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Master Gardener: Preventing, treating powdery mildew
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Master Gardener: Preventing, treating powdery mildew

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I went away for the weekend and came home to find some my plants covered with what looks like a white powder. What should I do? — L.H.

It sounds like you are describing powdery mildew. Many plants can be affected by powdery mildew including azalea, crabapple, dogwood, phlox, euonymus, lilac, snapdragon, dahlia, zinnia, crape myrtle, rose, pyracantha, rhododendron, spirea, wisteria, delphinium, oak, English ivy, photinia, blueberry, pecan, cucumber and squash. How severely it affects the plant depends on several factors including the variety of the plant, its overall health and the weather conditions. Unfortunately, there’s a lot of powdery mildew in our area right now due to our wet, relatively cool spring.

Powdery mildew becomes apparent when you notice a whitish substance covering leaves of your plant. This fungus gets its nutrients through small root-like appendages called haustoria. The haustoria penetrate the outer layer of the leaf’s surface allowing it to access nutrients in the leaves. This causes the leaves to turn brown, die, and eventually fall off.

The fungus sometimes spreads from one infected plant to another, depending on the strain of powdery mildew. Some are specific to only closely related plants.

Powdery mildew's fungi overwinter as small black spore bearing structures (cleistothecia) or as fungal threads called mycelium. Overwintering occurs in leaf debris, stems or dormant buds on plants. In the spring when temperatures get above 60 degrees, the overwintering structures begin to produce spores, which are moved by the air to an appropriate host plant. The disease is spread by not only wind but by splashing rain to other locations on the plant or other plants. It only takes about 48 hours for a newly germinated fungal spore to begin producing new spores, which contributes to its ability to spread quickly.

Even though we tend to expect powdery mildew in times when we have high humidity, this does not have to be the case. It tends to show up first in areas where plants are crowded together with poor air circulation that are located in damp or shaded areas.

To prevent or at least reduce powdery mildew, you can start by purchasing resistant varieties. If you have had a problem in the past, just ask your preferred nursery if they have varieties of your favorites that are resistant to powdery mildew.

If you have powdery mildew trouble with a particular plant each year, consider thinning it out or perhaps moving it to increase air circulation since this fungus prospers in locations with poor air flow. Also, it’s a good idea to clean up any plant debris at the end of the season which might play host to the overwintering spores.

Also, water in the morning and keep the water off the leaves so you don’t accidentally end up creating an environment conducive to this fungus.

The best way to approach powdery mildew control is to start by being actively involved in your garden. This disease, as is true with most plant disease, is easier treated and controlled the earlier you become aware of the problem.

Unfortunately, once your plant gets powdery mildew, there is no cure for the infected leaves. The best you can do is to try to prevent it from spreading. Usually, we recommend removing the leaves where powdery mildew is present. However, I was recently asked by a neighbor about her small tree whose leaves were covered with powdery mildew. If she were to remove all the infected leaves, there wouldn’t be any leaves left. You’ll want to begin using a fungicide on your infected plant, spraying with the frequency indicated in the fungicide’s instructions. Rather than repeat applications of the same fungicide, rotate back and forth between a couple different fungicides. This will help prevent the fungus from developing a resistance to a particular fungicide. If you do this, you will eventually eliminate the powdery mildew symptoms on your plant. Summer will also help since the heat minimizes spore production.

If you have a plant that has issues with powdery mildew each year, you can begin with your fungicide spraying schedule as soon as new leaves have appeared. In my yard I tend to have a problem with powdery mildew on our tall garden phlox. This makes sense because this plant grows in a cluster of vertical stems which by definition minimize the airflow making it an environment ripe for powdery mildew. So, I need to start treatment on this plant.

If you don’t see symptoms of powdery mildew until the fall, you may consider no treatment at all since the plant would likely be at the seasonal end of its life cycle. Just be sure to clean up the plant debris to minimize the opportunity for overwintering spores to plan a return visit to your garden next year.

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 St., or by emailing us at mg@tulsamastergardeners.org.

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