Lilacs (Syringa spp.), including common lilac (Syringa vulgaris), don't need much care but may have problems with bacterial and fungal diseases for which you can't do much to remedy other than to remove infected branches. Plant resistant varieties in your garden and provide good care to avoid disease problems. Lilacs grow 8 to 15 feet tall and 8 to 12 feet wide and generally grow in U.S. Department of Agriculture plant hardiness zones 3 through 7, depending on the species.
Lilac Bacterial Blight
Lilac bacterial blight affects all lilac species, although white-flowered lilacs are more vulnerable. Over-fertilizing young lilacs and fertilizing late in the growing season make lilacs more susceptible. The disease strikes in wet spring weather, affecting plants suffering from drought stress. Insects, unsterilized pruning tools, splashing rain and wind all spread the disease. Brown spots first appear on the leaves, causing them to drop. The disease moves to the stems, which turn black and wilt, killing shoots and blossoms. Prune off and destroy infected branches during dry weather. Make your cut approximately 12 inches below the infection. Dip your pruning shears in a solution of 1 part bleach to 9 parts water between cuts. To avoid the disease, plant only blight-resistant varieties like Syringae oblata var. dilatata "Cheyenne," hardy in USDA zones 4 through 7.
Other Bacterial Diseases
Dense clusters of short, thin twigs growing from one part of a lilac stem is a sign of witches’ broom, a bacterial disease. The “broom” twigs in these clusters stand straight upright and grow small, yellow distorted leaves. Dig out and destroy severely infected lilacs. If you've been watering your lilac with a sprinkler and you find stems turning black at the base and progressing up shoots growing up to 4 or 5 feet, your lilac may be suffering from shoot blight. Remove and destroy infected lilacs and replace them with a resistant cultivar.
Ascochyta blight causes new shoots and flower stalks to wilt and turn brown, encircled by dead areas that have gray to tan, shriveled bases. Pimplelike dark gray fungal spots dot the dead areas in wet weather. Round, olive green spots with definite edges form on the leaves. These spots turn tan in summer and fall. To avoid Ascochyta blight, do not use sprinklers. When you spot the disease, prune the infected parts, sterilizing your pruning tools between cuts. Powdery mildew leaves a whitish-gray film on lilac leaves, typically in late summer or fall. To discourage this disease, plant lilacs in full sun in areas where they can get fresh air. If your lilac plant is a thick tangle of branches, remove some branches for better air flow. Powdery mildew rarely causes permanent damage to lilacs, but you can avoid it by planting resistant cultivars.
To prevent bacterial and fungal diseases, spray lilacs with a Bordeaux mixture two to three times in spring, spacing treatments seven to 10 days apart. For a backpack sprayer with an agitator, mix 10 tablespoons of fresh, dry hydrated lime and 3 1/2 tablespoons of copper sulfate in 1 gallon of water and pour it into the tank. Other types of sprayers have different rates and mixing methods. Cover the lilac thoroughly with the spray. Wear protective eyewear, long sleeves, long pants, sturdy shoes and socks whenever you're working with garden chemicals. Use fresh slaked or hydrated lime for the mixture. You can buy both chemicals at specialist nurseries. Avoid bactericides, which generally don't work on lilac diseases and may kill the lilacs.