COMPARISON OF THE BEHAVIOR OF EPIPHYTIC FITNESS MUTANTS OF PSEUDOMONAS-SYRINGAE UNDER CONTROLLED AND FIELD CONDITIONS

The epiphytic fitness of four Tn5 mutants of Pseudomonas syringae that exhibited reduced epiphytic fitness in the laboratory was evaluated u nder field conditions. The mutants differed more from the parental str ain under field conditions than under laboratory conditions in their s urvival immediately following inoculation onto bean leaves and in the size of the epiphytic populations that they established, demonstrating that their fitness was reduced more under field conditions than in th e laboratory. Under both conditions, the four mutants exhibited distin ctive behaviors. One mutant exhibited particularly large population de creases and short half-lives following inoculation but grew epiphytica lly at near-wild-type rates, while the others exhibited reduced surviv al only in the warmest, driest conditions tested and grew epiphyticall y at reduced rates or, in the case of one mutant, not at all. The pres ence of the parental strain, B728a, did not influence the survival or growth of three of the mutants under field conditions; however, one mu tant, an auxotroph, established larger populations in the presence of B728a than in its absence, possibly because of cross-feeding by B728a in planta. Experiments with B728a demonstrated that established epiphy tic populations survived exposure of leaves to dry conditions better t han newly inoculated cells did and that epiphytic survival was not dep endent on the cell density in the inoculum. Three of the mutants behav ed similarly to two nonpathogenic strains of P. syringae, suggesting t hat the mutants may be altered in traits that are missing or poorly ex pressed in naturally occurring nonpathogenic epiphytes.