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 are  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’ leaves 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

Reader Question: Powdery Mildew on Lilac

Today’s Reader Question comes from Tommy in Halifax County, Virginia:

I live in an older home and there are lilacs growing on the property. Last year, their leaves turned almost white with some type of disease. Can you tell me what this was and if there is anything I can do to prevent it?

powdery mildew on lilac

Photo courtesy of ag.utah.gov

Tommy, it sounds like your lilacs (Syringa spp.) have a classic case of powdery mildew. Powdery mildew is a type of fungus that infects the leaves of many plants including dogwood, Monarda (bee balm), garden phlox and lilacs. It’s interesting to note that the parasitic fungi that infects one genus of plant will not necessarily infect other genera. Conditions that are favorable for powdery mildew growth include high humidity at night, low humidity during the day and temperatures of 70 to 80 degrees. That describes most springs in the Mid-Atlantic gardening region.

So what can you do to discourage powdery mildew on lilac?

  1. If you have the luxury of planting a lilac, pick a mildew resistant variety. ‘Miss Kim’ comes to mind immediately but I’m sure that there are other cultivars available.
  2. If you are going to be planting lilac, be sure to site them in an area with plenty of air circulation. If you have an area that always seems to be windy, this would be an ideal location for a new lilac.
  3. Lilacs will benefit from a sunny location to help the foliage dry before evening.
  4. In your case Tommy, one of the best things that you can do to prevent powdery mildew is thin the plant. By doing this, you’ll be helping the foliage to dry if you receive rainfall late in the evening or overnight.
  5. Speaking of rainfall and watering, don’t water in the evening. This watering rule pretty much applies to all plants, but it’s especially important with lilacs.

 

The last resort is to spray a fungicide. I don’t like chemicals in general so I don’t recommend going this route. Powdery mildew will rarely cause severe damage or kill the plant. Do everything that you can to promote healthy growth: add compost and mulch the plant to establish a deep root system. Enjoy the wonderfully fragrant blooms of the lilac and don’t worry so much about the powdery mildew. If you plant them in a mixed border, you’ll have other things blooming to distract your eye from the mildew.

If you like what we’re doing here at Mid-Atlantic Gardening, please subscribe to the website to receive updates to the latest posts as well as to be eligible for our subscriber giveaways. You can subscribe by joining our e-mail list on the top right of this page. Thank you for your support! If you have experience with powdery mildew on lilac, leave me a comment below or e-mail me at stacey@midatlanticgardening.com. Happy gardening!

 

March 29, 2012Permalink 1 Comment