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

News Release Distributed 08/11/10

A hot and wetter-than-normal summer has lead to an increase in the major disease problem on crape myrtles in south Louisiana, says LSU AgCenter horticulturist Allen Owings.

“The culprit is Cercospora leaf spot,” Owings says. “It usually begins appearing in late May to early June and continues in to the fall.”

“We generally don't see as much defoliation of crape myrtles this early as we have seen this year,” says LSU AgCenter plant pathologist Don Ferrin. “But with the high humidity and frequent rains we've been getting, the conditions have been ideal for disease development.”

Initial symptoms on crape myrtles are the appearance of dark brown spots that develop first on the lower leaves and progress upward in the canopy from mid-summer through fall, Ferrin says.

In most instances, infected leaves develop a yellowish to orangy-red coloration because of the production of a toxin by the pathogen, Ferrin says. These leaves then fall prematurely, particularly in highly susceptible varieties, and serve as a source to spread the pathogen and further disease development.

“Because of this, raking and destroying the fallen leaves should be a routine practice,” he says.

Older varieties of crape myrtles are more susceptible to this disease than newer varieties , Owings says. Hybrid crape myrtles are also less susceptible.

“The crape myrtle varieties most tolerant to Cercospora leaf spot are Natchez, Muskogee, Basham’s Party Pink, Sioux and Tonto,” Owings says. “This year though, even these varieties are showing some symptoms of the disease.”

Long term, this disease is not detrimental to a tree, although it will slow down growth on younger plants, the AgCenter experts say. And plants growing in conditions that are not ideal will be more affected by the leaf spot disease.

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