To investigate changes in bacterial communities associated with a fungal foliar disease, epiphytic bacteria from powdery mildew-infected and uninfected leaves of cucumber and Japanese spindle were analyzed using both culturedependent and -independent methods. Dilution plate counting suggested that powdery mildew-infected leaves likely accommodated larger populations of phyllosphere bacteria than uninfected leaves. Community-level physiological profiles (CLPP) also indicated that functional diversity, richness, and evenness of bacterial communities were significantly greater in the phyllosphere of powdery mildew-infected leaves. Genotype diversity and richness based on band patterns of denaturing gradient gel electrophoresis (DGGE) of the phyllosphere bacterial community were greater for leaves infected by powdery mildew. A principle component analysis of CLPP and DGGE patterns revealed a clear difference between infected and uninfected leaves of both plant species. These results suggest that powdery mildew-infection results in larger bacterial populations, and greater diversity and richness, and also changes the structure of the phyllosphere bacterial community. Furthermore, DNA sequences of the DGGE bands that showed greater intensity in the infected than uninfected leaves, differed between cucumber and Japanese spindle. This suggests that specific bacteria are associated with the plant species accompanying this fungal infection.Key words: powdery mildew, phyllosphere, epiphytic bacteria, denaturing gradient gel electrophoresis (DGGE), community-level physiological profile (CLPP)The phyllosphere is the above-ground surface of a plant, and is represented by the leaves. It provides habitats for various microorganisms including bacteria, fungi, and yeasts (20). Epiphytic bacteria are the main inhabitants of the phyllosphere, and are estimated to have a density of 10 6 -10 7 CFU cm −2 on a typical leaf (20). Culture-based studies have shown that such bacteria may include plant pathogens, ice nucleation-active bacteria, decomposers, phytohormoneproducers, nitrogen fixers, and antagonists of plant pathogens (8,10,11,14,25,35). Traditionally, bacterial communities in the phyllosphere were thought to be highly variable in both quantity and quality (9, 13, 33). Moreover, Yang et al. (37) demonstrated that based on culture-independent analyses of 16S ribosomal RNA gene (=16S rDNA), bacterial communities on leaves were more diverse than had been predicted from culture-based studies.In addition to bacteria, many fungal species have been isolated from the phyllosphere. These fungi are often observed as airborne spores on healthy leaves (3), and several of them also grow on the leaves. Some fungal species have evolved an ability to invade leaf tissues, causing foliar diseases. Although interactions between foliar fungal pathogens and their host plants have been well documented (21), little is known about the relationships between fungi and bacterial communities on leaves.In contrast, relationships between fungi and bacteri...