Lilacs are ornamental trees, prized for their attractive fragrant blossoms. Although lilac blossoms may look delicate, lilac trees are fairly hardy. They are resistant to a number of pests and susceptible to few diseases, if properly cared for. If you find that your lilac tree continually falls prey to a specific disease, consider planting a variety that is resistant to that disease.
Lilac trees are quite susceptible to the fungus that causes powdery mildew. Affected foliage slowly becomes covered with a white powder-like coating that is easily brushed away. Older leaves at the bottom of the tree are usually affected first, around July. If left untreated, powdery mildew will spread throughout the tree and eventually cause the leaves to yellow and drop before the end of the growing season.
Bacterial blight usually crops up early in the season after a period of hot, humid weather. Affected lilac leaves will look burned on their edges and may also develop dark spots with pale borders. As the disease progresses, young shoots and flowers may blacken and die. Bacterial blight does not kill lilac trees, but it will kill most of their new growth.
Leaf Spot Disease
Leaf spot is an umbrella term that describes a number of similar bacterial infections in lilac trees. The size and color of the spots largely depends on the type of bacteria at work; in general, the spots are brown or black in color. Some are surrounded by dark margins and others are marked by concentric rings. As the spots spread, they form large blotches centered around the veins of the leaves. If the disease is allowed to progress, the leaves will eventually yellow and drop.
Chlorosis can be caused by an iron, zinc or manganese deficiency in lilac trees and is often the result of overly alkaline soils. Lilac trees affected with chlorosis will have light green or yellow leaves with green, healthy looking veins. If left untreated, chlorosis may eventually kill a lilac tree.