If you are like me, you have some peonies in your landscape. Peonies are great because the produce these giant flowers pretty early in the season. But after that, they are a nice, green leafy plant for the rest of the growing season…a nice, green, leafy plant that has a pre-disposition to getting powdery mildew unfortunately.

While peonies are affected by powdery mildew, so are azaleas, crabapples, dogwoods, phlox, euonymus, lilacs, snapdragons, dahlias, zinnias, crapemyrtles, roses, pyracanthas, rhododendrons, spireas, wisterias, delphiniums, oaks, English ivy, photinias, blueberries, pecans, cucumbers, and squash. Their susceptibility to powdery mildew depends on several factors including the weather conditions and the overall health of the plant.

Powdery mildew is a fungus that is able to draw nutrients from the plant through small, root-like appendages knows as haustoria. These haustoria penetrate the outermost layer of the leaf to get access to the nutrients within the leaves. Over time, the removal of these nutrients from the leaves will cause the affected leaves to turn brown, die, and fall off.

This fungus overwinters as small, black, spore bearing structures, or as fungal threads called mycelium. Overwintering happens primarily in leaf debris or in dormant buds on the plant. As the temperature begins to warm up in the spring, the overwintering fungi start to produce spores which are then moved by the wind in search of a host plant. Rain can also splash the spores up onto the plant or spread it within a plant. The ability of this fungus to spread easily is further exacerbated by the fact that it only takes about 48 for a new fungal spore to be able to start producing additional spores. That’s a pretty short window between inception and reproduction.

High humidity can help to create an environment that helps powdery mildew develop, but so can plant over-crowding and poor air circulation. You can recognize powdery mildew when leaves on your plant appear to have been dusted with flour. Luckily there are some things you can do to help minimize and treat for powdery mildew in your garden.

First, you can start by purchasing disease resistant varieties of plants. To do this, read the labels or talk with your favorite plant provider. Unfortunately for most of us, that ship has already sailed. So, now that we have a plant susceptible to powdery mildew in our landscape, what do we do?

First, you can thin out the plant and plants around it to increase air circulation. Increased air circulation works against the development and spread of powdery mildew. Also, clean up plant debris at the end to the season. I know I usually try to encourage you all to leave plant debris clean up until the following spring to help provide a home for overwintering insects. However, we don’t want to provide an overwintering home for powdery mildew, so you have my permission to clean up that debris in the fall.

Since powdery mildew can spread via splashing water, pay attention to how you water your plants. It’s always best to water in the morning because this allows water that lands on the plants to evaporate. Watering in the evening (and I know we all have to do that sometimes) provides a moist environment throughout the night which can encourage the development of fungal diseases. In addition, try to just water the root zone without getting the entire plant wet. Again, this helps to minimize disease development.

When you first notice powdery mildew on your plant, it’s time to start a treatment program with a good organic anti-fungal such as copper fungicide following the directions on your particular product. Unfortunately, with continued use of one anti-fungal, fungal disease can develop a resistance over time, so it’s a good idea to rotate between a couple of anti-fungal products. Treatment tends not to cure the plant, but it will help to minimize the spread of the fungus within a plant or to other plants in your landscape.

If you have a plant or a certain area of your garden that seems to develop powdery mildew each year, you can begin your fungicide treatment as soon as the leaves appear on the plant in the spring. As for me, I know my peonies tend to get powdery mildew every year, so I should start treating with an anti-fungal early on… even though I tend to wait until I see evidence of the disease. This would be a case of “do what I say, and not what I do.”

If powdery mildew doesn’t develop on your plants until the fall, you probably don’t need to treat them at all since they will be going dormant soon anyway. Just remember to clean up the debris from the plant at the end of the season to minimize any overwintering fungus around the plant. Also, after cleaning up this debris, be sure to clean your garden tools with a 10% bleach solution to minimize the chance of you spreading the disease to other plants when using your tools. See you in the garden!

You can get answers to all your gardening questions by calling the Tulsa Master Gardeners Help Line at 918-746-3701, dropping by our Diagnostic Center at 4116 E. 15th Street, or by emailing us at mg@tulsamastergardeners.org. Photo: 

Mary Ann Hansen, Virginia Polytechnic Institute and State University, Bugwood.org