Frankly, I'm a bit surprised that I have to talk about this in April, as it usually rears its head much closer to June. Nevertheless, Powdery Mildew is already becoming a big problem on Crape Myrtles this spring. Powdery Mildew, or White Powdery Mildew is one of the most common problems of the crape myrtle, and it is caused by the fungus Erysiphe lagerstroemia. This fungus usually causes patches of a white to grayish powdery growth the surfaces of leaves, flowers and new shoots.
I've said for years on GardenLine, that while it is not a life-threatening problem, heavily infected flowers may fail to open. And infected parts of the plant are usually distorted and stunted and just flat out make the crape myrtle look bad. Usually the disease is most serious in shady, damp locations, especially where plants are crowded and air circulation is poor. However, you name the situation, and I've seen powdery mildew occur on crapes. Since development of the fungus is favored by cool nights and day time temperatures accompanied by high humidity, I think you can understand why we are having a serious outbreak right now. (If you've paid attention to the weather at all for the past two weeks.)
Prevention and Treatment: The most effective control measures include locating plants in full sun, removing sprouts from the base of the plant and planting resistant varieties. Still, I've seen supposed "resistant varieties" still get this nasty stuff. Removing diseased twigs and branches may be possible, if only a few shoots are infected. Remove sprouts (suckers) at the base of the plant as they occur, since they are very susceptible to powdery mildew. Once these sprouts become infected, the fungus easily spreads to the upper portions of the plant.
Once you need to "spray" with something, the best news is that there are many fungicides available on the market. Everything from Consan Triple Action 20 to Kocide, to Mancozeb to Banner-based fungicides will work at washing away the white powdery mildew and keeping it at bay at least for a few more days.
By far, the best news to come along for hard-core organic practitioners is how well Neem Oil-based products are working to wash away powdery mildew. Unfortunately, Neem Oil is not near as effective at long-term prevention as some of the other previously mentioned fungicides. But if staying "organic" is your main goal, at least there is finally something better than those baking soda concoctions that have historically burned the leaves off of crape myrtles.
Until next issue, here's to
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