ABSTRACT The in vitro expression of the syrB gene that controls the synthesis of syringomycin, a non-host-specific phytotoxin produced by Pseudomonas syringae pv. syringae van Hall, was studied using aqueous extracts derived from bark tissues collected from nitrogen-fertilized and nonfertilized peach trees. Expression of the syrB gene was quantified as beta- galactosidase activity expressed by P. syringae pv. syringae B3AR-132 containing a syrB::lacZ fusion. Gene expression was significantly less in three of four paired comparisons using extracts derived from fertilized versus nonfertilized trees; however, canker lengths were significantly different in only one of four comparisons. Expression was negatively correlated with plant tissue nitrogen content and positively correlated with a plant carbon/nitrogen ratio. Bark tissue from ring nematodeinfested trees had significantly higher concentrations of total soluble phenolic compounds and carbon/nitrogen ratios than bark samples from trees without nematodes, and canker size was significantly greater in trees growing in ring nematode-infested soil compared with noninfested soil. Nitrogen fertilization significantly decreased the plant carbon/nitrogen ratio, which was positively correlated with the concentration of total soluble phenolic compounds. Canker size developing after bacterial inoculation was positively correlated with higher plant carbon/nitrogen ratios and total soluble phenolic compounds. These results support the hypothesis that one reason why nitrogen fertilization decreases host susceptibility to bacterial canker is by either reducing the amount of plant metabolites that can induce syrB gene expression, or producing or increasing the concentration of compounds that antagonize syrB inducing compounds.