How To Solve Common Crepe Myrtle Problems From Diseases To Pests

For vibrant summer color, few flowering plants can compete with the crepe myrtle (Lagerstroemia indica). Just look around your neighborhood. Nearly every street in the South is lined with these blooming trees from July through September. There's plenty to love about them: They grow almost anywhere, are easy to maintain, and are available in varieties of all shapes, sizes, and shades of red, white, purple, or pink.

Though crepe myrtles (also referred to as a crape myrtle or crapemyrtle) are practically maintenance-free, problems can arise, such as sooty mold, powdery mildew, and leaf spots. Many can be avoided simply by making sure you’ve picked a tree that’s hardy enough for your area, gets lots of light, and has plenty of room to grow. But even the best preparation doesn’t prevent some common crepe myrtle issues that can arise. Read on for the problems we hear most about and how to fix them.

Sooty Mold

Sooty mold is a fungus that covers the leaves and looks like you just sprayed your crepe myrtle with asphalt. Fungal growth takes place from spring through early fall. Also called black molds, sooty molds are caused by several species of fungi. However, rather than feeding on the plant itself, sooty mold actually grows on sticky honeydew excreted by sucking insects like aphids, scales, and whiteflies that do feed on the leaves. So if you’ve got sooty mold, you also have an insect infestation. 

Some sap-sucking insects do not fully digest plant sap. The undigested portion is excreted as a sweet, sticky liquid called honeydew. If copious amounts of honeydew form on trees, sidewalks and other surfaces below may become coated with it and the sooty mold that follows. Ants add to the problem by collecting and tending to honeydew-excreting insects, such as aphids, scale, and mealybugs. They milk the insects for the honeydew, which they take back to other ants for food. Ants also transfer honeydew-excreting insects from plant to plant.

Because the sooty mold fungus feeds off insect secretions and not the plant itself, if you rub a leaf covered in sooty mold you’ll generally find a green and healthy leaf underneath. However, an extremely heavy infestation can block sunlight from reaching the leaves, which may yellow and fall prematurely.

Prevention, Treatment, and Control

Get rid of the insects and the black mold will go with them. Look for aphids, scales, or leafhoppers, which excrete the sticky honeydew on which sooty mold grows. Control these insects by blasting them off with water or by spraying them with insecticidal soap, horticultural oil, neem oil, or Natria Multi-Insect Control. All of these are safe, natural products available at home and garden centers. On small plants, wipe or wash off the molds with a small sponge and water. On large trees, use a hose-end sprayer to wash off the mold.

Splashing rain or water may spread the fungus to other plants so treat all your plants for insects and not just the ones currently exhibiting a sooty mold issue. That way if fungus spores do travel with water droplets, there’s less chance they’ll grow elsewhere.

Powdery Mildew

Powdery mildew is a filmy, white fungus that grows on leaves and flower buds. Round, white spots on upper leaf surfaces expand and merge, covering both sides of leaves. Powdery mildew thrives when cool nights follow warm days. Insufficient sunlight and poor air circulation encourage development. There are many different powdery mildew fungi. The fungus spores overwinter on fruit trees. In spring, the fungi begins to grow, and spores are released to travel on the wind to young leaves. 

Leaves infected with powdery mildew will turn yellowish or brown. New growth may be stunted, curled, and distorted. Fruit drops early or is dwarfed. Flower buds drop without opening.

Prevention, Treatment, and Control

Once you notice a powdery mildew outbreak, spray the foliage according to label directions in early summer with neem oil, horticultural oil, Natria Disease Control, Daconil, or Immunox. The first three are natural products. You'll probably have to spray more than once.

Although it won’t help with your existing crepe myrtle, should you be thinking about planting another one in the future, choose a mildew-resistant variety to reduce your odds of having a powdery mildew issue. Many of the newer ones are resistant, such as 'Acoma', 'Arapaho', 'Catawba', 'Comanche', 'Natchez', 'Sioux', 'Tonto', and 'Zuni'.

To prevent powdery mildew, give plants sufficient light and air circulation. Reduce spreading by cutting back on nitrogen fertilizer, and pick off and destroy infected leaves and flowers.

Cercospora Leaf Spot

Cercospora is a leaf spot fungus that used to be fairly uncommon but isn't anymore. The nearly universal planting of crepe myrtles in the South has made it easy for this fungus to spread. This fungus likes sheltered areas where breezes are blocked and the foliage stays wet for long periods. One tree I planted in front of the house gets eaten up by leaf spot every year, while the one growing in the middle of the lawn is hardly touched.

Fortunately, this disease is not detrimental to the overall health of the crepe myrtle, but it may stunt the plant's growth. You’ll know if your crepe myrtle has it if brown spots form on the lower leaves in mid to late summer. Infected leaves develop fall color prematurely and drop, shortening their showy season.

Prevention, Treatment, and Control

Remove the spotted leaves as soon as you see them. Give the tree a good spray down to wash away any remaining spores. This fungus spreads easily so it’s important that you be vigilant in raking up and disposing of any infected leaves that have fallen. That will keep the fungus from reinfecting your tree or others nearby. Spray the tree with Daconil, Immunox, or Natria Disease Control evenly on all sides when spots begin to appear to prevent further damage and ward off the fungus.  

Some websites claim certain selections are resistant, such as 'Apalachee', 'Sioux', 'Tonto', 'Tuscarora', and 'Tuskegee'. We have doubts, though, because the one that gets devoured every year is 'Sioux'. Your best bet is to prevent outbreaks by planting crepe myrtles in open, sunny spots where air circulates freely. 

Bark Scale

Crepe myrtle bark scale is an invasive insect in the family of Eriococcidae and have a similar look to mealybugs. They can be found clustered in branch crotches, pruning sites, and under loose bark. You’ll find them on all parts of the crepe myrtle but rarely on the leaves. Bark scale is a relatively new pest in the Southeastern US. It thrives when crepe myrtles are planted in shaded areas. 

They rarely kill crepe myrtles but produce honeydew which grows black sooty mold. This black mold can completely cover foliage branches and trunks. They can cause stunted growth and reduce the size and quantity of flowers.

Prevention, Treatment, and Control

Crepe myrtles grown in full sun have smaller infestations than those grown in shady locations. Mealybug destroyers are effective in controlling bark scale. The most effective control is a dinotefuran soil drench in the spring. In late summer spray the trees with a mixture of bifenthrin and  2% horticultural oil (5 tablespoons of horticultural oil per gallon of water). 

Asian Ambrosia Beetles

The Asian ambrosia beetle (Xylosandrus crassiusculus) is an invasive insect that now threatens peach, plum, pear, pecan, and many other trees in the South. Less than 1/4-inch long and cylindrical, it is dark brown, reddish-brown, or black and may have pitted wing covers. It entered this country in 1974 near Charleston, South Carolina. Since then, the Asian ambrosia beetle has spread into North Carolina, Georgia, Florida, Alabama, Mississippi, Louisiana, and Texas.

These tiny beetles attack both stressed and healthy plants. While beetles are present most of the year, major activity occurs in March. Females bore into stems, branches, or trunks of young trees. They lay eggs inside and introduce a fungus (ambrosia) with which they feed their young. Females remain with their young until they mature and exit the tree. Hatching females mate before leaving the tree to infest a new host. The fungus clogs the plant's water transport system, which results in wilting. It's much easier to detect the beetle's presence than the insect itself. A telltale clue is a white, toothpick-like spike of boring dust that protrudes about 1 1/2 inches from the trunk. Branches wilt and die back.

Prevention, Treatment, and Control

You can reduce stress on plants by making sure they are watered correctly, fertilized annually, and kept free of disease. Avoid wounding them. Once several beetles have invaded the plant, insecticides are not effective. Prune and destroy infested limbs. It's best to remove a tree with severe damage to the main trunk. For prevention, thoroughly spray trunks of susceptible plants nearby with diazinon, endosulfan (Thiodan), or chlorpyrifos (Dursban).

Japanese Beetles

Japanese beetles are a non-native invasive pest. Adult Japanese beetles are about ½ inch in length and are coppery-brown with metallic green heads. They emerge and feed from May until August on the flowers and leaves of many plants, including crepe myrtles. Both the adult and the larvae of the Japanese Beetle can cause extensive damage. The larvae primarily eat grass roots but can also feed on the roots of young crepe myrtle trees. 

Prevention, Treatment, and Control

Non-chemical control can be done by hand-picking the Japanese beetles off the tree and dropping them in soapy water. (This is a great way to keep children busy during the summer!) Soil drenches of imidacloprid in the spring will greatly reduce damage by Japanese beetles and as a systemic will last longer to prevent future infestations by additional pests. A liquid spray of Sevin is also effective.

Aphids

Crape myrtle aphids are a soft-bodied insect that is pale yellowish-green in color with black spots on the abdomen. They vary in length from 1/16- to 1/8-inch long. The aphids suck the plant sap out of leaves which causes two harmful effects. One is that as the aphids feed, they excrete a sugary liquid called honeydew that can completely coat the leaves. This honeydew is food for the sooty mold fungi that can diminish the aesthetics of the crepe myrtle. Also, aphids inject saliva into the leaf while it feeds. The saliva causes yellow spots to develop on the leaf.

Prevention, Treatment, and Control

Aphids are susceptible to insecticides like insecticidal soaps and horticultural oils. Another option for managing aphids on crape myrtles is to use one of the soil-applied imidacloprid products. There are many predatory insects that feed on aphids such a ladybug beetles and green lacewings so avoid spraying broad spectrum insecticides that would destroy these beneficial insects. 

No Blooms

No matter where you live in the South, your crepe myrtles should have bloomed in summer. If your crepe myrtle hasn't bloomed, most likely it's due to one of three reasons: 

1. Your plant is just too small to bloom. Give it time.
2. Your plant isn't getting enough sun. Too little sun can depress blooms or keep your tree from blooming at all. Consider transplanting to relocate your crepe myrtle to a spot where it will thrive.
3. Some crepe myrtles bloom better than others. You may have a slacker. If so, consider replacing it with another longer-blooming variety, such as ‘Enduring Summer’, ‘Summerlasting’, ‘Zuni’, or ‘Sioux’.

Poor Pruning ("Crepe Murder")

Don't chop your large crepe myrtles down to stubs each spring. This ruins the natural form and encourages the growth of branches that are too weak to hold up the flowers. Prune your crepe myrtle in late winter by removing suckers and any crossing or dead branches. Always cut back to another branch, to just above an outward-facing bud, or to the branch collar.