Two things to know about downy mildew right from the start: It’s not a fungus in the true sense of the word, and it’s not related to powdery mildew. In fact, it’s a good bit worse than powdery mildew, especially for farmers and growers. It spreads rapidly, and can kill vegetables, fruits, and flowers in as little as a week. Once a plant has downy mildew, it’s too late. Prevention is key.
Photograph by Edward Sikora, Auburn University, Bugwood.org, CC BY-SA 3.0, via Wikimedia Commons
Downy mildew is a class of diseases called water molds. There are thousands of them, each particular to a type of plant. While it can manifest differently depending on the genus of plant, generally it appears as angular yellow spots in the areas between leaf veins on the upper side of a leaf. On the underside there’s white, gray, or purple fuzz. The yellowing spreads and turns brown, and the leaf dies. Downy mildew appears not just on leaves, but on fruits and flowers, too.
The mildew travels through the air or in water. It also spreads via gardening tools and on gardeners’ hands. It doesn’t overwinter in the northern U.S., because it’s too cold, but in warmer regions the mildew overwinters on or inside the plant, and germinates once temperatures reach between 50 and 75 degrees F, with about 85% humidity. It spreads north with the warming weather.
Cucumbers and melons, grapes, basil, brassicas (broccoli, cauliflower, kale, Brussels sprouts, mustards), spinach, roses, impatiens, and sunflowers are just some of the types of plants vulnerable to downy mildew.
The difference in appearance between downy mildew and powdery mildew. Powdery mildew is a fuzzy gray or white mold on the top of leaves. It disfigures but usually doesn’t kill plants.
Once a plant has downy mildew, it can’t be cured. Prevention is the only option. Here are some tips to prevent it:
Buy seeds and plants labeled as resistant to downy mildew.
Provide good air circulation around and between plants.
Keep foliage from touching the ground when possible by pruning, staking or trellising.
Watch your plants and remove and destroy any you see with signs of mildew. Bag the plants and throw them away; never put them in the compost pile.
Don’t water plants from above. Water at the base of the plant whenever possible.
A thick (three-inch) mulch around plants will help prevent spores from splashing up onto foliage.
Practice good sanitation. Clean up fallen foliage throughout the growing season and remove spent plants once the season ends.
Photographs by Forest & Kim Starr, CC BY 3.0 US, via Wikimedia Commons
Fungicides are available to help prevent and control downy mildew, however they should be started before any signs of the disease appear in your garden. You will have to thoroughly cover both the top and undersides of the leaves with fungicide to be sure to get all the mildew.
Some strains of mildew don’t respond to fungicides, and some have even become resistant. Copper-based and other kinds of fungicides such as neem oil are effective, but can kill pollinators and soil-living beneficial insects and organisms. That’s why it’s far better to prevent downy mildew than to try to control it once it appears.
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