How to treat crepe myrtle powdery mildew?

KarenMay

Hi there, in Autumn this year I inherited a once very healthy potted lagerstroemia indica 'Rosea' pink crepe myrtle. I have been waiting in anticipation for it to flower. To my disappointment the new growth where buds are forming are wilting and seems to be powdery mildew forming. We have sprayed with milk solution as advised. Not sure if neem oil or white oil next. Please help.

EricL

Hello @KarenMay 

It looks like your crepe myrtle is dealing with a combination of stress and a fungal issue, most likely powdery mildew, and the wilted new growth is a classic sign that the infection has progressed into the tender tips where flower buds form. Powdery mildew is very common on lagerstroemia in cooler, still and shaded conditions, especially in autumn and early winter when days are mild and nights are cool.

The milk spray you used can help in the very early stages, but once there is visible distortion, browning and collapse of new shoots, it is not strong enough to bring the plant back to health. Powdery mildew will continue to damage new growth unless a more effective treatment is used and the growing conditions are improved, particularly airflow and sunlight exposure.

A horticultural oil can help, but it is important to choose the right one. Neem oil is useful as a preventative and mild treatment because it works as an antifungal, but it needs repeated applications and is less effective when the infection is already well established. White oil is more suited to soft-bodied insects rather than fungal problems, so it would not address powdery mildew properly. The most effective option at this stage is a sulphur-based fungicide Manutec 500 g Wettable Sulphur Fungicide or a dedicated powdery mildew fungicide labelled safe for crepe myrtles OCP 750 mL Ready To Use Eco-Fungicide. These products stop the fungus quickly and allow the plant to push out clean new shoots. Make sure to apply it in the cool part of the day and never combine sulphur products with oils, because the mix can burn foliage.

Because your plant is potted, it is also worth checking for underlying stress that might be making it more vulnerable. The pot looks adequate, but crepe myrtles prefer a position with good light and airflow. If it is close to the wall under the veranda, it may not be getting enough sun or air movement. Shifting it a little into brighter morning sun can make a significant difference. Ensure the potting mix drains freely and that the plant is not sitting in a saucer of water. Avoid fertilising while it is stressed; wait until you see healthy new growth.

Finally, you will need to remove the damaged tissue. Prune back the wilted tips to healthy wood to prevent the fungus from cycling back into new shoots. Crepe myrtles respond very well to pruning and will reshoot cleanly once the mildew is controlled.

Please remember to wear personal protection such as gloves, goggles and a mask when using chemical sprays.

If you need further assistance, please let us know.

Eric