.growingproduce.com/crop-protection/disease-control/whydisease-control-is-more-challenging-on-leafy-vegetables/.
3 Leonard P. Gianessi and Nathan Reigner, “Fungicides,” Fungicides (CropLife Foundation, September 2005), https://croplifefoundation.wordpress.com/resources/benefits-studies/ fungicides/.
4 China, 2012. Corteva data on file.