I’ll bet most of you are familiar with powdery mildew … at least you probably know what it looks like. It’s that funny white stuff that grows on some plants. No, a little garden fairy didn’t come during the night and throw flour on your plants!
Powdery mildew is a fungi and characterized by spots or patches of white to grayish, talcum-powder-like growth. It’s normally found on the top of leaves. Humidity alone is enough to make this fungus thrive, it doesn’t need free-standing water.
Although we don’t live in a humid environment, powdery mildew is definitely found in Colorado. (Some of the plants that are most susceptible are lilacs, phlox, mondarda / bee balm, roses, grapes, squash, and cucumbers.) Whether humidity is created by overhead watering, or from your sprinkler system, it can be easily combated.
The best defense against this disease it to space your plants properly when planting. Adequate airflow is important. You may need to prune out some branches to increase air circulation, or maybe you need to relocate some plants to another part of your garden. Damp, shaded areas are also common places for powdery mildew to thrive.
Powdery mildews are host-specific. That means they can’t survive without the proper host plant. The good news is the fungus that attacks grapes, for instance, will not grow on lilacs.
Many plants have been developed to be powdery mildew resistant, so look for that on the plant label when you’re shopping.
If you have a plant that’s been “attacked”, take action right away to prevent the spread of powdery mildew. Removed the affected leaves, or portion of the plant that’s affected. It’s important to disinfect your pruning tool between each cut so that you don’t transfer the disease. Clean your tool(s) with a simple solution of one part household bleach mixed with four parts water. Destroy the infected plant parts that you cut off, and definitely don’t compost them.
As a last resort, if improving air circulation doesn’t take care of the problem, there are chemicals that you can apply. The chemicals are plant specific, and always read the label and follow the instructions carefully. Chemicals work best when combined with good cultural controls.
Another important preventative step you can take is to clean-up fallen leaves and debris in the fall. The powdery mildew spores can live in the debris on the ground. Get rid of it in the fall so you don’t have to deal with powdery mildew again next spring.
Overall, powdery mildew is easily recognizable, can be avoided with good gardening practices and is relatively easy to treat. Check your garden regularly so you can be sure to spot it early. Don’t let the flour fairy visit you!
Learn more by reading CSU Extension Fact Sheet 2.902, “Powdery Mildews”
Contributed by Jan Roes, Certified Colorado Gardener
Photo courtesy of Ohio State University Extension