Lilacs are generally healthy plants, but some diseases and phenomenon that affect other plants can also harm lilacs. Among these are bacterial blight, phytophthora blight, verticillium wilt, freezing and mildew. Watch for the symptoms of these on your lilac bush or tree, and be sure to treat them properly.
Bacterial blight, or lilac blight, is the most serious disease of lilacs, according to John L. Fiala's "Lilacs: The Genus Syringa." Symptoms include discolouration and wilting of the flowers and foliage. Prune out stems using sterilised tools and burn them to prevent the bacterial infection from spreading.
Phytophthora blight is a soil-born fungus that can develop from too much wetness. An early sign can be brown striping along the new buds. Avoid planting lilacs too close to rhododendrons or elderbery, as these are vulnerable to the disease, and can spread it to the lilac.
Although verticillium wilt is not common in lilacs, it can cause the leaves to turn pale and wilt. There is no known treatment for this fungus, so the entire plant should be uprooted and burnt to prevent the disease from spreading.
Freezing and chilling can kill or damage leaves and buds. There is no cure to freezing, so use forethought and plant the bush in a place that is partially sheltered from wind.
This fungus germinates on the leaf's surface, and will appear as a white, powdery substance. Sulphur and water mixtures can be used to kill the mildew.