This time of year the tops of the leaves of many plants and shrubs become covered with a dull white dusty substance called powdery mildew. This frequently happens to roses, lilacs, phlox, honeysuckle and zinnias, but many other plants, shrubs and even trees are affected. This fungal disease usually occurs during periods of cool nights, humid days and no rain.
The good news is that the problem is mostly cosmetic; rarely does a plant die from powdery mildew. The other good news is that the cure for the problem is inexpensive and is probably already in your kitchen cupboard.
Just mix ½ teaspoon of baking soda with 1 quart of water in a spray bottle and spray the affected leaves every 7-10 days until the problem is gone. To make the spray stick to the leaves better, add ½ teaspoon liquid castile soap which you can purchase at health food stores. You can try using dish soap instead, but spray a small area of the plant first and wait a week to be sure the soap doesn’t harm the plant.
Use the spray as soon as you notice any mildew growth. If you had powdery mildew last year, you probably will have it again this year, and you may want to use the baking soda spray preventively on a 7-10 day schedule.
This same spray is helpful for two other more serious fungal plant diseases as well: blackspot which affects roses, and downy mildew which attacks many fruits, vegetables, flowers and grasses.
Blackspot is a common rose disease in which black spots with yellow margins appear on leaves. Eventually the leaves turn brown and the plant dies. Downy mildew is characterized by a white to purple downy growth on the underside of leaves and corresponding yellow spots topside. If left unchecked, the plant can eventually die.
My first experience with downy mildew was last year when my grapes were affected. I immediately began a baking soda spray program. It didn’t take long for the mildew under the leaves to disappear and for the yellow spots on the tops of the leaves to shrink to small brown specks. Japanese researchers have found that not only does baking soda prevent fungal spores from germinating and stops the development of the disease, but it even appears to help the plant repair fungus-damaged tissue.
Since downy mildew overwinters in affected plant parts and remains viable in the soil for several years, being spread by wind and rain, this year I am using the baking soda spray preventively. Since my roses are near the grapes, I usually give them a spray also to prevent powdery mildew and blackspot. So far, so good!
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