Characterization of Pseudomonas syringae ssp. savastanoi strains isolated from ash

Isolates of Pseudomonas syringae ssp. savastanoi from ash were examined for their ability to produce phytohormones in culture and for pathogenicity, in comparison with isolates from olive and oleander. Nineteen out of 20 ash isolates produced low levels of indole-3-acetic acid and its methyl ester but no cytokinins. In contrast, the remaining isolate, NCPPB3474, accumulated high levels of auxins and cytokinins in culture, comparable to those of olive and oleander strains. Hybridization of DNA preparations with tryptophan mono-oxygenase (iaaM) and isopentenyl transferase (qt) gene-containing probes showed sequences of DNA homologous to both probes only in isolate NCPPB3474, and in which the iaaM and ipt genes were located on the chromosome and on a plasmid of about 80 kb, respectively. When assayed for pathogenicity on ash, olive and oleander, 19 of the 20 ash isolates caused disease only on ash but NCPPB3474 caused knots on both ash and olive. Oleander isolates infected all three hosts whereas those from olive caused symptoms only on olive and ash. All the cultures were able to multiply in host plant tissues, but the growth rates and final population densities were correlated to the plant species inoculated and the host origin of the isolates. In particular, the highest population densities were reached by isolates capable of causing symptoms on the inoculated host. The phytohormone production shown by ash, olive and oleander isolates of P. syringae ssp. savastanoi was in accordance with the type of symptoms: cankers accompanied by wart-like excrescences on ash and knots on olive and oleander. Furthermore, the pathogenic features of these isolates and, in particular, their growth patterns in the different host tissues, support previous evidence on the existence of three distinct pathovars in P. syringae ssp. savastanoi.