Answer: Unfortunately, your crepe myrtle is probably suffering from mildew, a fungal infection. Some varieties are more susceptible to it than others. Powdery mildew, unlike other fungal diseases, does not need a film of water in which to develop. You may want to prune to remove the infected leaves, and to open the canopy up to better air circulation. You must also rake up and remove any fallen foliage as that will spread the disease. There are neem oil based fungicides that you can apply to the canopy of the tree, or you can make your own spray: Make a baking soda solution and spray it onto the affected leaves. To make this solution mix 1 tablespoon of baking soda and 2 1/2 tablespoons of ultra-fine horticultural oil with a gallon of water. The oil is slightly fungicidal and acts to help the baking soda coat the leaf and hang on longer. Apply as a spray as soon as the mildew appears and every 10-14 days thereafter. Be sure to coat all surfaces!
Q&A Library Searching Tips