Pests and Diseases: Powdery Mildew

powdery mildew
Photo courtesy of www.ag.utah.gov

Tis the season for the funkiness of powdery mildew to rear its ugly head. It seems that it’s already afflicting peonies, dogwoods and bee balm. All of the usual suspects have a coating of white fungal growth clamoring for space on their leaves. When you hear powdery mildew do you think of cool, wet conditions or dry, warm growing conditions? Let’s take a look at what conditions powdery mildew prefers.

It’s a common misconception that powdery mildew prefers wet leaves when in fact it prefers warm, dry days and cool nights. It loves the days when the humidity is high as that allows the spores to germinate. Once germinated though, it grows like mad when the leaves are dry…wet leaves actually inhibit germination of the spores. After the fungus is actively growing, the spores are spread by splashing water. It’s virtually impossible to not meet all of those growing conditions at some point during the growing season so don’t fret. It’s reassuring to know that most powdery mildews areĀ  plant specific; in other words, the powdery mildew that’s affecting your cucumbers won’t overtake your dogwoods. Do you feel better now?

So what can you do to prevent the likelihood of your ornamentals being infected with powdery mildew?

  1. Thinning – provide enough air movement around your plants so that the humidity doesn’t remain high and allow the spores to germinate. You can accomplish this with perennials by dividing them and planting them in areas with good air circulation. With woody species like dogwood, thin the canopy so that air can move freely. I know of a dogwood that was planted in a courtyard in a building whose leaves are white from powdery mildew all summer long. It’s not a particularly attractive sight.
  2. Resistant cultivars – plant breeders have been working their magic to develop cultivars of plants that are resistant to powdery mildew. Notice that I said resistant…they aren’t magicians so the plants that they’ve developed can still become infected but the infection won’t be as severe. Examples include Cornus florida ‘Cherokee Brave’, Syringa patula ‘Miss Kim’, Lagerstroemia ‘Tuscarora’ and Cucumber Marketmore 76.
  3. Spraying – as you all know by now, I don’t like conventional pesticides and I don’t make recommendations on them here. But what I can offer you is an organic alternative. If you have powdery mildew and would like to prevent it from spreading or are trying to prevent it in the first place, try this concoction: combine 1 TBSP baking soda, 1 TBSP vegetable oil and 1 TBSP dish detergent in a gallon of water. The oil will burn your plants’ leavesĀ if it’s applied during the heat of the day so apply it on overcast days or in the evening. Weekly applications will help deter powdery mildew.

 

If powdery mildew is allowed to infect the new growth on your plants, it can cause distortions of the leaves and even death in some cases. I recommend that you monitor your plants to survey the severity of the disease. If you notice powdery mildew late in the growing season, I wouldn’t bother treating it since the leaves will be dropping soon anyway. I probably wouldn’t bother with treating it all unless it was killing the new growth; but then again, I’m a lazy gardener who believes that the plants will either live or die trying. What has been your experience with powdery mildew? What have you done about it in your garden? Leave me a comment below or e-mail me. If you enjoy being part of the Mid-Atlantic Gardening community, join our e-mail list (upper right hand corner of this page), become a fan on Facebook and follow me at Twitter. Happy gardening!

 

June 12, 2012Permalink 1 Comment

One thought on “Pests and Diseases: Powdery Mildew

  1. Look up ampelomyces quisqualis. It’s removed pm from plants. It’s a living microorganism and a hyperparasite of pm. Also, milk at a 1:20 milk:water ratio will lower the ph of the leaf surface and restrict growth of the fruiting bodies.

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