Answer: It sounds to me like bacterial blight, a common problem in lilacs in your growing region. Bacterial blight of lilac is caused by the bacterium Pseudomonas syringae pv. syringae. The disease is also commonly referred to as Shoot Blight and Blossom Blight. The pathogen is capable of causing damage to all types of lilacs including Japanese, Chinese, Persian and common varieties. Some researchers suggest that white flowering varieties may be more susceptible to infection than other varieties. The disease is usually associated with plants that have been stressed by drought conditions, improper fertilization, and/or have been wounded.
Infections result in the appearance of brown spots on the leaves and stems on the plants. The spots may enlarge and cause malformations of the leaf tissue. Leaves may die and drop from the stems. The symptoms may move from the leaves to the stems as the disease progresses, turning the tissue black and causing it to wilt. The stems infection causes girdling of the tissue resulting in the death of shoots and blossoms.
Infected branches should be pruned 20 to 25 cm (10-12 inches) below the visible infection. Pruning should be done during dry weather to minimize the chance of spreading the pathogen. Pruned branches should be destroyed or discarded. Always sterilize pruning tools between cuts to prevent spreading the bacterium to other areas on the tree.
To prevent further outbreaks, take the proper actions required to keep the plant as healthy as possible. Practice proper fertilization and water management. Stresses caused by the lack of nutrients and/or water can predispose the plants to an infection. Avoid wetting the foliage and overhead irrigation to minimize splashing of the bacterium on to the host plants. Prune plants to allow for increased air circulation through the canopy. Also proper spacing of plants is recommended.
If there are holes in the bark, there may be beetle damage, and the pests will need to be identified before treatment strategies can be recommended.
It sounds as though your lilac may be too far gone to try to save - dealing with both diseases and insect pests can be difficult. If you decide to replace your lilac, try to choose one with some resistance to bacterial blight. Here are a few suggestions:
Syringae oblata var. dilatata 'Cheyenne', S. vulgaris 'Edith Cavelle', 'Fr. John L. Fiala', 'General Sheridan','Katherine Havenmayer', 'Krasavitsa Moskvy', 'Montaigne', 'Nadezhda', and 'President Grevy', S. chinensis 'Red Rothamagensis' and 'Saugeana' and S. meyeri. Some varieties that show poor resistance characteristics include S. hyacinthiflora 'Annabel', S. vulgaris 'Agincourt Beauty', 'Bridal Memories', 'Burgundy Queen', 'California Rose', 'Charles Joly', 'Charm', 'Edward Boissier', 'Edward Gardner', 'Etna', 'Firmament', 'Lavendar Lady', 'Little Boy Blue', 'Miss Ellen Willmott', 'Monge', 'Olimpiada Kolesnilova', 'Paul Thirion', 'Royal Purple', Ruhm van Horstenstein', 'Wonderblue', and Yankee Doodle'.
Best wishes with your landscape!
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