The longest day of the year has come and gone. As gardeners try to capitalize on every hour of sunlight, they are starting to find a powdery white growth taking over many of their plants. The warm weather mixed with ample rain has set the stage for a family of fungi known as powdery mildew. If you are in the garden this week and notice green leaves turning white, here is what you need to do to help these plants turn healthy again.

Powdery mildew often appears as a superficial white or gray powdery growth of fungus over the surface of leaves, stems, flowers or fruits. These patches may enlarge until they cover the entire leaf on either one or both sides. Young foliage and shoots may be particularly susceptible, and leaf curling or twisting may be noticed before the fungus is seen. Severe powdery mildew infection will result in yellowed leaves, dried and brown leaves, and disfigured shoots and flowers. Although not usually a fatal disease, powdery mildew will cause the infected plant will look extremely unsightly.

Powdery mildew fungi infect almost all ornamental plants. Although each plant can be attacked by different species of fungi, all of the powdery mildew diseases are similar in appearance. Susceptible woody plants include some deciduous azaleas, buckeye, catalpa, cherry, a few of the flowering crabapples, dogwood, English oaks, euonymus, honeysuckle, horse chestnut, lilac, privet, roses, serviceberry and sycamore.

Powdery mildews are also common on certain herbaceous plants, such as chrysanthemums, dahlias, delphiniums, kalanchoes, phlox, Reiger begonias, snapdragons and zinnias. Remember that each species of powdery mildew has a very limited host range. Infection of one plant type does not necessarily mean that others are threatened.

Before using fungicides to control powdery mildews try these practices:

l Purchase only top-quality, disease-free plants of resistant cultivars and species from a reputable nursery, greenhouse or garden center.

l Rake up and destroy all dead leaves that might harbor the fungus.

l Maintain plants in a high vigor.

l Plant properly in well-prepared and well-drained soil where the plants will obtain a minimum of six hours of sunlight daily.

l Space plants for good air circulation. Do not plant highly susceptible plants, such as phlox, rose and zinnia in damp, shady locations.

l Do not handle or work among the plants when the foliage is wet.

l Water thoroughly at weekly intervals during periods of drought. The soil should be moist 8 to 12 inches deep. Avoid overhead watering and sprinkling the foliage. Use a soil soaker hose or root feeder so the foliage is not wetted.

To prevent damage to plants, begin spraying as soon as mildews are detected. Spray on a regular schedule, more often during cool, damp weather. Use a good spreader-sticker with the fungicides. Be sure and cover both surfaces of all leaves with the spray.