One of the most widely planted bushes in residential landscapes, lilacs briefly display their fragrant showy blossoms every spring. The rest of the year their mid-size bushes provide carefree boundary hedges or lawn specimens. Lilacs can be susceptible to diseases, which can be held in check by good cultural practices. For extra protection from disease, plant lilac bushes far enough apart so that they are not crowded.
Bacterial blight, a common and widespread disease of lilacs, begins as brown spots on the leaves and stems of the bush. The spots eventually enlarge, causing the leaves to become malformed, die and drop off. The spots then move to the stems, which are eventually girdled by the bacteria. Shoots and blossoms die as a result of the stem infection. Clean up debris around lilacs to prevent the spores from entering the shrub through pruning cuts. Prevent bacterial blight by proper fertilizing and watering of lilacs. Water at soil level to keep leaves dry. Prune lilacs yearly to allow air and light into the center of the plant, which will decrease the likelihood of an infection.
Botrytis blight affects many ornamental flowering plants, including the lilac. Initially, it infects the bush through its tender flower petals or buds. If your lilac bushes are heavy with buds that turn dark brown or black and never open, chances are it is infected with botrytis blight. As the infected flower petals and buds drop off the bush, they spread the spores to the leaves and stems. Prune lilac bushes yearly to keep the center of the bush open to light and airflow.
Powdery mildew is a widespread fungal disease that is commonly seen on fruits, vegetables, annual and perennial flowers, trees, and shrubs. It is spread by wind-blown spores which grow best at temperatures between 60 and 95 degrees F. A white powder attaches itself to the leaves, flowers and stems of the affected plants, giving them a pronounced whitish cast. Shrubs and flowering plants that grow in full sun are more resistant to powdery mildew because it prefers shady conditions to become established. To reduce the number of spores that take hold, give individual lilac bushes enough room for air to circulate around them.