Diversity Among Pseudomonas syringae Strains from Belgian Orchards

Abstract

Conventional identification tests and rapid tests based on phytotoxin and pyoverdin production were used to determine the diversity in 170 fluorescent and non-fluorescent oxidase-negative strains isolated from diseased plant material in pear, sweet cherry, sour cherry and plum orchards in Belgium. Determination of phytotoxin production was based on a biological test for the detection of toxic lipodepsipeptide production and PCR tests to detect the syrB, syrD and cfl genes. The pyoverdin tests were visual, spectrophotometrical and isoelectric focalisation tests; they detected the Fe(III)-chelated atypical pyoverdin produced by Pseudomonas syringae. The P. s. pv. syringae and P. s. pv. morsprunorum were almost exclusively detected in pear and sweet cherry orchards and the phytotoxin tests proved useful in identifying these pathovars, although caution was needed regarding negative results because a few strains were unable to produce phytotoxins. In contrast, many plum and sour cherry strains could not be affiliated to a pathovar. All the non-toxin-producing strains investigated in this study were fluorescent. The pyoverdin tests classified them in the siderovar of P. syringae. The results therefore underscored the great variation in the P. syringae strains isolated from plum and sour cherry orchards. The pyoverdin tests were particularly useful in identifying the non-toxin-producers and they showed a real potential as general identification tools of fluorescent P. syringae strains, but they were less discriminatory than the phytotoxin tests. Also, the pyoverdin tests could not identify non-fluorescent strains comparable to 34 of the 45 P. s. pv. morsprunorum strains in this study.