University of Illinois Extension

University of Illinois Extension

Hort Answers

Fungal Disease

Powdery Mildew [Shrubs and Trees]
Many genus and species involved

Powdery mildew of lilac.
Powdery mildew of lilac.
4 (1 = rare 5 = annual)
2 (1 = very little damage 5 = plants killed)
There are many fungal species that cause powdery mildew. The disease affects many herbaceous and woody plants.

Plants Affected
White powdery patches develop on leaves, young stems, buds, flowers and even fruit. Patches may enlarge until the entire area is covered. With time, these patches become mealy or felt like and turn gray to tan. Leaves may stunt, curl, become chlorotic and drop early. Flower buds maybe deformed. Damage is more unsightly than harmful on most shrubs and trees.

Life Cycle
Some species of powdery mildew fungi infect only a few closely related hosts, where as others have a broad host range. Powdery mildew fungi overwinter on plant tissue and dormant buds. Spores are released in damp spring weather and move to uninfected tissue in water or wind. The fungus growing on the surface of leaves, gives the foliage a white, powdery look. The disease cycle may continue throughout the growing season as long as days are warm and dry followed by cooler, damp/humid (but NOT wet) nights.

Most powdery mildews on shrubs and trees do not have a significant impact on plant health. There are times, however, when management is needed, especially when plants have suffered repeated infections for several years. In many of these cases, cultural practices will control the disease. Plant in areas that are proper for the plant species. Place plants in sunny locations with good air movement. Do not crowd (allow for good air circulation) plants. Dense, shady, or damp areas favor disease development. Remove and destroy dead foliage around the plants in the fall to decrease the level of infection next spring. Maintain healthy plants, but avoid excessive fertilization and watering as these practices encourage succulent new growth, which is more susceptible to powdery mildew. Resistant varieties continue to offer the best source of disease control. If infections occur early in the growing season, fungicides can be used to reduce infection, but treatment must be started as soon as the first symptoms are seen. Once the disease becomes widespread, it cannot be controlled in that year. Chemical control is not recommended for infections that take place late in the growing season.

Related Resources
Home, Yard & Garden Pest Guide
Illinois Commercial Landscape and Turfgrass Pest Management Handbook
U of IL - Distance Diagnosis through Digital Imaging
U of IL - Plant Clinic