Issue 4, May 14, 2010
Japanese Tree Lilac Diseases
Japanese tree lilac (Syringa reticulata) has become a very popular plant in Illinois. I am not going to try changing that concept. Questions merely prompt me to write this article about the disease problems one might see on this host. The good news is that so far the disease problems seem few and sparse. This is a lilac, so we should see the same diseases we see on other lilacs. I think the difference is that the tree growth form allows better air movement among and around branches.
As an example, powdery mildew is a common problem on shrub lilacs. It occurs on tree lilac, but only where the plant is shaded. Since the recommendation is to grow it in full sun, and since the tree form dries more quickly, powdery mildew is less of a problem. Interestingly, the powdery mildew species that affects lilac also infects ash and privet. You may see more disease near those hosts.
Bacterial blight is probably the most serious disease on tree lilacs, and is most severe on the white flowering types. The bacterium, Pseudomonas syringae pv. syringae infects in wet weather when new shoots are developing. The same bacterium may cause shoot blight on blueberry, cherry, maple, and pear. Symptoms include brown/black spots on leaves, shoots, petioles, buds, and flowers. Stems may be girdled and die beyond the point of infection. Leaves may be crinkled along the edge or along the midvein. You will want to work with such plants only in dry conditions. Remove infected sections, disinfecting pruners between cuts. Prune plants to allow good air movement (rapid drying) and avoid high nitrogen fertilization that promotes infection. Infection is often worst following storms with high winds. Copper fungicides may help reduce spread of this bacterial pathogen.
Crown gall and Verticillium wilt can also infect tree lilacs, but we have not seen these diseases at the Plant Clinic to date. Balanced fertility is promoted as a means of avoiding Verticillium wilt on this species. Crown gall may infect injured tissue, so mulch around the base of plants to keep mowers and trimmers at bay.--Nancy Pataky