Cercospora leaf spot causes spots and premature leaf drop on crapemyrtles.
August 21, 2024 - Crapemyrtles are beloved for their vibrant summer blooms and attractive bark, making them a popular choice for many East Texas gardens. However, like any plant, they are not immune to diseases. One common issue gardeners face with crapemyrtles is Cercospora leaf spot, a fungal disease that can occasionally detract from the plant's beauty.
Cercospora leaf spot is caused by the fungus Cercospora lythracearum. This disease typically manifests in late summer and early fall, particularly during periods of high humidity and rainfall, conditions common in East Texas. The fungus infects the leaves, leading to the appearance of small, circular to irregular brown spots with a yellow or orange halo. As the disease progresses, the spots can coalesce, causing significant leaf damage and premature leaf drop.
The initial symptoms of Cercospora leaf spot are small, reddish-brown to black spots on the lower leaves of the crapemyrtle. These spots gradually enlarge and can cover the entire leaf surface. Infected leaves often turn yellow or orange and fall off, leading to defoliation. While this defoliation rarely kills the plant, it can weaken it a bit and reduce its aesthetic appeal.
Cercospora leaf spot thrives in warm, humid conditions. The fungus overwinters in fallen leaves and other plant debris, releasing spores in the spring and summer. These spores are spread by wind, rain, and overhead irrigation. When they land on wet leaves, they germinate and infect the plant, starting the disease cycle anew.
Managing Cercospora leaf spot involves a combination of cultural practices and rarely chemical treatments. Here are some effective strategies to control and prevent this disease:
- Watering Practices: Avoid overhead watering, especially in the late afternoon or evening. Watering the base of the plant helps keep the foliage dry and reduces the risk of infection.
- Proper Spacing: Ensure proper spacing between plants to improve air circulation. Good air circulation and full sun helps the foliage dry quickly after rain or irrigation, reducing the humidity around the leaves and making it less conducive for fungal growth.
- Sanitation: Remove and dispose of fallen leaves and plant debris regularly. This practice helps eliminate the primary source of spores that cause new infections. Infected leaves should be bagged and discarded, not composted.
- Mulching: Apply a coarse layer of organic mulch such as pine straw around the base of the plant. Mulching helps maintain soil moisture, reduces water splashing onto the leaves, and minimizes the likelihood of spore transmission from the soil.
- Fungicides: In severe cases, fungicides may be necessary. Products containing chlorothalonil, myclobutanil, or thiophanate-methyl can be effective when applied early in the disease cycle before the problem starts. Follow label directions for application rates and timing. Remember, fungicides are most effective when used as a preventive measure rather than a cure so once it is noticed, it is too late to do anything about it. To be honest, I have never sprayed crapemyrtles with fungicide in my life.
- Resistant Varieties: Consider planting crapemyrtle varieties that show resistance to Cercospora leaf spot. While no variety is completely immune, some are less susceptible than others. According to the LSU Ag Center, the crapemyrtle cultivars most tolerant to Cercospora leaf spot are Natchez, Muskogee, Basham’s Party Pink, Sioux, and Tonto all hybrid cultivars developed by the late Donald Egolf at the U.S. National Arboretum.
Cercospora leaf spot can occasionally be a frustrating issue for gardeners, but it is difficult to control, does not kill the trees, and to be honest is best ignored. It is just a sign that we had rainfall early in the season.
Greg Grant is the Smith County horticulturist and Master Gardener coordinator for the Texas A&M AgriLife Extension Service. He is the author of Texas Fruit and Vegetable Gardening, Texas Home Landscaping, Heirloom Gardening in the South, and The Rose Rustlers. You can read his “Greg’s Ramblings” blog at arborgate.com, read his “In Greg’s Garden” in each issue of Texas Gardener magazine (texasgardener.com), or follow him on Facebook at “Greg Grant Gardens.” More science-based lawn and gardening information from the Texas A&M AgriLife Extension Service can be found at aggieturf.tamu.edu and aggie-horticulture.tamu.edu.
