Stone fruit and nut trees "stressed" as a result of feeding by ring nematode are more likely to be affected by Pseudomonas syringae than nonstressed trees.
Symptoms of bacterial canker are typically most evident in the spring in the form of dead or dying limbs and lesions oozing dark colored sap. Wounds or lesions oozing clear sap are more likely caused by insects or physical abrasions than by bacterial canker.
This disease may occur throughout an orchard or be concentrated in pockets called "bacterial canker holes". The seriousness of the disease can vary from the death of a few small twigs to the death of entire trees. In some instances, trees replanted in the same location will die year after year. Farm Advisors have developed a numerical rating scale to score the seriousness of the affliction for experimental purposes.
Although this disease has been studied extensively by nematologists and plant pathologists, there is still much that is not understood. Other stresses can also result in serious problems. For example, in a recent experiment in Winters, CA studying the effects of various levels of fertilization, a correlation has developed between low levels of fertilization and the development of bacterial canker in an orchard not affected by ring nematode.
This is a disease which appears to be increasing in severity in California since the loss of use of DBCP (dibromochloropropene) as a nematicide approximately 15 years ago.