The Japanese tree lilac is a unique, asymmetrical tree that, while a lilac, is shaped far more like a conventional arboretum inhabitant than like a climbing vine. This showy, brightly flowered tree is a popular landscape and street tree in many areas of the United States, although it is not native to North America. While this tree will grow in poor, clay and alkaline soils, it is not impervious to diseases and must be closely monitored in order to prevent infections and keep the plant healthy and "happy."
Leaf spot on Japanese tree lilacs is caused by the fungus pseudocercospora. When humidity is high and temperatures are around 70 degrees Fahrenheit, look out for this infection, which creates gray, brown and black splotches on the leaves of the tree and eventually leads to serious defoliation if left unchecked. Sterile pruning is essential to controlling this disease, and you must be sure to destroy all infected clippings, as the spores of this fungus can survive in leaf debris for up to two years. You can treat the fungus with fungicides, but as of 2010 none have been specifically approved for Japanese tree lilacs.
Borers and Miners
Japanese tree lilacs are as vulnerable as any other type to lilac borers and lilac miners, insects that tunnel in the leaves and branches of these trees. Borers cause serious wilting and branch breakage, and they thrive in drought conditions. Miners eat the interior of the leaf, usually along the pattern of the veins, and then emerge from the leaf as caterpillars that will finish off your foliage. Both can be controlled in the early stages of infection by "hand picking" and the pruning of infected limbs and branches. However, should the infection progress, you may have to remove the tree to prevent the spread of the problem.
Tree scale in Japanese tree lilacs is caused by scale insects, which are hard to spot because, until they burrow in, each is only about the size of a flake of black pepper. Leaf wilting and general droopiness often indicate a scale problem, as does the appearance of mold or mildew on the plant. You should treat your tree with mineral oils if you suspect a problem with scale insects, or you can use a stronger chemical pesticide if necessary.