Downy Mildews

You may not know it, but you’ve probably seen downy mildews on several plants in your garden. The symptoms can be different for each type of plant, but the first thing that most people notice is angled dead areas on the tops of leaves. Closer inspection reveals gray, blue, white, or lavender fuzzy areas on the underside of the leaf. That’s downy mildews.


Most of us are familiar with the fuzzy white spots on leaves and stems that indicate powdery mildew. Powdery mildew is a fungal disease. The white areas we see are colonies of tiny fungi. Until recently, scientists thought that downy mildews were simply a different breed of fungi. Now we know that downy mildews are actually a whole new collection of tiny algae-like microbes (oomycetes) that parasitize vascular plants to complete their life cycle (obligate parasites or holoparasites).

White downy mildews branch with spore attached (PSU Extension)

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Figuring out if your plants are infested with powdery or downy mildews is important because the treatments are different.

Downy and powdery mildews on grape leaf (Wikipedia)

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Downy mildews symptoms

As downy mildews sets up household on your plants, you may not notice those gray, white, blue, or lavender fuzzy areas (sporulations) that eventually turn gray or black, or threadlike growths (mycelium) on lower leaf surfaces. Only after the lower leaf tissue is dissolved and absorbed by the downy mildews organisms will it become obvious, as the upper leaf surfaces turn yellow and die. These leaf lesions commonly have angular shapes that stop at leaf veins.

Downy mildews leaf lesions on spinach (Steven T. Koike)

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Some varieties of downy mildews can also grow inside your plants, leaving black streaks visible in stems and flowers. Other downy mildews symptoms are mistaken for gray mold (Botrytis). Remember, downy mildews isn’t a single life form. There are dozens of downy mildews. Another issue with downy mildews is that each host plant species may show different symptoms. In fact, sometimes the symptoms on different cultivars of the same species look very different. The only consistency is the fuzzy areas on the underside of leaves and discolored upper surfaces.

Downy mildews symptoms on different rose cultivars (Janna Beckman)

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Host plants

I’m sure there are some vascular plants that are not impacted by downy mildews, but I don’t know what they are. Downy mildews have evolved in tandem with countless host plants, pushing the evolutionary envelope ever forward. Here is just a small sample of the downy mildews you may encounter, and their host plants (and don't let the Latin scare you!):

• Peronospora sparsa - roses
• Peronospora lamii - mint family, including salvia, basil, coleus, oregano, and many others
• Pseudoperonospora cubensis - cucurbits and melons (biggest global cucurbit disease)
• Peronospora belbahrii - basil (first reported in 2004, now a global problem)
• Peronospora parasitica - cole crops, including broccoli, Swiss chard, 
• Plasmopara halstedii - daisy family (Asteraceae), including asters, sunflowers, artichokes, dahlias, yarrow, Echinacea, and more
• Plasmopara viticola - grapes (CA growers lost $189 million in 2011 to downy mildews!)
• Bremia lactucae - lettuce
• Peronospora antirrhini - snapdragons
• Peronospora farinosa - spinach
• Peronospora manshurica - soybeans
• Pseudoperonospora humuli - hops
• Plasmopara obducens - impatiens (another new one that could pose a big threat to impatiens)

Researchers believe that some species of downy mildews, especially the one that attacks cucurbits, actually follow ripening crops northward every growing season. Most of these microorganisms cannot handle the cold, though some overwinter in leaf litter, soil, or other debris.


Downy mildews control

Unlike true fungi, which cannot swim in the water that collects on leaves, downy mildews can and will. This means that the longer your plants’ leaves are wet, the more likely they are to become infested. Wet morning leaves provide the perfect growth medium, while dry, warmer afternoons allow spores to catch a ride to nearby plants on the slightest breeze. Because of this, sanitation and air flow are your best friends when battling or preventing downy mildews. These microorganisms prefer cool, damp conditions. When temperatures are between 50-75oF and relative humidity is above 85% (welcome to California winter), these microorganisms start reproducing like crazy. These tips can help reduce the likelihood of downy mildews in your garden and landscape:

• Prune plants to increase air flow
• Avoid overhead watering; use soaker hoses 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 air flow
• Rotate crops every three years
• Ensure proper drainage
• UCANR states that some fungicides may be effective in preventing the disease, but many of these chemicals are only available to licensed professionals
• Some copper sprays may be effective

Plants affected by downy mildews should not be eaten. I don’t know if it’s dangerous or not, but the pros say the flavor is altered and unmarketable. Doesn’t sound very good, does it?

Do any of your garden plants show signs of downy mildews?