In addition to the mildew that may appear in your house in damp, dark places, you may soon see mildew on your outdoor plants! Both downy and powdery mildews are likely to infest Western North Carolina gardens this summer. Should you be concerned? Is there anything you can do to stop them?
Identifying mildews
If you see spots on leaves, you may have plant mildew. The spots may appear on the top surface of the leaf, the underside, or both. Powdery mildews tend to start out looking like white powder on the top of leaves, while downy mildews often first appear on the underside of leaves as downy-looking spores that range from white to quite dark, depending on the plant. As the disease progresses, downy mildews may produce yellow lesions on top of leaves that are defined by leaf veins.
Mildews may also appear on other plant parts, such as flowers. Although mildews affect many plants, the good news is that most mildews on shrubs, trees, and perennials are unsightly, but not fatal to the plant. The bad news is that mildews are much more serious on annuals and cucurbit vegetables, which include cucumbers, squash, pumpkins, gourds, and melons.
What causes plant mildews?
Many different strains of fungi cause powdery mildews, but downy mildews are caused by different fungus-like organisms. Although you may find both types of mildews on susceptible plants, the strains that cause mildews on one type of plant will not spread to other types. So, no need to worry that powdery mildew on your dogwood tree will spread to your cucumbers!
Prevention is key!
When you buy plants, you may find selections bred for resistance to mildews, or to better tolerate these diseases. If mildew-resistant varieties aren’t available, carefully examine the plants you buy for signs of infection.
If you’ve had problems with mildews in the past, be sure you’ve removed any diseased plants or weeds that may serve as a refuge for the pathogens. Do not save seed from downy mildew-infected plants as they may produce diseased plants.
Give your plants plenty of space for air circulation and avoid overhead watering. Cool-to-moderate humid weather promotes the growth of mildews, so that’s when you’re first likely to spot problems.
Treatment?
Although fungicides are available to help prevent mildews, for home gardens preventive spraying requires vigilance and may not be worth the effort. Instead, remove diseased plants from your garden before the disease spreads.
More information
– Clemson Cooperative Extension: Charts of plants resistant to powdery mildew and examples of fungicide products.
– Perdue Extension: List of disease-resistant annuals and perennials.
– NC State Extension: Curcurbit powdery mildew.
Article written by Debbie Green, Extension Master GardenerSM Volunteer.