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Powdery mildew on a bee balm plant. Photo by www.MelindaMyers.com

By Melinda Myers
Guest Columnist

Don’t panic when you find a powdery white substance covering the leaves of some of your flowers, vegetables, and shrubs. This is a good clue that your plants are infected with the fungal disease, powdery mildew. Most otherwise healthy plants can tolerate the damage and perennials will return the following year.

You may see this disease on a variety of plants including zinnia, phlox, bee balm, lilacs, roses, cucumbers and squash. Powdery mildew is caused by several different fungi with each attacking specific host plants. This means the powdery mildew on one plant won’t necessarily infect its neighbors. But when the weather conditions are right, high humidity at night and low humidity during the day, powdery mildew can develop on a variety of unrelated susceptible plants.

This fungus grows on leaf surfaces, preventing sunlight from reaching the leaves. The infected leaves eventually turn yellow, brown and may wither. The disease usually won’t kill the plant, but it does ruin the beauty of ornamental plants and severe infestation can reduce the size and quantity of fruit when growing cucumbers, squash and other susceptible vegetables.

Consider living with the damage. Mask the view of the diseased leaves with slightly shorter nearby plants, allowing you to still enjoy the flowers.

If you decide to treat the plants, you must start at the first sign of the disease and then every seven to 14 days for effective control. Consider using one of the organic fungicides labeled for controlling this disease on the plants you are treating.

Cornell University found baking soda was effective at managing powdery mildew. You will find variations of the mixture using 1 to 1 1/2 tablespoons of baking soda and 2 1/2 to 3 tablespoons of lightweight horticulture oil mixed into one gallon of water.

Be sure to read and follow label directions when using any organic, natural, or synthetic product. Pretest any fungicide selected including the Cornell mixture. Apply the fungicide to a couple of leaves and monitor for any toxic side effects before spraying all the infected plants.

Reduce the risk of powdery mildew in future gardens. Remove and dispose of mildew-infected plants and leaves in fall.

When adding new plants to the garden, select those noted for powdery mildew resistance. They aren’t immune but less likely to develop the disease than more susceptible varieties.

Give plants plenty of room to reach their mature size. This increases airflow and light reaching the plants, decreasing humidity and the risk of infection. Avoid excess fertilization that promotes lush succulent growth that is more susceptible to this and other diseases.

Train vining crops like cucumber, squash, and melons onto a trellis. You’ll not only save space but also increase light and air penetration around the plants for greater productivity and less risk of disease.

Do a bit of pruning on susceptible phlox and bee balm next spring. Remove one-fourth of the stems of susceptible perennials when the plants are several inches tall. This increases light penetration and airflow, resulting in stronger stems and less risk of this disease.

A bit of prevention goes a long way in boosting the beauty of your garden and reducing the time spent managing this disease.

Melinda Myers has written more than 20 gardening books, including the recently released Midwest Gardener’s Handbook, 2^nd Edition, and Small Space Gardening. She hosts The Great Courses “How to Grow Anything” instant video and DVD series and the nationally syndicated Melinda’s Garden Moment TV & radio program. Myers is a columnist and contributing editor for Birds & Blooms magazine and her website is www.MelindaMyers.com.