A: Your crape myrtle has a disease called powdery mildew. I have seen and heard about it a good bit the last couple of weeks. This disease is common on ornamental plants such as dogwoods, rose and phlox, and many bedding plants. The fungi infecting these plants are all different. In other words, the disease generally does not spread from one type of plant to the other.
Powdery mildew usually begins to show up in the landscape in April. This year, it began to appear in March because plants leafed out much sooner because of our warm winter. It starts to appear on the new growth of the plants. It grows over the surface of tender leaf and stem tissues where it produces spores that give the leaf a white to gray appearance.
Powdery mildew develops when daytime temperatures are warm (70-85 degrees) and nighttime temperatures are cool (50-65 degrees). Infection is favored primarily by the cooler night temperatures and high humidity (80-100 percent). Unlike most other fungal pathogens that require free moisture to germinate and infect plant tissues, infections for powdery mildew fungi is decreased in the presence of moisture on the plant surface.
The crape myrtles I see that have the most powdery mildew are ones that don’t tend to get enough sunlight. Ideally, a crape myrtle should get full sun most of the day or at least a minimum of six hours. If practical and you can reach the leaves most affected, remove that growth and discard it. Rake up and destroy any fallen leaves. The white, powdery growth is the fungus producing thousands of spores that can continue to spread. Prune out dead and infected branches and twigs. Improving air circulation around the plant can help. Increase sunlight penetration around the tree by removing overhanging branches from other trees.
If you or anyone else is considering planting a new crape myrtle in the landscape in the foreseeable future, select varieties more resistant to powdery mildew.
The older varieties are much more susceptible. The newer varieties with the Indian names (Natchez, Muskogee, Comanche, Tuscarora) are all resistant.
Unfortunately at this point in the summer, it would not do any good to spray your tree with a fungicide. The spraying must be done when the leaves first come out in March or April to protect them before they get it. The fungicide cannot take away the powdery mildew and the leaf distortions that are already there.
There are many fungicides labeled for control of powdery mildew but probably two of the best to use would be myclobutanil, sold as Immunox, and tebuconazole, which is sold as Bayer Advanced Disease Control for Roses, Flowers, and Shrubs. Begin spraying when leaves emerge and continue applications according to the label directions until weather conditions are no longer favorable. You must get good coverage of the tree when you spray. For a larger tree, this might be difficult.
Also, many of you may not want to go to all this trouble of spraying, particularly if you have lots of trees. In this case, you might consider only the one or few you may have that you consider a valuable specimen in high impact areas of the landscape.
For those of you who practice organic gardening, natural fungicides such as Neem oil extract and potassium bicarbonate have been shown to be effective in some cases.
Although it is too late to spray for what powdery mildew is already there, it can actually get worse during late summer and early fall when the weather conditions return that favor their development. So spraying fungicides can help keep it from getting worse.
It is hard to evaluate the long-term affects that powdery mildew has on crape myrtles. In the short term, it should not bother them very much, but in the long run, the loss of photosynthesis and water due to leaf infections could weaken the trees.