Powdery Mildew Affects Many Plants

Powdery Mildew

Brian Jervis: Ask a Master Gardener

Sunday, May 10, 2020

Q: Some of my plants are developing this white powdery substance on them. What is going on? AM

A: What you are describing is called powdery mildew and can occur on most ornamental plants such as crape myrtles, lilacs, photinias, zinnias, tall phlox, and others. Humid conditions over a variety of temperatures such as cool nights followed by warm days are conducive to powdery mildew growth. Have we been having any of those? Yes, we have. Younger growth also tends to be more affected.

The spores of this disease tend to overwinter on plant debris from the previous season. The spring awakens these dormant spores, which then begin making their way to your new plants. You will likely first notice powdery mildew on the leaves of your plants with them taking on a somewhat dusty appearance.

To prevent this disease from occurring, there are a few things you can do. First, give your plants some breathing room. Do not crowd them. The air circulation will help mitigate the conditions that encourage its development. You may even need to do some pruning to allow for greater air flow.

Also be mindful of when and how you are watering. The leaves of your plant do not need to be watered. The water needs to be placed on the root zone. Now, a lot of times there are sprinkler systems that are watering a large area at once and you cannot keep the water off the leaves. But if you are hand watering, water the roots, not the leaves.

Also, water in the morning. This allows time for water to evaporate off the leaves during the day rather than watering in the evening when water can remain on the leaves all night making a perfect breeding ground for disease.

You also do not want to fertilize your plants in late summer, which would encourage new growth that could become susceptible to powdery mildew.

If these strategies do not prevent development of powdery mildew, your plants can be sprayed with a fungicide or neem oil. Before spraying, remove as many of the infected leaves as is practical. You may say all the leaves are infected. If that is the case, do not remove all the leaves. But if the disease is limited to a certain area of the plant, then by all means, remove that branch or stem because you will be eliminating the source of the problem. You may need to repeat these treatments every two weeks for a while until you don’t see any more evidence of the disease. Chemical treatment will not repair damaged leaves, but it will stop its spread.

Garden tips

• Prune and feed all the spring blooming shrubs, such as azaleas and forsythia immediately after blooming, if needed. Azaleas need less fertilizer than many shrubs and often a yearly addition of mulch, as it decays, it will add all the nutrients they need.

• Seeding and sodding of warm-season grasses such as bermudagrass, Buffalograss and zoysiagrass is best performed in mid-May through the end of June. The soil temperatures are warm enough for germination and growth. These grasses need a long summer growing season to promote winter hardiness.