Powdery mildews are caused by over 1,000 species of fungi that affect most landscape plants. Most 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 control can prevent severe damage to plants.
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 fail to open, leaves can become distorted, turn yellow, brown, or show chlorosis, or they may drop prematurely. Fruits develop blemishes or abort early.
Powdery mildew grows primarily on leaf surfaces and does not require water to infect the plant. Powdery mildew fungi overwinter in tiny black bodies, known as fungal threads, which are located in leaf litter, stems, and dormant buds. In spring, the fungal threads 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 80oF). 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.
Some of the more susceptible trees and shrubs to exhibit powdery mildew symptoms include alder (Alnus), azalea (Rhododendron), birch (Betula), bittersweet (Elastrus), catalpa (Catalpa), a few crabapple cultivars (Malus), dogwood (Cornus), elm (Ulmus), euonymus (Euonymus), holly (Ilex), lilac (Syringa), magnolia (Magnolia), oak (Quercus), privet (Ligustrum), and viburnum (Viburnum). Powdery mildews are also common on certain herbaceous plants, such as chrysanthemums, dahlias, delphiniums, phlox, snapdragons, and zinnas.
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
- Purchase disease resistant plants
- Plant and space properly in well drained soil where plants receive all day sun and good air circulation
- Avoid working among plants with wet foliage
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. Check labels to be sure the plants you want to treat are included. Sulfurcontaining products can burn some foliage.
Refer to the Illinois Urban Pest Management Handbook (University of Illinois Cooperative Extension Service) for a complete listing of chemical recommendations. Use pesticides safely and wisely; read and follow label directions
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.