Bacterial isolates from blast lesions and stem cankers on various species of citrus trees were characterized as Pseudomonas syringae and were pathogenic when inoculated into petioles and axial buds of healthy Washington navel orange trees in both orchard and greenhouse tests. With one exception, B-455, the citrus isolates were host-specific. Inoculations of the citrus isolates into young Lovell peach stems and maize leaves caused no disease symptoms in contrast to the stone fruit ecotype, B-3A, which caused stem cankers on peach and the grass ecotype, 5D430, which caused holcus spot symptoms on maize. The citrus isolates produced variable patterns in their production of bacteriocins and sensitivities to bacteriophages; however, the production of syringtoxin (ST), a wide spectrum biocide and Phytotoxin produced by these isolates, was unique to the citrus ecotype of P. syringae. Healthy petioles of navel orange trees when treated with ST developed symptoms typical of citrus blast lesions. The biocidal spectrum and amino acid composition of toxin preparations from cultures of the citrus isolates, when compared to known ST and syringomycin, established the identity of the Phytotoxin from citrus isolates as ST.