For Immediate Release

Contact: Robert Waggener, Editor

Phone: (307) 766-3571

E-mail: robertw@uwyo.edu

Date: Aug 14, 2006

Landscaping Tips

Weather unfortunately great for powdery mildew this summer

By Professor Gary Franc

Plant pathology specialist

University of Wyoming Cooperative Extension Service

The weather this summer has unfortunately been great for powdery mildew development in Wyoming, and many homeowners are starting to see this very common plant disease on grass, shrubs and trees around the home landscape.

Powdery mildew often does not make its appearance until August. An infected plant becomes weakened because the fungus shades the leaf, reducing the plant’s ability to photosynthesize and store energy for the coming winter. The fungus also directly robs nutrients from the leaf on which it is growing in order to support its own growth.

In Wyoming, the good news is that powdery mildew usually appears late enough in the season so no real damage is done to the plant.

Powdery mildew is one of the most easily recognized plant diseases in Wyoming. It appears as a white to grayish powdery coating on tree, shrub and flower leaves. It can also be seen on grass blades where the lawn is growing in shaded and humid areas. Good places to find diseased plants are where plants are shaded or where grass grows around the base of tree trunks and along hedges.

As this disease progresses, tiny black specks, barely visible to the naked eye, appear in the powdery areas. These specks are the structures that survived the winter, which allow the fungus to live from season to season.

Powdery mildew is host specific. When it appears on one type of plant, it usually does not spread to other species of plants. It will be more severe in situations where plants are closely spaced and crowded or where leaves are shaded.

The powdery mildew fungus is an obligate parasite, which means it needs a plant host in order to reproduce and survive. Like all good obligate parasites, the fungus rarely kills its host plant.

 Homeowners experiencing a powdery mildew problem this summer should carefully rake and discard, or compost, infected leaves and other plant debris this fall. This practice will remove the little, black overwintering structures clinging to leaves and will reduce disease development next summer. Unlike with many of the other plant diseases, disinfecting tools after raking powdery mildew-infected leaves is not necessary.

Thinning overgrown plants and other steps to aid penetration of sunlight and improve air circulation also will reduce the disease in areas of the landscape where it chronically reappears each year. Planting shade-tolerant plant varieties often reduces powdery mildew development where sunlight penetration is poor.

Powdery mildew is easily controlled with fungicide, and there are several effective fungicides that can be used. Fungicides, however, are recommended only if infection is severe early in the summer. Simply read the label to determine if powdery mildew is listed.         

Sulfur also can be used to suppress powdery mildew, but be careful as sulfur can damage plants if not used properly. Always read the fungicide label for proper application instructions and safety precautions.