Bacteria are common inhabitants on leaf surfaces and are both of practical importance and of scientific interest because they can cause plant frost injury and plant disease, produce phytohormones, fix nitrogen, and have other direct and indirect effects on plants and other organisms. Considerable interest exists in modifying the microbial composition of leaves and roots by application of microorganisms or by other means to control plant diseases or frost injury. There is also considerable interest in predicting the effects of intentionally or inadvertently introducing recombinant microorganisms with unique traits into natural habitats such as on plants. This study will emphasize factors that are important in intraspecific competition of bacteria on leaves. Competition for limiting environmental resources of leaves best explains the exclusion of ice nucleation active (Ice+) Pseudomonas syringae strains from leaves previously colonized by recombinant Ice- strains of this species both in greenhouse and in field tests. Ice+ P. syringae strains that either coexisted with or which could not coexist with reference Ice- P. syringae strains on potato leaves have been identified and will be compared for relative competitive ability against Ice- P. syringae strains both in the laboratory and under field conditions and will be contrasted with competition exhibited by isogenic Ice+ P. syringae strains. Spatial and nutritional heterogeneity of different P. syringae strains on leaves, and other features that might condition competition of bacteria on leaves, will be related to different patterns of competition on and "invasion" of leaves exhibited by these strains. Tests will be also made of hypotheses that different strains avoid direct competition by inhabiting spatially distinct sites or utilize different resources at the same physical site on leaves.
