Are the leaves of some of your favorite plants covered with a strange powder that makes them look like they were dusted with flour? You have powdery mildew.
A number of plants are susceptible to powdery mildew, such as phlox, lupine, hazel, birch, maple, roses, lilacs, monarda, grapes, gooseberries, squash and cucumbers. Over the past couple of years here, maples have been particularly affected. Squash and cucumber plants almost always have it in the early fall when the temperatures cool and the humidity rises.
The good news is that powdery mildew rarely kills or seriously damages a plant. If a plant is hit hard over several years the disease can stunt its growth. It usually just makes plants look bad, and some gardeners can’t stand that.
The best offense for dealing with powdery mildew is a good defense. First, select cultivars or varieties of plants that are mildew-resistant. Plant breeders have been working hard over the past 20 years to create more resistant lines of plants, so they are out there. Ask about it when you buy plants, or do an Internet search for varieties.
Powdery mildew is always present in the environment. It overwinters in garden debris and begins spreading spores in the early spring through wind, splashing rain and insects. Damp weather or periods of high humidity without a lot of rain seem to favor its development. It doesn’t do well in hot, dry weather.
This is why we see it here in the late spring and again in the fall. Powdery mildew is host specific – lilac mildew will not spread to your roses, maples or squash.
While we can’t control the weather conditions, we can control the growing conditions to help reduce the presence of powdery mildew. Overhead watering creates a lot of humidity. To reduce this, use soaker hoses or drip irrigation instead. This will also save on your water bill. Crowded and overgrown plantings reduce air circulation and as a result raise humidity. Prune out overgrown plants and thin others. If plants are too close together, have a digging and moving party in September to thin them out. Tell your friends if they come and help, they get to take the excess home. Lastly, just pick the offending leaves off and live with it.
If you feel a more aggressive treatment is needed, start early. The time to spray for powdery mildew is before the white powder appears on your plants. While this isn’t helpful for attacks you weren’t anticipating, it is good for those plants that are prone to it.
There are many different fungicides on the retail market to treat existing problems but no systemic or early-season sprays that will give season-long protection. Once you select a fungicide, plan on making several spray applications according to the label directions.
The fungicides will not make the existing damage disappear but will slow the spread to other parts of the plant.