White Mold on a Lilac

Signs of Infection

• Patches of white to a grayish tinge will appear on the lilac plant, usually on the leaves. Buds, flowers and young stems may also be infected with this fungus. The overwintering bodies of the fungus, known as cleistothecia, show up on the plant as pinhead-sized, ball-shaped fruiting structures that shift in color from white to black.

Favorable Conditions

• The conditions in which the fungus thrives are dependent on the condition and age of the lilac plant and on weather conditions during the local growing season. Powdery mildew exists in warm, dry climates and needs high relative humidity only for the germination of spores. Lilacs in shaded, damp areas where air circulation is low offer ideal conditions for the infestation of powdery mold. A relative humidity as high as 90 percent will increase the infestation.

Fungi Characteristics

• Species of fungus are host specific, with Microsphaea alni affecting lilacs in addition to oak, catalpa and elm. Powdery mildew produces fungal threads known as mycelia that do not invade plant tissues but only grow on the plant surface. The fungus send out haustoria, rootlike structures, with which to feed. The haustoria overwinters on plant debris; in the spring, spores reproduce and reach susceptible hosts via the wind, splashing raindrops or insects.

Control

• Attempt natural means of controlling the fungus on the lilac before resorting to chemical fungicides. Remove and destroy damaged or dying lilac branches by pruning off the injured sections, or even the entire plant if the infestation is severe. Always use well-drained soil preparations and avoid damp, shady areas. If all else fails, try a fungicide containing the active ingredients Bacillus subtilis, myclobutanil or neem oil in a spray form.