Downy Mildews

You may not know it, but you’ve probably seen downy mildew on several plants in your garden. 

The symptoms can vary, depending on which plant and downy mildew species are involved. The first thing that most people notice is angled dead areas on the tops of leaves. Closer inspection reveals gray, blue, white, or lavender fuzz on the underside of the leaf. That’s a downy mildew.

Downy and powdery mildews on grape leaf (David B. Langston, USDA ARS) Public Domain

Is it powdery mildew?

Most gardeners are familiar with the fuzzy white areas on leaves and stems that indicate powdery mildew. Powdery mildew is a fungal disease. Those white areas are colonies of tiny fungi. Until recently, scientists thought downy mildews were just another fungus. Now we know that downy mildews are a whole new collection of tiny algae-like microbes (oomycetes) that parasitize vascular plants to complete their life cycle.

Figuring out if your plants are dealing with powdery or downy mildew is necessary because the treatments are very different.

Downy mildews symptoms

You may not notice downy mildew at first as gray, white, blue, or lavender fuzz grows on the underside of leaves. Eventually, those growths will turn gray or black as the tissue dissolves. By then, angular lesions that stop at leaf veins start forming on the upper surface. Next, the upper leaf surfaces turn yellow and die.

Some varieties of downy mildew can also grow inside plants, leaving black streaks visible on stems and flowers. Other symptoms are mistaken for ​gray mold (Botrytis). Remember, downy mildew isn’t a single life form. There are dozens of them.

Host plants

Nearly all vascular plants are susceptible to downy mildew. Here is a sample of plants affected by downy mildew:

• Cabbage family, including broccoli
• Cucurbits, such as squash and melons
• Grapes (CA growers lost $189 million in 2011 to downy mildew.)
• Hops
• Lettuces
• Mint family, including salvia, basil, and oregano
• Spinach
• Soybeans
• Sunflower family, including sunflowers, artichokes, and yarrow
• Swiss chard

Researchers believe downy mildew species follow ripening crops northward every growing season. Most of these microorganisms cannot handle the cold, though some overwinter in leaf litter, soil, seeds, or plant debris.

Downy mildews control

Unlike true fungi, which cannot swim in the water that collects on leaves, downy mildews can and will. So the longer your plants’ leaves are wet, the more likely they will become infected. Wet morning leaves provide the perfect growth medium, while dry, warmer afternoons allow spores to catch a ride to nearby plants in the slightest breeze. Because of this, sanitation and good airflow are your best friends when battling or preventing downy mildews. These microorganisms prefer damp conditions in temperatures between 50-75°F.

Use these tips to reduce the likelihood of downy mildews occurring in your garden and landscape:

• Prune plants to increase airflow.
• Avoid overhead watering; use soaker hoses or drip irrigation instead.
• Remove and trash any diseased plant tissue.
• Use resistant cultivars.
• Remove nearby weeds that may act as temporary hosts.
• Space plants to provide good airflow.
• Rotate crops every three years.
• Ensure proper drainage.
• Some fungicides may prevent the disease, but most are only available to licensed professionals.
• Some copper sprays may be effective.

To be safe, do not eat plants affected by downy mildew. It might not be dangerous, but experts say the flavor is altered and unmarketable.