Powdery Mildews

Vulnerable plants
Many plants are susceptible to infection by powdery mildew fungi. Though the dusty coating looks similar, there are many different species of powdery mildew fungi that infect different plants. For example, the powdery mildew on rose is a particular species that only infects rose and not dogwood, and vise versa.

What powdery mildews look like
Powdery mildew fungi can be seen as a dusty, gray to white coating growing on the surface of leaves and shoots and occasionally flowers and fruit. These fungi grow primarily on the outer surface of affected plant parts. Powdery mildews are parasites and obtain their food from living host plants using small tube-like structures called haustoria. As the fungus grows over the surface of the plant it extends haustoria into the plant cells where it absorbs material from plant cells for nourishment. Infected young leave and shoots curl, and become yellowed and brittle.

Premature leaf loss may also occur. On highly susceptible varieties of roses, powdery mildew often prevents normal flowering. Late in the growing season, numerous spherical, black over-wintering structures are visible on the surface of leaves of some host plants.

How infection occurs
Powdery mildews produce spores that blow through the air and infect plants when temperatures are moderate (60-80º F). Humid conditions, overcrowding and excessive shading that keeps plants cool and damp promotes powdery mildew development. Spores are produced on the leaf surface periodically throughout the growing season. And spread the disease to other vulnerable parts of the plant. Many, but not all, powdery mildew fungi begin to form numerous over-wintering structures in the late summer on infected leaves. Other powdery mildews over-winter by infecting buds on susceptible plants. Powdery mildew over-winters on roses in this manner. As the shoots expand the following spring, the powdery mildew fungus resumes growth. Remember that each species of powdery mildew can only infect certain kinds of plants. Infection of one plant species does not necessarily mean that that particular powdery mildew fungus threatens other species nearby.

Management strategies
Although powdery mildews are some of the most conspicuous plant diseases in the landscape, generally the damage caused is of minor consequence to healthy plants and does not warrant chemical control if unsightliness is not a critical concern. Furthermore, limiting powdery mildews by chemical applications alone is seldom successful. Employing proper cultural practices is important, and often all that is needed, to manage the disease effectively.

Resistant varieties - Grow cultivators and species of host plants that are resistant to powdery mildew. This is the easiest way to reduce incidence of the disease.

Sanitation - Prune and dispose of all dead and diseased twigs and branches. This practice is particularly important on plants such as rose and crabapple, where powdery mildew over-winters in infected buds. Where the fungus over winters on fallen leaves, collect and dispose of these leaves to reduce inoculum available to start infections next spring. Prune plants for good air circulation and space them well to provide adequate sunlight penetration.

Modifying the growing environment - Avoid growing susceptible plants in shaded sites. These plants should receive sun all day or for a minimum of 6 hours daily. Avoid excess fertilizer applications, especially nitrogen. This practice encourages tender, succulent growth, which is more prone to powdery mildew infection.

Chemical control - Where appearance is critical or serious damage is occurring, chemical treatments are recommended along with suitable cultural practices to achieve acceptable control. When selecting chemical control of powdery mildews consider products such as horticultural oils, commercial baking soda preparations (sodium bicarbonate and potassium bicarbonate), and anti-transpirants, as well as conventional fungicides. Chemical spray programs are most effective when begun just as symptoms of powdery mildew begin to show. If powdery mildew is extensive on the plant, there will be little benefit from chemical control that season. Applications need to coat both surfaces of all leaves to prevent infection. Spray on a regular schedule, and repeat more often during mild, damp weather. A spreader-sticker mixed with the fungicides improves coverage and adhesion to plant surfaces.

Dan Gillman
UMass Extension Plant Pathologist
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Diseases index:
Apple Scab
Cedar-Apple Rust
Crown Gall
Dogwood Anthracnose
Fire Blight
Gynosporangium rusts
Juniper Tip Blight
Powdery Mildew
Seedling Root and Crown Rots of Woody Plants
Spore Shooting Fungi
Verticillium Wilt of Woody Landscape Plants
Volutella Blight and Stem Canker on Pachysandra

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