Evaluation of Pathogenicity and of Cultural and Biochemical Tests for Identification of Pseudomonas syringae Pathovars syringae, morsprunorum and persicae from Fruit Trees

Comparative studies were based on 216 isolates of the Pseudomonas syringae pathovars syringae, morsprunorum and persicae , mainly originating from fruit trees in several European countries, but also from USA, USSR, South Africa, New Zealand and Turkey. All the identified morsprunorum isolates were obtained from stonefruit trees, which were also a source of some syringae isolates. Fluorescent strains HR negative on tobacco leaves were regarded as saprophytic pseudomonads. Nearly all the HR positive but none of the HR negative isolates caused lesions on sweet cherry shoots and fruitlets. The largest lesions on sweet cherry shoots were caused by the morsprunorum isolates. Syringae strains always caused lesions on pear fruitlets, whereas the morsprunorum strains never did so. Some degree of specialization within the pathovar syringae seemed to exist. Cultural and biochemical tests well suited to differentiate the three pathovars were as follows: colour after growth in sucrose nutrient broth, liquefaction of gelatin, activity of β‐glucosidase (arbutin) and tyrosinase, use of L‐leucine, L(+)tartrate and DL‐lactate as sole carbon source, fluorescent pigment production, crystaline inclusions in the nutrient agar medium, and growth rate in 0.2% yeast extract nutrient broth. Tests for longevity on nutrient agar with 5% sucrose turned out to be fairly unreliable. A tube assay gave the most consistent results when studying utilization of organic substrates. The capability for gelatin liquefaction correlated with virulence, and mucoid isolates showed a tendency for higher aggressiveness than rough colony variants. Page of protein extracts obtained from freeze‐pressed bacterial cells revealed that the pathovars syringae and morsprunorum could be differentiated due to specific isoenzyme patterns for esterases and acid phosphatases. It was concluded that several pathogenicity, cultural and biochemical tests allowed an unequivocal differentiation of the Pseudomonas syringae pvs. syringae and morsprunorum. Some of the discriminating biochemical characters may, play a role during pathogenesis.