What's That White Stuff On My Crepe Myrtle?
For years, I've been railing against crepe murder -- that odious practice of chopping down crepe myrtles into ugly, thick stumps each spring -- and many of you have wisely listened. Some of you, however, persist in this cretinous pursuit and now you're paying the price. Not in just hideous, chopped trees. You're seeing your crepes covered with this weird, white stuff. And it's all your fault.
The weird, white stuff is a fungus called powdery mildew. Its spores are everywhere, but they don't germinate unless the conditions are right. And that's what crepe murderers have helped to do this year. FYI, there are lots of different kinds of powdery mildew and they attack many different plants -- bee balm, lilacs, euonymus, phlox, squash, roses, etc. But they all look pretty much the same -- a powdery white coating or spots on leaves, stems, and flower buds.
What Harm Does Powdery Mildew Do?
On older leaves, powdery mildew mainly forms whitish spots that lay on the leaf surface. In doing so, they interfere with photosynthesis, because the leaf surface they're laying on can't get sunlight. They also produce heaps of mildew spores that go on to infect other leaves.
Powdery mildew is much more injurious to young growth. New, fast-growing leaves pucker and become distorted. They may even die. Flower buds covered with mildew fail to open. Powdery mildew won't kill your crepe myrtle. It'll just make it so ugly that you'll wish it were dead.
So How Is Powdery Mildew My Fault?
This fungus favors new, young, succulent, crowded growth that prevents good air circulation. That's exactly what you get when you commit crepe murder. A million stems grow like mad from the end of every stump. And powdery mildew says, "Yum!"
"But wait!" you say. "I bought a crepe myrtle that was mildew-resistant. How can it get mildew?" Grumpy attributes this to our wacky spring weather that featured a warm February, cool March, hot April, and cooler May. Although powdery mildew is generally associated with hot, wet weather, this is not its perfect world. In fact, its spores cannot germinate on a wet leaf surface and will actually die if the leaf is covered with water for too long. What powdery mildew likes best is fresh, new growth brought on by an early spring and stupid pruning combined with cool, humid, dry weather. Just like we got.
FYI, Grumpy has two crepe myrtles that he never prunes. Neither has a spot of powdery mildew.
How Can I Prevent Powdery Mildew?
1. Do not commit crepe murder! To see how to prune a crepe myrtle correctly, read "Crepe Myrtle Pruning Step-By-Step." By providing good spacing between the trunks and major limbs, you'll improve air circulation and have a healthier plant.
2. Go easy on the fertilizer. Unless you have sandy soil, crepe myrtles don't need regular fertilization. Over-feeding them produces lush, new growth that's prone to disease. A crepe myrtle growing in good soil and full sun naturally resists disease.
3. Plant a disease-resistant crepe myrtle. Most of the newer selections resist mildew. Many of them have Native American names, like 'Natchez,' 'Osage,' 'Acoma,' 'Sioux,' 'Miami,' 'Catawba,' and 'Zuni.' Other good ones include 'Dynamite,' 'Red Rocket,' and 'Pink Velour.'
4. If you have a sprinkler system, use it in early morning only. This will give crepe myrtle foliage and the surrounding garden time to dry before it cools off at night.
How Can I Get Rid of Powdery Mildew Once It's There?
In Grumpy's opinion, the best fungicide for preventing powdery mildew is Immunox, because it's systemic. This means the leaves absorb it, so its good effects last longer. For those of you who prefer organic or natural products, you can use neem oil or horticultural oil. All of these products are available at garden centers. OxiDate, a broad-spectrum organic fungicide, also works, but it's expensive. With all of these products, follow label directions carefully.