|Many of my plants, including bee balm, phlox, and squash, have a white, powdery coating on the leaves. I have researched this and have concluded it is powdery mildew. Do you agree? What can I do to control it and keep it from spreading to other plants?|
|It does, indeed, sound like powdery mildew. The bad news is that this fungal disease overwinters on living plants, and can be difficult to eradicate. There's some good news, though. First of all, specific strains of the mildew affect different types of plants, so, for example, the powdery mildew on your squash won't spread to your perennials. Also, plant breeders have developed powdery-mildew-resistant varieties of many types of plants, so consider this when choosing new plants.
There are some things you can do to minimize problems with powdery mildew. This disease is unique among plant diseases in that it doesn't require a wet leaf surface to spread. It can thus thrive during hot, dry weather. The general advice to inhibit the spread of fungal diseases is to avoid wetting leaf surfaces. In the case of powdery mildew, however, you can actually inhibit infection with periodic strong sprays of water (not so strong as to damage the plant.)
Here are some general rules for control. Start by making sure that your plants are getting enough direct sunlight. (Eight to ten hours a day is generally the minimum for plants that flower or bear fruit.) You'll also want to make sure that there's enough room between plants for air to circulate freely. Overcrowding not only makes plants more susceptible to diseases, if leaves touch other plants, those diseases can easily be spread.
Some people report success with this home-made spray: Mix 1 tablespoon of baking soda and 2 1/2 tablespoons of ultra-fine horticultural oil in a gallon of water. Apply as a spray as soon as the mildew appears and every 10-14 days thereafter. Be sure to coat all surfaces.