It seems to spring up overnight: Powdery mildew.
The splotchy white and gray dust covers the leaves and stems of many of the most popular plants. Woody perennials like lilacs, roses, azaleas, serviceberry and buckeye are especially susceptible. So are flowering fruit trees like cherry and cra
bapple, along with squash, cucumbers and herbaceous perennials such as chrysanthemums, phlox, dahlias and zinnias.
While rarely fatal, powdery mildew is very unsightly and can stress and weaken plants. If enough leaf surface is covered with the mold, photosynthesis is inhibited and the leaves will fall off prematurely. For food crops like grapes and beets, this may reduce their flavor and nutritional value. Infected blossoms may not open or develop fruit at all.
There are many species of powdery mildew, and each is highly specific about what it attacks. The infection on your tomatoes won’t spread to your lilacs or roses. Unfortunately, these fungi are everywhere. They overwinter in plant debris and start forming spores in the early spring.
The spores are carried by wind and insects and can even jump to leaves via water splashed up from the soil. They thrive in damp, humid conditions and in the still air found among plants growing too close together.
There are a number of products that can help control powdery mildew, but practicing good gardening culture is the first line of defense. Buy only top-quality, disease-resistant species and their cultivars from a reputable nursery or garden center. Your county extension office can help you choose the right plants for your area. And in every case, be sure the plants you bring home are disease-free.
Give them the best start by planting in properly prepared, well-drained soil in an area that gets the correct amount of sun each plant requires, usually a minimum of six hours a day. Give the plants plenty of space for good air circulation, and keep mildew-susceptible plants like roses and zinnias out of damp, shady locations. Don’t work around the plants when the foliage is wet; give everything time to dry out in the morning.
Water deeply and thoroughly. The soil should be moist 10 to 12 inches deep. Use a soaker hose and irrigate in the morning. Let the water percolate slowly into the soil, and avoid wetting the foliage. Less desirable is overhead watering, which will wet the foliage and possibly splash soil-born fungi onto leaves. This watering method is especially problematic if done in the late afternoon or evening, due to the greater chance of any transplanted fungi infecting the plants during the warm, moist overnight hours.
Prune out all dead and obviously diseased stems from crabapples, roses and other woody plants and shrubs. The idea is to get the spore-producing mold off the plant and open up the structure to allow better air circulation.
All debris should be removed from the area and discarded. Remove any leaves or other fallen material and destroy it, too. Don’t add any infected materials to the compost pile.
A homemade protectant can be mixed from 1 tablespoon of baking soda and 1/2 teaspoon of liquid soap (like Ivory, but not dishwashing liquid or other detergents) in 1 gallon of water. It’s been shown to be effective in preventing powdery mildew in hot, humid conditions, but doesn’t have much effect once the infection has started.
Water the plants thoroughly a couple of days before applying this mixture, and don’t apply in direct sun. It could burn the leaves. Try it on a small area of foliage first to see the plant’s reaction.
There are many effective “natural” eradicants available too. The least toxic include Neem oil, jojoba or horticultural oil, potassium bicarbonate, copper, sulfur and the biological fungicide Serenade. Use at the first sign of infection. Spray on all surfaces, including the undersides of leaves. Apply to new leaves as the plants grow, and reapply every seven to 10 days.
Finally, as with any disease issue in your garden, the best defensive treatment to protect the rest of the landscape is to break the powdery-mildew life cycle by removing infected plants from the area and destroying them.
Joe Lamp’l, host of “Growing a Greener World” on PBS, is a master gardener and author. For more information, go to www.joegardener.com.
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