Lilac (syringa), in its many forms, graces gardens with its classic beauty throughout the United States. However, according to over a decade of data compiled at the Penn State University Plant Disease Clinic, Syringa is vulnerable to several common diseases. Knowing how to spot early signs, or better yet, how to prevent them, will help ensure a healthy, rewarding garden.
Powdery Mildew
Lilacs are most commonly affected by this fungal organism, Microsphaera penicullata, which produces white powdery patches on the leaves. According to Plant Pathology professor Gary W. Moorman, fungal spores tend to form in high humidity but disperse well in low humidity. Therefore, the problem is most evident in spring and late summer, when the daytime brings hot humid conditions and nightfall turns cooler and dryer. This enables the fungus to complete its life cycle in as little as 48 hrs and thus spread quickly and efficiently. Ensuring adequate air circulation around and throughout the plant helps reduce the risk of powdery mildew. Planting in a sunny location and pruning plants before they become overgrown or too dense are common sense preventative measures. The problem is mainly aesthetic, as the leaves may become deformed and drop prematurely. Seldom is damage to the plant's overall health significant.
Bacterial Leaf Blight
Caused by Pseudomonas syringae, this blight turns leaves brown, then black, and can kill off entire shoots. To control the disease from rapidly spreading, prune back the diseased areas, leaving a generous margin of healthy plant tissue. Disinfect pruning tools between cuts with alcohol or a 1:9 bleach to water (10 percent) solution. To reduce the risk of leaf blight avoid overhead watering and do not encourage plant overgrowth with overuse of fertilizers.
Witches' Broom
Phytoplasmas are small bacteria that spread from plant to plant via the insects that feed on those plants. This condition is manifested by broom-like clusters of thinly malformed twigs with distorted leaves. The leaves may also maintain their green hues, forgoing the normal change in color during the fall. Pruning the affected parts of the plant is recommended unless the plant is so diseased it must be removed to reduce the risk of spreading witches' broom.
Shoot and Root Blight
The effects of Phytophthora were most dramatically demonstrated during the famine caused by the Irish potato blight in the 19th century. When your lilac's branch shoots and base roots are black you may have root and shoot blight caused by the fungal organism, Phytophthora. Remove the plant and do not replace it with any other plant susceptible to Phytophthora. Rhododendron and azalea are quite vulnerable to this blight. As with the other fungal problems discussed in this article, avoidance of overhead watering and providing for proper soil drainage will help prevent the problem.
