A long rainy spring, such as many of us experienced this year, creates the perfect conditions for lilac blight. There are two types of lilac blights, a fungal variety, Ascochyta syringae, and a bacterial variety, Pseudomonas syringae p.v. syringae (the p.v. stands for pathogen variety and indicates a subspecies). Both varieties affect new growth on lilacs, and, since the symptoms can appear somewhat similar, we’ll look at each blight separately.
Ascochyta is a disease that usually affects grains and grasses; it causes large, wheat-colored patches of dead grass on bluegrass, for example. The fungus can also appear as leaf spots on various trees, including ash, boxelder, dogwood, and walnut, among others. Before being renamed Phoma clematidina, Ascochyta clematidina was the name of the pathogen responsible for clematis wilt. So Ascochyta is a well-distributed plant pest.
On lilac, Ascochyta is sufficiently severe to cause a blight, in which the leaf spots coalesce into blotches. Eventually the blight girdles new twigs and kills the leaves. In the spring, look for symptoms on new growth, including a brown or black shepherd’s crook. This is one of the confusing symptoms, since shepherd’s crooks are more commonly seen with bacterial diseases, such as fireblight on crabapples.
Another symptom of the fungus is leaf spots. Generally, Ascochyta will have a tan center with a different colored margin of decaying tissue—typically a darker brown—with indistinct, rounded edges. Bacterial leaf spots are usually more angular in structure.
Fungal diseases are spread by splashing water, wind, insects, gardening tools, and touching affected plants while they are wet. Natural disease control methods include proper plant spacing and pruning to allow rapid drying of foliage, pruning out infected branches (use an antibacterial spray on secateurs between cuts), avoiding high nitrogen fertilizers, controlling powdery mildew, raking up and disposing of all dead leaves, and avoiding overhead watering. There are also systemic fungicides that can help bring Ascochyta syringae under control while pursuing the non-chemical approaches to improved lilac health. As with any chemical control, always read and carefully follow instructions on the label.
Pseudomonas syringae p.v. syringae
The more commonly seen lilac blight is the bacterial blight Pseudomonas syringae p.v. syringae. Symptoms of bacterial blight also include blotchy brown patches on leaves, but the edges of the damage will appear more angular and may be surrounded by a thin, yellow halo. Twigs will also appear dark brown and dead, similar to the symptoms of fireblight, but, instead of forming a shepherd’s crook, the twig appears broken.
Windy, rainy weather just prior to flowering favors development of Pseudomonas. The disease can be spread by splashing water, wind-driven rains, insects, birds, humans handling plants, and tools which haven’t been disinfected. Unlike many fungi, bacteria reproduce most effectively as parasites of the plant host rather than in plant litter or in the soil.
The best method of control is to plant lilac varieties that are resistant to bacterial blight. Dr. Leonard Perry, a noted horticulturist at the University of Vermont (I recommend his excellent Perry’s Perennial Pages for plant specific information) has put together an extensive list of lilac trials in Vermont for susceptibility to blight. Although the common wisdom with lilacs is that white-flowered varieties are more prone to blight, Professor Perry’s list contains several white-flowered varieties with minimal bacterial blight issues.
If you already have a favorite lilac that you want to keep blight-free, follow the same cultural procedures for lilac health mentioned under Ascochyta Blight. To control existing bacterial blight, use an antibacterial spray on your secateurs and cut away the affected twig at least 6” (15 cm) below the damaged area. Repeat spraying the secateurs between each cut. For severe problems, or if you live in an area with especially wet spring weather, you may wish to purchase a fungicide with copper sulfate, using it before new growth begins each year, up to flowering, and after flowering until new growth ceases. Carefully follow all label directions.