Disease name: Powdery mildews
Pathogen name: Several species of fungi
Hosts: Numerous species of plants. Hosts vary by the species of fungus.
Powdery mildews are caused by over 1,000 species of fungi that affect a wide variety of landscape plants. Most of these diseases grow only on the upper leaf surface and stems of plants, and do not invade the leaf tissue of the host plant. In most cases, powdery mildew is not a serious problem and prompt recognition and management can prevent severe damage to plants.
Some of the more susceptible trees and shrubs to exhibit powdery mildew symptoms include apple and crabapple (Malus), some cultivars of ninebark (Physocarpus), elm (Ulmus), rose (Rosa) and lilac (Syringa). Powdery mildews are also common on certain herbaceous plants, such as beebalm, phlox, and zinnias.
Powdery mildews are often observed in late summer and early fall as a white or gray powdery growth on the top surface of leaves, stems, flowers, and fruit. As the infection advances, buds may fail to open, leaves can become distorted, turn yellow or brown, or they may drop prematurely. Fruits develop blemishes or abort early. Powdery mildew grows primarily on plant surfaces, rather than penetrating into the tissues. Unlike most other fungal infections, it does not require water on plant surfaces to cause infection.
Powdery mildew fungi overwinter in tiny black fruiting bodies which are located in leaf litter, stems, and dormant buds. In spring, the fruiting bodies produce spores that start the initial infection, especially during periods of high humidity when days are warm and nights are cool (ideal temperatures range between 60 to 80 degrees F). Susceptible plants are most vulnerable while new shoots and leaves are expanding. The specific fungi causing damage varies with each individual host; for example, the fungus that causes powdery mildew on lilacs does not infect viburnums.
Cultural Management:
Many powdery mildews, especially those that attack trees and shrubs, are more unsightly than harmful. Good sanitation is very important to reduce infections next spring and summer. Powdery mildews do not grow on dead tissue, but they have structures that carry them through the winter on dead and living plant tissue. Remove diseased leaves as soon as they drop off. Do not compost or use as mulch. Plant and space properly, in well drained soil, where plants receive adequate sun and good air circulation.
Resistant cultivars:
There are disease resistant cultivars for many species of plants. These are the first line of defense against powdery mildew.
Chemical management:
Since most powdery mildew symptoms occur late in the growing season, it is usually not considered serious enough to justify chemical control. However, some plants may warrant protection and successful chemical control requires applying a fungicide properly and at the right time. Mildews develop rapidly so spray when symptoms first appear.
The pesticide information presented in this publication is current with federal and state regulations. The user is responsible for determining that the intended use is consistent with the label of the product being used. The information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement made by The Morton Arboretum.
For current pesticide recommendations, contact The Morton Arboretum Plant Clinic (630-719-2424 or plantclinic@mortonarb.org).