What you need to know about powdery mildew
Powdery mildew is an easily identifiable fungus characterized by white, powdery areas that spread across a plant's foliage (usually the top side of leaves) and often its flowers as well. On the plus side, powdery mildew is host specific. So, just because one plant becomes infected doesn't mean neighboring plants will, too. However, powdery mildew consists of a wide range of species that infect an even wider range of host plants. Every group of plants, from ornamentals to edibles, shrubs to trees, has members that are commonly affected by powdery mildew. Make sure to do some basic research on the plants you choose for your garden to determine which pests and diseases you're likely to come up against. According to Greenhouse Grower, some of the edibles most prone to powdery mildew include squash, cucumber, tomato, Swiss chard, basil, sage, mint, as well as strawberry, raspberry, and blueberry plants.
Powdery mildew rarely kills a plant. But that doesn't mean it isn't a big deal if a plant becomes infected. The fungus pulls nutrients from the host plant and stunts productivity, which usually leads to a decrease in the number of flowers and fruit produced. UC Master Gardeners of Santa Clara County warns that the development of powdery mildew is favored by high humidity and moderate temperatures during the day, followed by cool nights. So, it's especially important to monitor plants during late spring and early fall.
- As your first line of defense, choose plant varieties that are resistant to powdery mildew. Resistant varieties may still become infected, but should be able to avoid the degree of damage done to susceptible varieties.
- Place plants in full sun since, according to Rodale's Organic Life, direct sun exposure prevents powdery mildew spores from germinating.
- Prune plants appropriately and allow for adequate spacing as a way to increase airflow and decrease humidity.
- During optimal conditions, consider spraying susceptible plants with preventative fungicides or homemade sprays. Rodale's Organic Life recommends spraying plants with a mixture of 1 teaspoon baking soda to 1 quart water, which will increase pH and inhibit powdery mildew from forming.
- Remove and destroy infected plant material immediately. The Old Farmer's Almanac stresses how spores are easily carried by wind from one host to another, and that they readily overwinter on plants or in the soil.
- Spray infected plants with a fungicide. The Old Farmer's Almanac suggests sulfur, lime-sulfur, or neem oil. As an alternative to fungicides, Rodale's Organic Life recommends spraying infected plants with a mixture of 1 part milk to 9 parts water, citing milk's ability to slow the spreading fungus. You will want to apply the milk mixture frequently, and reapply after rainfall.