Powdery mildew is a fungal growth, generally white and powdery that appears on the leaves. It usually is found on the upper surface of the leaf, but on some instances it may appear on the lower side of the leaf or on both. A slight purple discoloration may develop. Leaves will yellow and drop prematurely.
Trees, shrubs, roses, perennials, annuals, bulbous plants, vegetables, fruits, and grasses may all be affected by this fungi. Spores are spread by wind and rain splash. The fungus may over winter on host plants and re-infect new growth the following year.
Almost all landscapes have plants that become diseased with one of the powdery mildew fungi. Although the fungi that cause powdery mildew are usually different on different plants, all of the powdery mildew diseases are similar in appearance. In most cases, prompt recognition and control actions can prevent severe damage to plants from powdery mildew diseases.
Powdery mildews are host specific— they can't survive without the proper host plant. For example, the species Uncinula necator, which causes powdery mildew on grape vines and linden trees, does not attack lilac shrubs. Similarly, Microsphaea alni affects elm, catalpa, lilac and oak but not turfgrass.
Powdery mildew appears as a dusty white to gray coating over leaf surfaces or other plant parts. In most cases this fungal growth can be partially removed by rubbing the leaves. It might be identified incorrectly as dust that has accumulated on the leaves. Powdery mildew, however, will begin as discrete, usually circular, powdery white spots. As these spots expand they will coalesce, producing a continuous matt of mildew (similar to dirt or dust).
Turfgrass looks as though it is sprinkled with flour. Kentucky bluegrass and shade areas are the most susceptible. Grass will wither and die.
The fungi which cause powdery mildew are spread by spores produced in the white patches. These spores are blown in the wind to other parts of the plant or to other plants during the growing season. Generally each species of fungus will be limited in the number of plant species that can be attacked. For example the species of fungus infecting lilacs will not cause powdery mildew on apples.
During the winter the fungus survives on infected plant parts and in debris such as fallen leaves. It may produce resting structures known as cleistothecia, which resist harsh winter conditions. These will appear as small black dots within the white powdery patches.
The following spring, sexual spores (ascospores) are released from the cleistothecia, shot into the air, and carried by air currents to leaves of plants where new infections will begin. During the growing season, the fungus produces asexual spores (conidia) that help the fungus to spread and infection to build.
Before using fungicides you should attempt to limit powdery mildews by other means. The following cultural practices should be beneficial for controlling powdery mildews.
Purchase only top-quality, disease-free plants of resistant cultivars and species from a reputable nursery, greenhouse or garden center. Consult horticulturists in the green industry and Extension offices about the availability of resistant varieties for your specific area.
Prune out diseased terminals of woody plants, such as rose and crabapple, during the normal pruning period. All dead wood should be removed and destroyed (preferably by burning). Rake up and destroy all dead leaves that might harbor the fungus.
Try to maintain plants in top health using correct pruning techniques for the plant as well as regular fertilization at appropriate times, but don't over-fertilize.
Water only in the morning; reduce shade by pruning, aerate and check drainage in the area. Remove infected leaves promptly and destroy them. Do not compost infected leaves.
In many cases, powdery mildew diseases do little damage to overall plant health, and yearly infections can be ignored if unsightliness is not a major concern. For example, lilacs can have powdery mildew each year, with little or no apparent effect on plant health. On some plants, powdery mildews can result in significant damage. Thus, fungicides must be used to achieve acceptable control. For best results with fungicides, spray programs must begin 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.
Neem oil (Rose Defense, Shield-All, Triact),
Triforine (Ortho Funginex), ornamental use only, or
Potassium bicarbonate (Kaligreen, First Step)