Bacterial blight, pseudomonas syringae, pv. syringae, attacks shrubs, trees, and tree fruit. The more susceptible lilacs are Chinese, Japanese, Persian, and common lilacs, especially the white flowered varieties. This pathogen can cause problems on many other plants, including stone fruits, peach, forsythia, mock orange, and pear. The pathogen causes leaf spots, blossom blast, shoot blight and canker diseases.
Symptoms
On leaves, the bacterial spots will be brown with a yellowish halo. Spots may merge and cause a blighted dark brown to black appearance and die quickly, giving the plant the appearance of a fire blight infection. Stems may become infected through the leaves or directly through the bark. Young shoots may have black stripes or a one-sided effect because only part of the shoot is infected. Black lesions result on infected stems. Flowers turn brown and limp, whereas flower buds turn black. Flower buds may be infected before they open. Flower buds may then die (bud blast). Succulent new growth may be girdled causing the tips to droop and die. The infected area is usually black while the tissue above turns brown as it withers and dies. The disease tends to attack the new succulent growth. More mature growth is more resistant. Plants infected with disease tend to be more prone to winter injury.
Life Cycle
As with most bacterial diseases, this one is prevalent in rainy seasons or with frequent overhead irrigation while tissues are succulent. The bacteria overwinter in infected host tissue and invade succulent tissues or wounds any time cool, wet conditions prevail. Bacteria need an injury (another disease, insect feeding, pruning cut, and so on) or a natural opening (stomata, lenticels, pollen tube) to get in.
Management
Remove infected shoots several inches below visible cankers. Prune when dry, and disinfect pruners between cuts. Prune lilacs and surrounding plants to allow good air flow within and around the plants. Rapid drying of the plants will discourage infection. Avoid overhead irrigation, especially in spring. Because succulent tissue is more susceptible to infection, use balanced but not excessive (especially nitrogen) rates of fertilizer. This will reduce the amount of succulent tissue. Copper fungicides may provide some benefit in disease prevention if used two or three times, starting when new growth appears. Spray every seven to ten days when conditions are favorable for infection.