Pseudomonas syringae: A Common Bacterial Pathogen on Woody Plants

The bacterium Pseudomonas syringae is an opportunistic pathogen that attacks a wide variety of woody plants, especially when they are damaged by frost or mechanical injury. Among its many pathovars, P. syringae pv. syringae is the most important on woody ornamentals, occurring in nurseries and landscapes across the U.S. and worldwide. It causes shoot and flower blights, cankers, and diebacks.

Symptoms

• Lilac: Round to irregular brown spots with yellow halos on leaves; spots enlarge and blight entire leaves. Succulent twigs are girdled, killed, and turn black.
• Maple (Red, Norway, Japanese): Leaf spots, blackened veins, and branch tip dieback.
• Other susceptible plants: 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.

Sources of the Bacteria

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

How the Bacterium Damages Plants

Pseudomonas invades damaged tissue and produces a toxin that kills surrounding cells, allowing the bacteria to multiply. Additionally, when present on plant surfaces, it produces an ice-nucleation protein that promotes ice crystal formation. As ice crystals enlarge, they pierce and severely damage plant cells, which are then colonized by the bacterium.

Spread

• Wind-driven rain
• Insects
• Infected budwood and nursery stock
• Contaminated pruning tools
• Aerosols from cutting bacteria-harboring herbaceous weeds or crops (e.g., alfalfa) with weed-eaters, rotary mowers, or harvesters

Management

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

Resistant Lilac Varieties

Common lilac cultivars with reduced susceptibility: ‘Edith Cavell’, ‘Glory’, ‘Pink Elizabeth’. Species with greater resistance than Syringa vulgaris: S. josikaea, S. komarowii, S. microphylla, S. pekinensis, S. reflexa.

Chemical Control Notes

Fall applications of certain bactericides can reduce bacterial populations. Use copper sprays in spring with extreme caution—young tissue is easily damaged, and some P. syringae populations are copper-resistant. Some plant defense activators (chemicals that induce systemic resistance) may offer protection.

References

• 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