The leaves on my tall garden phlox are covered with something white and they are looking bad. What can I do about this? NK

Sounds like your phlox has come down with a disease called powdery mildew. I battle this one each year on my garden phlox. It also seems to really likes my peonies. In reality, powdery mildew is not too picky. Many plants are susceptible to 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.

The severity of a powdery mildew infestation depends on the overall health of the plant and the weather conditions. We have had a lot of rain and relatively mild temps in the spring so we should get ready for powdery mildew.

Powdery mildew is a fungus that gets its nutrients via small, root-like appendages called haustoria. It’s the haustoria that penetrate the leaves, giving it access to the nutrients found in the leaves. Left untreated, powdery mildew will cause the leaves on your plant to turn brown, die, and fall off.

While there are a variety of powdery mildews, their life cycle is the same. Small black spore bearing structures called cleistothecia overwinter and begin to get active in the spring when temperatures rise above 60 degrees. These now active structures begin to produce spores which are moved by the air, eventually landing on an appropriate host. Splashing rain can also help circulate these spores.

While high humidity can be a contributing factor to its development, plants that are crowded together or growing in clusters with poor air circulation are also good hosts for powdery mildew. Once it finds a home, it tends to spread quickly. Damp shaded areas are also good incubators for powdery mildew.

Because of this, giving your plants room to breathe with air circulation is a good first line of defense against powdery mildew. In addition, if you had a problem with powdery mildew in previous years, removing the leaf litter from last year will also help reduce its ability to spread.

The way you water your plants can also make a difference. Since we know moisture is a contributor to the spread of powdery mildew, be sure to water your plants in the morning rather than the evening. Plants that are watered in the evening, oftentimes remain wet all night, making a perfect breeding ground for powdery mildew to spread. Also, the leaves of your plants do not need any water. It is the roots that need the water. So, try to only water the roots. This will help minimize the conditions necessary for powdery mildew to thrive.

This might be a good time to review what we call the “disease triangle.” The disease triangle is a way to remember what is needed for plant disease to thrive. For plant disease to occur, you need three things (the three sides of the disease triangle.) You need an appropriate host, a active pathogen, and the proper environment. If you remove even one side of the triangle, you are well on your way to minimizing plant disease in your garden. In this case, if you can minimize the environmental conditions such as overcrowded plants, or wet, humid conditions, you are removing a necessary element for powdery mildew to thrive.

One of the downsides to powdery mildew is that once your plant is symptomatic, there is no cure for the infected leaves. At that point your prime strategy becomes one to prevent the disease from spreading. Removing the infected leaves is a good place to start. When you are removing the leaves, you are removing one of the necessary elements of the disease triangle - removing the pathogen. Unfortunately, if you don’t catch it early, removing all the leaves is not an effective strategy for long term plant health.

Once you have removed the infected leaves, you’ll need to start a fungicide treatment program that will involve multiple applications of the fungicide on a schedule recommended by the manufacturer. It’s also a good idea to rotate back and forth between a couple of different fungicides to prevent the pathogen from developing any resistance to the fungicide. Also, as the summer heats up, the heat helps minimize spore production of the fungus.

If powdery mildew is a recurring issue in your garden each year, you might consider starting your fungicide treatment program as soon as your plant pushes out leaves in the spring.

As the growing season nears its end in the fall, if powdery mildew begins to appear then, you might not need to treat at all since the plant is likely at the end of its seasonal life cycle. If this is the case, be sure to clean up the infected plant debris in the fall to minimize the overwintering spore population. 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: Charles Averre, North Carolina State University, Bugwood.org