Our recent spate of hot and cold weather has brought out a late outbreak of powdery mildew. Leaves on roses, lupines, phlox, squash, melons and even maple trees have all picked up the telltale whitish coating. Maple trees have been particularly hard hit this year.
Typically, powdery mildew covers leaves and tender shoots with a powdery, grayish-white coating dotted later in the season with small brown or black spots. With a heavy infestation, the leaves can be distorted, curled and stunted, reducing their ability to make food for the plant.
Powdery mildew is actually a group of diseases caused by a number of types of fungi. Some of the fungi affect only certain plants while others aren’t particular and will invade any susceptible plant. They are parasites that grow on the surface of the leaf or stem and send root-like structures into the leaf tissue to draw nutrients.
The white, powdery covering that we see is actually fungus strands, or mycelia, and summer spores. The summer spores form chains that are easily picked up by the wind and moved to new hosts. As the mildew gets established, it begins to produce black or brown fruiting bodies throughout the mycelia. These tiny spheres then overwinter on buds, bark and in leaf debris around the plant, emerging the following spring to spread the disease. Cold will kill some but not all types.
Getting rid of powdery mildew is a process and not a quick cure. Mildew thrives in crowded plantings where humidity is higher and air circulation is poor, so thin out or move sensitive plants to improve circulation. Sanitation in the fall is important to reducing next year’s problem. All mildew-infected plant material should be thrown in the regular trash and not in the compost pile or the clean green container.
On shrubs and perennials, pick off affected leaves and stems. Rake up all fall leaf debris under shrubs and trees, especially those that were infected this year, and put them in the trash. Vegetable plants like zucchinis, melons or squash need to be pulled and thrown in the trash. Lastly, if the same plants seem to get mildew every year, consider replanting with mildew-resistant varieties.
Surprisingly, one of the best tools for reducing the presence of mildew is to spray your plants with a hard stream of water several times a week. The fungus can’t tolerate being soaked with water and the spores will burst if they get too wet. Without spores, the fungus can’t spread. In the process, you might just knock off a few aphids and spider mites and reduce another garden challenge.
There are a number of fungicides available to the homeowner to treat powdery mildew, but they often work best when they are applied before symptoms appear in the spring and then reapplied every 10 to 14 days. Fungicides do work after an infestation, but it isn’t going to restore the looks of your plants.