It’s the time of year when you’re likely to notice the leaves of your lilac shrubs (Syringa) covered in a powdery, white substance known as powdery mildew. Powdery mildew on lilac is a fungus whose causal organism is Microsphaera penicullata. There are over 1100 separate types of powdery mildew, and each is plant specific. So even if you see powdery mildew on, say, some roses located near your lilac bush, the organism causing the rose mildew is distinct and specific to roses. The roses didn’t acquire the disease from the lilac.
Powdery mildew in Zones 5 and 6 usually makes its appearance in July or August. Suitable conditions for fungus growth include: humid air, high daytime temperatures, overcast skies, lack of airflow between branches and leaves, insufficient sunlight, and over-fertilization. The fungus spores overwinter on affected leaves and are easily spread by water. Fortunately, since most of the lilac’s required photosynthesis has already taken place by the time the powdery substance begins to coat the leaves, the only real damage to the plant is aesthetic. However, some lilacs, such as the Mt. Baker variety shown in the picture, produce colorful fall foliage, so you may want to treat the lilacs to prevent the fungus from taking hold.
There are many fungicides available to control powdery mildew on lilac; however, if you’d like to try a less expensive, safer approach, consider using monthly sprays of a summer weight horticultural oil, such as Bonide® All Seasons Horticultural Spray Oil. Although horticultural oil is generally confined to treating plants for insects rather than disease, researchers at the University of Maryland Cooperative Extension, testing horticultural oils against other fungicides for control of lilac powdery mildew, discovered that horticultural oil was equal to or more effective than other recommend treatments. During the first year of the trial, the researchers sprayed with horticultural oil every two weeks, beginning in mid-July. The second year of the trial, the researchers sprayed with horticultural oil only once a month, and they concluded that monthly sprays offered the same protection as using the spray every other week.
To make a preventative spray for your lilacs, use the horticultural oil in either a 2% or 3% solution. To make a 2% solution, pour 1/3 cup of oil into a spray container, add 1 gallon of water and mix thoroughly before spraying. To make a 3% solution, use 1/2 cup oil to 1 gallon of water and, again, mix thoroughly before spraying. Wait until annual flowering is completely finished before beginning your spray program, and then spray late in the day when no rain is forecast for 24 hours. Also, be sure to rake up all lilac leaves in the fall and burn them, bury them, or send them off to your municipal garden waste collection site. Do not compost infected leaves.
All common lilacs (Syringa vulgaris) are subject to powdery mildew, as are most Syringa x hyacinthiflora varieties. In my experience, the most classically fragrant lilacs are those within these two species. However, there are lilacs of many colors, of many species and varieties, and for many uses, including those specifically bred to be mildew resistant. So if you are looking for appearance more than fragrance, you might want to check out the mildew resistant varieties listed by the Virginia Cooperative Extension here. For more information on lilacs, generally, the University of Minnesota Cooperative Extension has a comprehensive listing here. Finally, for online sources of lilacs, including photos of numerous varieties, take a look at these websites, here and here.
Update, September 1, 2014: Unfortunately, the University of Minnesota Cooperative Extension article is no longer available. An excellent alternative is available here from the University of Wisconsin Cooperative Extension.
Update, September 1, 2015: The Virginia Cooperative Extension listing is no longer available. I will try to find another listing or develop one from available data.