The bacterium Pseudomonas syringae, is an opportunistic pathogen that can attack a wide variety of woody plants especially when they are damaged by frost or injured in some way.

While there are a number of strains or pathovars (pv.) of this organism, pv. syringae is most important on woody plants. Pseudomonas syringae pv. syringae occurs on nursery and landscape plants throughout the U.S. and much of the world and can cause shoot and flower blights, cankers, and diebacks.

Symptoms

• Common, Persian, Chinese, and Japanese lilac leaves develop round to irregularly shaped brown spots with yellow halos. These spots enlarge and blight entire leaves. Succulent twigs are girdled, killed, and turn black.
• Red, Norway, and Japanese maple leaves can be spotted, their veins blackened and branch tips killed.
• Other plants known to be susceptible include apple, pear, cherry, plum, basswood, saucer magnolia, poplar, dogwood, golden-chain tree, forsythia, and some herbaceous plants. In nurseries, even Monterey pine seedlings have been killed by this bacterium.

Sources of the bacteria

• On or in bud and twig tissue
• In cankers formed the previous season
• In or on grasses and herbaceous weeds

Effects of the bacteria

Pseudomonas invades damaged tissue and produces a toxin that kills surrounding cells where the bacteria can then multiply. Also when it is present on plants, it produces a protein around which ice crystals form. As the ice crystals enlarge, they pierce and severely damage the plant cells. These damaged cells are then colonized by Pseudomonas.

Spread of the bacteria

• Wind-driven rain
• Insects
• Use of infected budwood and nursery stock
• On pruning tools

In aerosols of plant debris, sap, and water created as bacteria-harboring herbaceous weeds or crops (such as alfalfa) are cut (weed-eaters, rotary mowers, harvesters)

Management

• Do not use infected plants as stock or sources of budwood.
• Avoid planting susceptible species in frost-prone areas.
• Avoid fertilization practices that result in very succulent growth in the early spring or in the fall.
• Prune in the winter or very early spring.
• Disinfect pruning shears between plants.

Resistance

Edith Cavell, Glory, and Pink Elizabeth common lilac varieties are less susceptible than many other cultivars. Syringae josikaea, S. komarowii, S. microphylla, S. pekinensis, and S. reflexa have greater resistance than common lilac (Syringa vulgaris).

Fall sprays of certain bactericides are reported to reduce bacteria populations. Great care must be exercised if copper sprays are used in the spring because young tissue is easily damaged by copper. *Some populations of Pseudomonas syringae are resistant to copper. In some plants, the application of chemicals that induce a resistance response in the plants that helps protect plants from some pathogens.

References Used In Preparing This Fact Sheet

• Jones, R. K. and Benson, D. M., eds. 2001. Diseases of Woody Ornamentals and Trees in Nurseries.. APS Press, St. Paul, MN. 482 pp.
• Sinclair, W. A., Lyon, H. H., and Johnson, W. T. 1987. Diseases of trees and shrubs. Cornell University Press, Ithaca, NY. 574 pp.

Prepared by Gary W. Moorman, Professor of Plant Pathology