Powdery Mildew of Lilac

Photo and Narrative by Curtis. Swift, Ph.D., Area Extension Agent (Horticulture)

Photograph of Lilac with Powdery Mildew

The Above photograph of a Powdery Mildew infected lilac shows patches of gray-white material which resembles powder or dust on top of the leaves.
Green tissue of non-infected tissue can still be seen.

Introduction:

Powdery milew is caused by fungi belonging to several genera of the family Erysiphaceae. Many occur on hardwoods as well as flowers and grasses. Gymnosperms (i.e. spruce, pine, juniper, etc.) are not attacked. Most of these fungi are host specific attacking specific plants and in some cases specific cultivars of plants.

Some of the trees and shrubs commonly infected with powdery mildews include:
african violet alder apple ash
beech begonia birch black locust
bluegrass boxwood buckeye catalpa
cherry chrysanthemum cosmos crabapple
dahlia delphinium elm euonymus
hawthorn honeylocust honeysuckle horse chestnut
hydrangea kalanchoe lilac linden
maple mesquite oak pear
phlox planetree plum poplar
rose snapdragon sweetpea sycamore
tuliptree Virginia creeper walnut willow
zinnia

Powdery mildew is more prevalent from mid to late summer when cool evenings follow warm days. The patches of fungus are more abundant on lower leaves and leaves that are in the shade. Heavily infected foliage may be discolored, distorted or stunted. Severe infections can severly limit the plants ability to photosynthesize materials necessary for the health of the plant.

Control:

During the fall small brown to black bodies (cleistothecia) will form on the underside of the leaves. These often appear to be droppings of insects or specks of plant debris. The cleistothecia are the major overwintering (survival) stage of this fungus in cooler regions. Removing fallen leaves from around infected plants prior to spring may help reduce infections the following year. Powdery mildew also overwinters under the bud scales of the terminal buds on some woody plants. These buds will be unthrifty appearing and less plump than non-infected buds. such buds should be noted and removed when pruning woody plants.

Many powdery mildew fungi in warm regions do not produce cleistothecia but survive from one year to the next as mycelium on stems and leaves and in buds of perennial plants.

Fungicides are also available for the control of powdery mildew. The Cooperative Extension office in your state should also be able to provide you information on grape production and insect/disease control options.

References Used: Additional References

Boyce,J.S. 1961. Forest Pathology. McGraw-Hill Book Company, Inc.
Forsberg, J.L. 1975. Diseases of Ornamental Plants. University of Illinois Press.
Sinclair, W.A., H.H. Lyon & W.T. Johnson. 1987. Diseases of Trees and Shrubs. Cornell University Press.
Tattar, T.A. 1978. Diseases of Shade Trees. Academic Press, Inc.

July 31, 1996
Updated on June 18, 2009