ECOLOGICAL SIMILARITY AND COEXISTENCE OF EPIPHYTIC ICE-NUCLEATING (ICE(-SYRINGAE STRAINS AND A NON-ICE-NUCLEATING (ICE(-)) BIOLOGICAL-CONTROL AGENT()) PSEUDOMONAS)

De Wit replacement series were used to study competitive interactions between epiphytic Ice(+) Pseudomonas syringae strains and the biologic al frost control agents Ice(-) P. syringae TLP2del1 and Pseudomonas fl uorescens A506. Mixtures containing two strains in different proportio ns but at a constant total population size were inoculated onto potato leaves. The population sizes of each strain and the total population size were determined when the community had reached equilibrium. A nea r-isogenic P. syringae strain pair exhibited an interaction similar to that expected for strains competing equally for limiting environmenta l resources. Replacement series with nonisogenic Ice(+) and Ice(-) P. syringae strain pairs suggested that these strains competed for limiti ng resources according to their relative competitive abilities. There was no evidence of any niche differentiation between the Ice(+) P. syr ingae strains and the Ice(-) P. syringae strain. The growth responses of epiphytes following addition of nutrients to the phyllosphere indic ated that the epiphytic P. syringae populations were nutrient limited and that, under growth chamber conditions, the populations were more l imited by the availability of carbon than by the availability of nitro gen. Determination of in vitro carbon source utilization profiles prov ided further evidence for the lack of niche differentiation between th e Ice(+) and the Ice(-) P.syringae strains. Niche overlap indices calc ulated for the Ice(+) P. syringae strains with respect to Ice(-) P. sy ringae TLP2del1 were uniformly high, indicating ecological similarity, and were consistent with the observed low level of coexistence. The b iological frost control agent P. fluorescens A506 replaced P. syringae . This was correlated with a high degree of niche overlap between thes e species.