Phytopathogenic pseudomonads were placed into four major groups on the basis of nutritional and physiological characteristics. Group I consists of 86 strains of phytopathogens distinguishable from other fluorescent pseudomonads by low growth rates, ability to induce hypersensitivity on tobacco, absence of arginine dihydrolase, and relatively limited ranges of carbon sources. Most of these strains cannot utilize benzoate, 2-ketogluconate, spermine, β-alanine, l-isoleucine, l-valine, and l-lysine. Most of the organisms in group I clustered into a small number of subgroups, each of which generally corresponded to a previously recognized nomenspecies. These subgroups differ with respect to the number of substrates used. As a rule, the organisms that utilize the fewest substrates have the most limited host ranges. The fluorescent pseudomonads of group II are arginine dihydrolase-positive and utilize a considerably larger number of carbon sources. Most pathogens of group II are similar to Pseudomonas fluorescens biotype A. Groups III and IV consist of nonfluorescent pseudomonads. These two groups can be distinguished by the number of carbon sources used and by pigmentation. An amended description of the flurescent pseudomonads and their internal subdivision is presented.
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