Q: What is this white stuff on the new growth of my crape myrtles?
A: After seeing the tree clippings brought into the office, it was easy to diagnose the problem as Powdery Mildew, Erysiphe lagerstroemiae. Powdery Mildew first appears on new shoots as a whitish powder that later spreads to the surface of leaves, stems, and flowers. Powdery mildew causes leaves, stems and flowers to become distorted and stunted. In severe cases, leaves may drop prematurely and flower buds may fail to open. Shady, humid locations and cool nights encourage powdery mildew in addition to frequent wetting of the foliage by irrigation or rainfall. Powdery mildew is most prevalent in spring and fall.
The best way to avoid powdery mildew is to plant one of the cultivars bred and selected for resistance to powdery mildew. Additionally, crape myrtle should be planted in sunny locations allowing free air movement so that wet foliage dries quickly. The following cultivars are showing excellent or good resistance to powdery mildew: Semi-dwarf (15 feet) – Acoma (white), Caddo (pink), Hope (blush-white), Pecos (pink), and Tonto (red). Intermediate (up to 20 feet) – Apalachee (orange), Centennial Spirit (dark red), Christiana (deep red), Comanche (coral pink), Hopi (pink), Lipan (red-lavender), Near East (pink), Osage (pink), Osage Blush (pink), Sioux (pink), and Yuma (lavender). Full tree (over 20 feet) – Basham’s Party Pink (lavender pink), Biloxi (pink), Choctaw (pink), Fantasy (white), Kiowa (white), Miami (pink), Muskogee (lavender pink), Natchez (white), Townhouse (white), Tuscarora (coral pink), Tuskegee (pink), Twilight (dark purple), and Wichita (lavender).
One other point I want to discuss is the importance of having a confirmed diagnosis before applying any pesticide. The condition on your crape myrtle is caused by a fungus therefore using insecticides would not be beneficial. The improper application of pesticides means we are not following the guidelines set by the Federal government on the pesticide label. In essence, we are breaking the law. Improper pesticide application wastes time and money and can contribute to the pest resistance. I know it is sometimes difficult to drop specimens by the Extension office but it is essential for us to provide the correct chemical for management. For any of your plant problems attend the free plant clinics at the Yulee office (86026 Pages Dairy Rd., Yulee) – the dates are listed on our website at http://nassau.ifas.ufl.edu.