Differences in the leaf colonization pattern of two bacterial species.
S. SABARATNAM and G.A. Beattie. Department of Microbiology, Iowa state University, Ames, IA 50011-3211.

Two bacterial species, Pseudomonas syringae pv. syringae and Pantoea agglomerans, were compared both for their population dynamics on leaves of bean and maize and their ability to colonize internal leaf sites. Following spray inoculation of plants with GFP-labeled bacterial strains, bacterial cells were enumerated by plate counts and visualized in situ by fluorescence microscopy. Populations of the saprophyte P. agglomerans BRT98 increased on maize incubated at 95% relative humidity (RH) but decreased on maize at 45% RH and on bean at both humidity levels. Based on direct visualization and enumeration of cells recovered from surface-sterilized leaves, BRT98 remained exclusively on the leaf surface and showed no evidence for sub-epidermal colonization. In contrast to BRT98, the population sizes of the pathogen P. syringae pv. syringae B728a increased considerably on bean under both low and high humidity conditions, but decreased to undetectable levels within 3 days on maize. Interestingly, on bean leaves, B728a colonized sub-epidermal sites almost exclusively and showed little to no evidence for survival on leaf surfaces.