Garden Q&A: Why is my crape myrtle turning white?

Q: There’s white stuff all over my crape myrtle foliage and flowers. I’ve never seen it this bad. What is it, and what can I do about it?

A: Powdery mildew is the culprit, and it seems to be abundant this summer. crape myrtle is one of many plants susceptible to this group of fungi. Some cultivars do have improved mildew resistance (many from the U.S. National Arboretum breeding program from a few decades ago), but none are perfectly immune. There are well over a hundred crape myrtle cultivars overall on the market these days, but some of the recent introductions seem focused more on novel leaf colors (near-black), richer flower colors (true reds), or greater rebloom (being seedless) than they are on improved resistance to disease. Something to consider if you’re selecting varieties to grow.

Fungal pathogens usually infect a plant in a way that keeps the main fungus body, or mycelium, hidden inside the plant’s tissues. Powdery mildew is more unusual in that the mycelium primarily grows on the outside of the leaf. From there, the fungal strands intermittently punch through plant cell walls to enter the cells and absorb food. You can picture it like a sprawling perennial ground cover that roots as it creeps, where it stays on the soil surface but produces roots along the stem to anchor it to the ground and obtain nutrients.

The chains of wind-dispersed spores produced by the mycelium is what gives powdery mildew its characteristic appearance on most plants. Due to this, infections actually began a couple days or longer prior to this sign of disease. Symptoms in the foliage itself include yellow or reddish blotches discoloring the leaf and sometimes leaf wrinkling.

The more exposed nature of powdery mildew fungi makes them comparatively easier to treat, though as with many diseases, fungicide intervention is a preventive measure to protect healthy growth. It’s not curative, and it cannot reverse leaf damage from foliage already infected. Many fungicides are labeled for powdery mildew suppression; follow their directions. Only resort to chemical intervention when outbreaks are severe or if they threaten significant weakening of a plant already at risk of other problems. Consider the ease of treatment too — a tall crape myrtle whose foliage you cannot reach is not worth treating, or at the very least should involve a certified pesticide applicator.

Chemical-free approaches to reducing the risk of a powdery mildew outbreak include improving air circulation around and through dense plants (not planting too close to a wall or solid fence and reducing plant crowding) and selecting varieties of species vulnerable to mildew that have improved resistance, when possible. The most immediate method to reduce infection is to simply trim off buds or branch tips that have heavy fungal growth since infected flower buds tend not to open normally anyway (no great loss then), though this can delay rebloom. This late in the summer, though, flowers are finishing-up and you want to avoid trimming crape myrtle in autumn for the sake of not affecting winter hardiness, so it would be simplest to live with a white-mottled plant and begin monitoring for an outbreak the following year when there is more time to take action.

Q: Some of my hosta leaves are yellowing or browning. Is it an infection, and do I need to do anything besides cut those leaves off?

A: It might simply be heat or drought stress, or it could be an infection like anthracnose or alternaria. If a disease, then no, cutting off symptomatic leaves is sufficient intervention to slow infection progression since a fungicide would not be able to cure existing infection. Heat and drought stress can exacerbate plant vulnerabilities to pathogens.

If you water the plants with a sprinkler or garden hose, try to avoid wetting the leaves and only dampen the soil around its roots instead. Otherwise, water early enough in the day so that the hosta foliage can dry by nightfall. This will reduce their susceptibility to infection since many plant pathogens have an easier time infecting tissues when leaves stay wet for long periods.

University of Maryland Extension’s Home and Garden Information Center offers free gardening and pest information at extension.umd.edu/hgic. Click “Ask Extension” to send questions and photos.