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Pseudomonas Syringae: A Common Pathogen on Woody Plants

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 of this organism, syringae is most important on woody plants.


  • Common, Persain, 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
  • 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)


  • 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
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