Book excerpt: In a recent survey, it was observed that many woody plant species grown in Pacific Northwest nurseries exhibited disease symptoms typical of a bacterial infection and Pseudomonas syringae was commonly isolated from these tissues. The distribution of the ice nucleation phenotype among P. syringae strains recovered from these infected woody hosts was examined. More than eighty-five percent of the P. syringae strains from linden, lilac, dogwood and oriental magnolia samples were ice nucleation active (INA) at 5°C; 76% of the P. syringae strains from aspen were INA at -5°C; but only 30% of the P. syringae strains from Japanese pear and 24% of the red maple strains were active ice nucleators at this temperature. The P. syringae strains isolated from these seven plant hosts were variable relative to their ability to induce a hypersensitive response in tobacco leaves and their ability to induce pathogenic changes when injected into immature tomato fruits. The range in hypersensitivity response by P. syringae strains isolated from a particular host varied from 100% in aspen strains to 57% in Japanese pear while the range in potential pathogenic ability on tomato fruit varied from 100% in aspen to 36% in saucer magnolia. In November 1983, tissue samples were also obtained from fields of diseased sudan grass used as green manure, from fields of symptomless cereal rye grass grown as cover crops as well as from roadside grass species growing around the perimeter of nursery production areas. Large populations of pathogenic and INA strains of P. syringae were isolated from these grass strains with populations of fluorescent pseudomonads exceeding 109 cfu/g fresh tissue from sudan samples whereas populations of 109 cfu/g were obtained from cereal rye grass and roadside grass samples. Eighty-one randomly selected strains from these isolations were tested using the L0PAT determination scheme for fluorescent pseudomonad identification. Fifty-eight of the 81 strains (72%) were similar to P. syringae, whereas 34 (59%) of the 58 strains were ice nucleation active at -5°C. Thirty-one of the 58 strains induced a hypersensitive response in tobacco leaves, and 29 (50%) were pathogenic to green fruit of tomato. Several P. syringae strains isolated from sudan and cereal rye grass were pathogenic when inoculated to greenhouse grown sudan seedlings; however, none of the strains tested were pathogenic to cereal rye grass seedlings in the greenhouse. Three of six P. syringae strains tested were also pathogenic to young shoots and leaves of peach trees maintained in a greenhouse chamber at high humidity.