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Diseases of fruit trees caused by pathovars of Pseudomonas syringae van Hall are of major concern in fruitproducing areas worldwide, are exceedingly difficult to control, and result in significant economic losses. The pathogen has the ability to kill both young and older trees. Systemic infection and death of young trees is a perennial problem in nurseries, and canker development leading to the girdling and death of scaffold limbs and entire trees is a common event that can lead to the rapid demise of older orchards. For example, bacterial canker of plum caused by P. syringae pv. syringae causes annual tree mortality rates as high as 30% in Germany (43), and bacterial canker of hazelnut has resulted in the mortality of large numbers of trees in Italy and other European countries (77). Yield losses due to lesions on fruit are typically more sporadic in occurrence and variety dependent, but significant yearly losses have …