Michelle Grabowski, UMN Extension Educator
Recent wet weather has favored the growth of a bacterial pathogen of lilac called, Pseudomonas syringae pv. syringae. This bacterial pathogen can infect shoots, young twigs, leaves and occasionally flowers. Dark brown to black spots can be seen on infected leaves. The spots are often surrounded by a pale yellow halo. With age, the center of the leaf spot often falls out, resulting in a shot hole appearance to the leaves. Often several spots grow together into large irregular black blotches on the leaves. Even more dramatic is infection of shoots and young stems. Sunken black lesions can be seen on green stems. If the infection encircles the stem, all of the leaves beyond the infection, turn black and wither. This often results in 6-8 inches of blackened withered leaves.
Although most gardeners are willing to tolerate a few small leaf spots, many blackened withered shoots inspire gardeners to look for solutions. Sanitation is the first step in clearing up this bacterial disease. Choose a cool dry day and prune out infected shoots. Be sure to closely examine the branch before making a cut. Often oblong sunken black lesions can be seen on the stem just below the severely infected shoot. It is important to make the cut low enough on the branch to remove these infections as well. Severely infected leaves can be pinched off if desired but remember to never remove more than 1/3 of a plants leaves. Sterilize pruners between cuts and when the job is done with a 10% bleach solution, Listerine or Lysol. Bacteria are sticky and can be transferred to healthy plants on pruners used to clean up an infection unless the tools are properly cleaned.
Bacterial pathogens thrive in moist conditions. The next step in reducing damage from bacterial blight of lilac is to reduce moisture in the plant canopy. Reposition nearby lawn sprinklers or irrigation systems so that water does not spray onto the lilac leaves. If the plant is dense, prune out several branches to allow better air movement through the bush. This will help the leaves dry quickly after rain and dew. If the bush is crowded by nearby plants, remove weeds and relocate overgrown perennials or other plants to improve air circulation around the lilac.
Often these two simple steps combined with warm dry summer weather, reduce bacterial blight of lilac to the point where only a few small leaf spots remain.