Cercospora leaf spot on crape myrtle leaves. (Photo by Allen Owings. Click on photo for downloadable image.)

Suckers on crape myrtle trees. (Photo by Allen Owings. Click on photo for downloadable image.)

News Release Distributed 07/29/11

By LSU AgCenter Horticulturists Dan Gill, Kyle Huffstickler and Allen Owings

It’s the time of the year, or at least one of the times of the year, when home gardeners have crape myrtle questions. We will answer a few here.

Crape myrtles are battling Cercospora leaf spot. In addition, aphids and the resulting sooty mold can be prevalent this time of the year. And finally, it’s time to practice sucker control – do you need to cut them off and can you spray them with something to keep the suckers from growing or coming back after they are pruned off?

Leaf spot

The weather was hot and very dry this spring and this past fall, but Cercospora leaf spot can still be found on crape myrtles, especially in south Louisiana. This fungal disease usually begins appearing in late May to early June and continues into fall. Initial symptoms are the appearance of dark brown spots that develop first on the lower leaves and progress upward in the canopy from midsummer through fall.

In most instances, infected leaves develop a yellowish to orangey red coloration due to the production of a toxin by the Cercospora pathogen. These leaves then fall prematurely, particularly in highly susceptible cultivars, and serve as a source of inoculum for spreading the pathogen and further disease development. Because of this, raking and destroying the fallen leaves should be a routine practice.

Older varieties of crape myrtles are more susceptible than the newer varieties to this disease. Hybrid crape myrtles are also less susceptible. The crape myrtle varieties that are most tolerant to Cercospora leaf spot are Natchez, Muskogee, Basham’s Party Pink, Sioux and Tonto.

Long term, this disease is not detrimental to the plant. It will slow down growth on younger plants, and if plants are growing in conditions that are not ideal, the leaf spot will weaken individual plants more than if the growing conditions are ideal.

The use of fungicides to control this disease has not been very effective because they would have to be applied repeatedly throughout the growing season. Getting adequate coverage on larger crape myrtles is also problematic.

Aphids

Another problem that is frequent on crape myrtles is insect damage. Actually, most insects do not do much damage except for aphids that may feed on the new shoot growth in spring. White flies also are sometimes a problem on crape myrtles. Left unchecked, these insects will release bodily fluids onto the foliage, and the resultant honeydew leads to sooty mold on the leaves. This black discoloration occurs normally in early summer through fall.

If you control the insects, no sooty mold will develop. Most insecticides control aphids. You can apply a systemic material early in the spring to control aphids before they appear or spray a contact insecticide once aphid problems become apparent.

Sucker control

Tired of suckers on crape myrtles? Many home gardeners and landscape professionals ask about sucker control on this popular tree.

Suckers are more prone on juvenile, young trees. In addition, mechanical damage by weed trimmers to the lower stem and upper portion of the root system can cause sucker development. Trees with surface roots also have more suckers.

When removing suckers by pruning, use sharp pruners and do not leave a stub. Most sucker control products work best when suckers that are present are cut off, then the sucker-control product is sprayed on the cut-over areas.

You can spray napthaleneacetic acid (NAA - an organic auxin) to control suckers. Products available with this active ingredient are Sucker Stopper from Monterey Lawn and Garden Products and Fertilome Prune Smart Sprout Inhibitor.

Visit LaHouse in Baton Rouge to see sustainable landscape practices in action. The home and landscape resource center is near the intersection of Burbank Drive and Nicholson Drive (Louisiana Highway 30) in Baton Rouge, across the street from the LSU baseball stadium. For more information, go to www.lsuagcenter.com/lahouse or www.lsuagcenter.com/lyn.

Rick Bogren