What’s That White Stuff On My Crepe Myrtle?

June 11, 2012 | By | Comments (12)

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.

Mildew_phixr
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.

COMMENTS

  1. Steve Bender

    Missy,

    It sounds like your crepe myrtle is infested with a serious pest called the Asian ambrosia beetle. The “worms” are actually strings of sawdust pushed out from the insects inside. At this stage, there is nothing to do but cut down the crepe myrtle. Burn the infested wood or put it out with the trash. Sorry.

    June 25, 2014 at 9:43 am
  2. Missy

    My crepe myrtle has something on the trunk/limbs that looks like a white worm sticking out of it. The trunk is turning black and it has very few leaves.

    June 20, 2014 at 8:47 pm
  3. Grumpy Gardener (His Magnificence)

    SHB,
    If the plant is healthy and getting plenty of sun, my advice is to be patient. Sometimes new plants put most of their energy into growing new leaves and stems, instead of flowers. But you’ll get blooms eventually.
    Cindy,
    Thanks for the tip about mildew!

    July 6, 2012 at 4:04 pm
  4. Cindy

    I used a mixture of ivory soap and water on the powdery mildew and it took care of it. We purchased an older home that had an overgrown yard with leaves piled around everything. Once I cleaned up arund the base of the trees they have been growing fast and I have not cut them back at all.

    July 4, 2012 at 6:28 am
  5. SHB

    Grumps-
    I live in the DFW area and planted a Natchez in a full sun area in front of my house in early December. It was about 12′ tall when planted and looks to be very healthy. The tree has yet to bloom and it almost July 4th. We do not see any sign of budding either. Should we be concerned? Is there anything you would suggest we do?

    July 3, 2012 at 3:01 pm
  6. Grumpy Gardener (His Magnificence)

    DJ,
    Sorry, but that isn’t true this year. Many of the crepe myrtles I see covered with mildew this year are those hybrids. This isn’t supposed to happen, I know. But as I said, the fact that they were severely pruned in spring coupled with the unusually warm weather caused a massive flush of new growth that is now covered in white. I think that if they’d been left alone, there wouldn’t be any mildew on them. My unpruned ‘Miami’ and ‘Sioux’ are mildew-free.

    June 18, 2012 at 10:33 am
  7. DJ/Meander Mountain

    If you plant crepe myrtles named for American Indians (Miami, Zuni, Natchez, etc., as Steve said), you can dispose of fungicide products. These crepe myrtles were developed by the U.S. Arboretum and are outstanding. Beautiful, exfoliating bark is a bonus. They are trees, rather than shrubs and need to be allowed to grow like trees– i.e. no crepe murder!

    June 16, 2012 at 1:09 am
  8. Grumpy Gardener (His Magnificence)

    Colin,
    These treatments will work on your peony too.

    June 14, 2012 at 11:51 am
  9. Colin

    I have a powdery mildew problem on peonies. Will these treatments work on that, too? My default response has been to cut off the foliage and get it out of the garden.

    June 14, 2012 at 9:55 am
  10. Grumpy Gardener (His Magnificence)

    Thanks, Jason! Check it out, everybody! Randall, don’t prune! That will just make things worse. Spray your plants according to label directions. They will eventually send out more leaves. Spray the new ones too.

    June 13, 2012 at 11:53 am
  11. Randall Reynolds

    I got a chocolate crape myrtle here in Lancaster SC that has the mildew on it. I did not commit crepe murder!(I didnt even cut it at all) I sprayed the tree real good last week. It looks better but I cut off about four 8″ tree limbs some of the parts that where the worst. Was that ok? Are should I just spray and leave alone next time?

    June 12, 2012 at 4:06 pm
  12. Jason Pohl

    Hi Grumpy,
    Thanks for the mention of our organic broad-spectrum OxiDate fungicide! Just thought I should note that we do sell OxiDate in smaller (and more affordable) quantities – OxiDate Ready-to-Use (RTU) 32oz hand sprayer & OxiDate Ready-to-Spray (RTS) 32oz bottle with garden hose attachment.
    Thanks and good riddance to powdery mildew!
    Jason
    http://www.enviroselects.com

    June 12, 2012 at 3:17 pm

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