LSU AgCenter horticulturist Allen Owings said, "Some years, the leaf spotting will result in significant defoliation of trees by the time late summer and early fall arrive. This year, the high humidity and weekly rainfall have made conditions ideal for disease development."
This leaf spot usually starts appearing in May and will continue into fall. Owings said, "And this may be worst case of leaf spotting on crape myrtles in many years."
According to AgCenter plant pathologist Raj Singh initial symptoms include the appearance of dark brown spots that develop first on the lower leaves and progress upward in the canopy from midsummer through fall.
Singh said, "In most instances, infected leaves develop a yellowish to orangy-red coloration because of the production of a toxin by the pathogen. Those 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."
Older varieties of crape myrtles are more susceptible to this disease than newer varieties.
Owings said, "Natchez, Muskogee, Tonto, Sioux, Basham's Party Pink and Tuscarora are more tolerant varieties, but even these are showing some symptoms this year."
New crape myrtle varieties with burgundy or black foliage seem to have less disease.
Long term, this disease is not detrimental to a tree, although it will slow down growth on younger plants. Plants growing in conditions that are not ideal will be more affected by the leaf spot disease.
Singh said, "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."
Getting adequate fungicide coverage on larger crape myrtles is also a problem, Singh said. You can, however, achieve some control of the Cercospora leaf spot fungus with applications of a systemic fungicide in the spring.
Another, new bacterial leaf spot disease is being reported in the South and is causing similar symptoms on crape myrtles, Singh said. AgCenter scientists are currently surveying trees and noting the differences between the diseases.
Owings said, "Crape myrtles are Louisiana's most popular summer flowering tree, but we should not be discouraged about their performance this summer."
In addition to the leaf spots, trees have not flowered as well because of the cooler-than-normal spring and summer. Crape myrtles prefer a warm spring, which results in a growth flush that yields more abundant summer flowers.
Owings said, "Our spring flush was lacking this year. Also, rainy summer conditions slowed bloom development and resulted in a shorter blooming cycle than we would normally see."
Copyright 2016 Nexstar Broadcasting, Inc. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.