Toward the beginning of August, I noticed that the pods and leaves of my snow pea plants had taken on a white coating. It looked like a dusting of confectioners sugar was covering the plants.
This is a fungal disease known as powdery mildew. Standing near the pea plants on a hot afternoon, the plants smelled “moldy.” The disease rendered the pods inedible. Since pea plants are a spring crop, I was surprised that I was able to still harvest a few pods into summer. I had thought that planting a single row of the peas, with a two-foot clearance on either side, would provide enough air circulation. I removed the plants and did not compost them. The fungus can survive the winter on such plant debris and then become a source of infection next year.
Many species of fungi are classified as powdery mildew. Usually, a specific fungus infects a specific host. The fungus begins as small spots of white, which eventually grow to cover entire leaf surfaces, flowers, and fruits. Unlike some fungal diseases, powdery mildew does not require wet leaves. The warm days and cool, humid nights are enough for growth of this disease.
Over 10,000 plants can be affected by powdery mildew. You may find your tomatoes, carrots, beans, and several other crops troubled by powdery mildew. Flowering Dogwood is susceptible, as are Bee Balm, Garden Phlox, Black-eyed Susan and Zinnia. The native, red-flowered Bee Balm that grows in a wet area on our property gets the disease while the cultivar in my garden does not. Besides sanitation and providing good air circulation, using resistant varieties helps to reduce the occurrence of this disease.
Most of the time, powdery mildew looks unsightly but will not kill the plant. But if it becomes severe enough, the fungus will block sunlight from the leaves, and they will turn yellow and drop off. To be effective, a fungicide must be started before the fungus becomes widespread. If you opt to use a fungicide on food plants, read labels carefully and be aware of the application to harvest interval.
Don’t mistake this disease for some white-colored insects. Raised spots of white on Eastern Hemlock are likely to be Hemlock Woolly Adelgid. Mealy bugs are typically found on house plants. Wooly Aphids or Flatid Planthoppers (which found my lilac this summer) will look like bits of white fluff. These are sucking insects and they attach themselves and don’t move around. All of these will appear as discrete white spots, not as a coating of white on the surfaces of your plants. Even if the insects are clustered, you can still discern the outline of the individuals in the group. Dealing with insects will require different tactics.
Cornell University provides a list of disease resistant pea varieties at: https://www.vegetables.cornell.edu/pest-management/disease-factsheets/disease-resistant-vegetable-varieties/disease-resistant-pea-varieties/. This site will also provide a link to resistant varieties of other vegetables.
To reduce the use of fungicides, look for Jean’s Appalachian Snow Flowering Dogwood, Jacob Cline Bee Balm, David Garden Phlox and State Fair Zinnia. Many more disease resistant varieties are for the taking. Read catalog and plant labels to be in the know.
Donna Gates is a retired lab technician for the University of Maryland whose work focused on the identification of stream invertebrates. Her current gardening interests are centered around her rural Garrett County, Maryland, home.