INVASION AND EXCLUSION AMONG COEXISTING PSEUDOMONAS-SYRINGAE STRAINS ON LEAVES

The invasion and exclusion abilities of coexisting Pseudomonas syringa e strains were quantified on leaves. Twenty-nine P. syringae strains w ere inoculated onto plants in 107 pairwise combinations. All pairs wer e duplicated so that each strain was inoculated both first as an antag onist strain (day 0) and second as a challenge strain (day 3). The pop ulation size of each strain in a mixture was quantified on day 6 follo wing incubation under moist conditions. For P. syringae strains, the p resence of an established population often significantly reduced the g rowth of subsequently arriving challenge strains on the leaf surface. Invasion and exclusion abilities, quantified by contrasting population sizes of challenge strains in the presence and in the absence of anot her strain, varied significantly among P. syringae strains and were pa rtly a function of the particular strain pair. The population size of a strain when present alone on a leaf was not predictive of invasion o r exclusion ability. Successful invaders were significantly less likel y to exclude challenge populations than were nonsuccessful invaders. P opulation sizes of successful excluders were negatively correlated wit h population sizes of coexisting challenge strains, while population s izes of successful invaders were positively correlated with those of c oexisting antagonist strains. The patterns of interaction among coexis ting strains suggest mechanisms for successful invasion and exclusion among P. syringae strains on leaves.