SUMMARY: A study of phytopathogenic pseudomonads was begun, but it was found that they could not easily be differentiated from the commonly occurring soil- and water-inhabiting fluorescent pseudomonads. A collection of 169 isolates, including 24 named cultures from various collections, was studied. Both old and new kinds of diagnostic characters, cytological, physiological and biochemical, were investigated under standard conditions. Recordings were made at frequent intervals during a 28-day incubation period. Each character was investigated at least three times during a period of at least five years in order to assess its stability.

A clear differentiation between (NCTC ‘7810) and P. icthyosmia (NCTC 8049) and the rest of the collection was apparent; these two isolates belong to the genus A m m a s Kluyver & van Niel. The use of the vibriostatic pteridine derivative 0/129 revealed a close relationship between them and PL 1.

Of the 169 isolates (including at least 20 named species) 165 had 26 common definable and stable characters; these are offered for an extended definition of the genus . A further 43 characteristics were not common to all 165 isolates. No two of these characters were correlated. One isolate had 42/43 positive characters and one had only 4, and there was a very great range of variation. Among 58 selected isolates there were 21 small groups of identical isolates, but the largest number in any group was 5; the remaining 107 isolates each had a different combination of the 43 characters. Even 10 fluorescent isolates, originally obtained from a single plating of 1 loopful of river water, showed 9 different combinations of characters.

A small group of 5 isolates appeared to conform with as defined by Haynes (1951) and Gaby (1955) but many fluorescens-type isolates were closely related when considered on the basis of all the characters investigated. Several of the named phytophathogens showed a close relationship, or apparent identity, with soil and water isolates; many phytophathogens, however, had a conspicuous number of negative characters, suggesting loss of adaptability in a more selective environment.

The 134 soil- and water-type isolates had 11 positive characters, and 9 others almost always positive. On this basis, and adopting the view that all these characters merit equal emphasis (see Tanner, 1918; Sneath, 1957a, b) it is thought that further divisions into subgroups (species?) are not justified, and that these characters should be used to describe accurately P. Migula 1894. A definition of this species is suggested.


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