Landscape and Nursery - Powdery Mildew Season

Powdery mildew season is upon us. The following is information on this disease group that attacks many of our ornamental plants.

General Information

The fungi which cause diseases known as powdery mildew attack a wide variety of ornamental plants grown in Delaware.

The damage due to infection by the fungi causing powdery mildew can be slight to severe. The disease generally occurs in Delaware during late spring, late summer and fall in cool to warm weather with high relative humidity and in shady areas.

Symptoms

A powdery, fluffy white to light gray-colored fungus growth on succulent stems, leaves, buds and flowers is the characteristic symptom of powdery mildew. Young plants and actively growing shoots are more severely damaged than older plants, leaves or branches. Infected leaves may be dwarfed, curled or deformed. Powdery mildew can destroy the blossoms on crape myrtle. Small dead flecks in the leaves and defoliation may occur on some varieties of azalea and rhododendron that are infected with powdery mildew without the obvious development of white fungus growth.

Disease Cycle

The white mildew on the plant surface is actually composed of the threads (mycelium) and asexually produced spores (conidia) of the powdery mildew fungus. These spores are wind-blown to other parts of the same plant or other plants of the same species. Powdery mildew fungi are quite host-specific, so for example the mildew on zinnia will not spread to dogwood or sycamore, and vice-versa. Also, they are obligate parasites, meaning that they can only grow on living plant tissue.

Some powdery mildew fungi survive the winter as colonies of mycelium, but many switch over to sexual reproduction in the fall, producing minute brown to black specks amid the old mycelium on the dying leaf or other plant part. These survive the winter and in the spring release another type of spore to start the cycle over.

Control

Some highly susceptible plants, such as Chinese photinia or euonymus should be replaced with a similar plant that is not susceptible to powdery mildew. On many trees, the disease causes little or no damage, therefore, control is not necessary. Plants in the landscape which may require fungicide applications to prevent powdery mildew damage include crape myrtle, phlox, rose and zinnia.

If damage from powdery mildew is severe and a susceptible plant(s) must be grown in the landscape, prune out severely diseased portions and use one of the following fungicides according to label instructions:

Propiconazole (Banner Maxx) is registered for control of powdery mildew and numerous other diseases on ornamentals.

Myclobutanil (Systhane, Eagle, and Immunox) is registered for control of powdery mildew and numerous other diseases on ornamentals.

Sulfur as a spray or dust as needed. Sulfur may cause some plant injury if applied when air temperature exceeds 90 F.

Triforine (Funginex) is labeled for use on roses and several other ornamentals. Follow manufacturer's instructions on the label. This product is only available in small packages.

Triadimefon (Bayleton and Strike) is now registered for use on a number of ornamentals.
Fenarimol (Rubigan) is a locally systemic fungicide for the prevention or therapeutic control of powdery mildew of field or landscape grown ornamentals.

In some situations copper fungicides will provide control of powdery mildew.

Note: The powdery mildew fungi can become resistant to any one of the fungicides listed above except sulfur.

Information modified from "Powdery Mildew of Ornamentals and Shade Trees" Ornamental Disease Information Note 4, R.K. Jones, Extension Plant Pathologist and D.M. Benson, Plant Pathologist, North Carolina State University Plant Pathology Extension