What is the white powdery substance on my crape myrtle leaves?

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Question What is this white powdery substance on my crape myrtle leaves?

Answer Unless you recently hosted a baby powder party, it is probably powdery mildew. This fungal growth is most common and damaging in the spring on new growth. In severe cases, it can infect flowers and stems as well as leaves. Several factors contribute to its occurrence and severity.

Environmental conditions have the most effect on powdery mildew occurrence. Hot days and cool nights with high humidity is the combination where this disease thrives. Ironically, it doesn’t like rain. Dry leaf surfaces are more conducive to development of this disease.

Luckily there are steps we can take to lessen its effects since we can’t change the weather. First, start with disease resistant plants. Not all crape myrtles grow the same height, the same color blooms or have the same amount of resistance to powdery mildew. For a comparison of these characteristics check out http://edis.ifas.ufl.edu/MG266 .

Second, plant crape myrtles in the sun. They should get at least 8 hours of direct sunlight each day. Shady conditions increase potential for disease infection. Shade also reduces the amount of flowers a crape myrtle will produce.

Third, ease up on the fertilizer. Excessive, luscious new growth tends to be more disease prone. While excessive nitrogen is problematic, the lack of potash (potassium) and other nutrients may also contribute to the severity of a disease problem. Have your soil tested. Get your pH and nutrients in balance.

Fourth, prune responsibly. Pruning crape myrtles is not complicated, but it can be confusing. When managing powdery mildew, prune your crape myrtles and surrounding trees to increase air flow. The “lollypop” effect, created by shearing crape myrtle trees, does not allow air to flow freely through the tree canopy. For more info on improper pruning, often referred to as “crape murder,” visit http://sfyl.ifas.ufl.edu/hot_topics/lawn_and_garden/crape_murder.html

Lastly, fungicides can be used to manage powdery mildew. Spray at the first sign of disease development. Be sure to rotate pesticides to prevent disease resistance. If the first four steps of cultural management are taken, then pesticides should not be necessary every year. For a list of pesticides, visit http://www.ces.ncsu.edu/depts/pp/notes/Ornamental/odin004/odin004.htm If you don’t have internet access, call me at 910-893-7533 or email me at gpierce@harnett.org

Have you perused the baby powder isle lately? They have all sorts of additives and smells now. Remember, the rule of thumb is lotion in the winter, baby powder in the summer, and no baby powder on babies.

http://www.babycenter.com/406_is-it-safe-to-use-baby-powder-on-my-baby_1519559.bc

Gary L. Pierce

Horticulture Extension Agent