Powdery Mildew Attacks Many Garden Plants

It may be caused by any one of several closely related fungi and attacks about 300 different ornamental plants. A few common plants attacked are rose, phlox, chrysanthemum, dahlia, lilac, zinnia, hollyhock, crape myrtle, apple, oak and delphinlum. It becomes particularly troublesome when cool weather and heavy night dews of fall provide good growing conditions for it.

The name of the disease is quite descriptive, since it produces a while covering on infected leaves and stems. In early stages the powdery material may appear in spots but eventually may cover the entire upper or lower surface of the leaf. Colorless spores are produced on the white surface. They are easily detached and carried by wind or water to spread the disease to other leaves and other plants. Later in the fall, small black spherical bodies can be seen on the surface of the powdery material. These structures serve to carry the fungus over the winter.

Along with the powdery appearance, leaves that are infected can become stunted, discolored and deformed. In some plants, such as zinnia, phlox and hollyhock, the condition can become so severe that it can kill the plant. On a tree such as English oak, the condition appears late in the season when growth has almost ceased, so little or no permanent damage is done. Several steps can be taken to reduce the mildew problem next year. Sprays applied now can help stop its spread to uninfected areas, but generally much damage has already been done where the disease is present.

Sulfur as a dust or spray has long been used to control powdery mildew, particularly in the spring. It may cause plant damage if applied in summer when temperatures are too high. It is also not effective once the disease has become established on the plant and, therefore, is most effective as a preventive material. Sprays need to be applied at about 10-day intervals. Other spray materials are also available for the control of mildew on ornamental plants. These can provide some control even after the fungus has attacked the plants. Included are Actidione PM, Benlate and Karathane. Be sure to follow the label instructions carefully. Applications need to be made every 10 to 14 days throughout the fall to keep the disease under control. In future years remember to begin spraying in early August.

Sanitation is extremely important to reduce the source of infection for next spring and summer. Remove badly diseased plants promptly, and when cleaning out the garden later in fall, destroy all plants that have been infected. Tree leaves containing the disease should be burned or removed, and not used as a mulch. There is some difference in plant susceptibility to powdery mildew. Note varieties you have grown that seem more resistant so that they may be used in the future. Good air movement around plants also helps reduce the mildew problem, and those well spaced are often less severely attacked.

American Horticultural Society Pests and Diseases, by Pippa Greenwood.