ABSTRACT. The spread of the epiphytic population of Xanthomonas campestris pv. vesicatoria and the disease it causes, bacterial leaf spot, were studied in field plots of pepper near Gainesville, Florida. In the summer of 1989, the epiphytic population of X. campestris pv. vesicatoria was dispersed to the west-northwest from point sources of diseased plants. Winds from the southeast during rainstorms were essential for the spread of bacteria in the field. In the autumn of 1989, a focus of bacterial leaf spot developed naturally near the centre of the experimental plot. The epiphytic population of X. campestris pv. vesicatoria increased sharply after a 2-day rain accompanied with strong wind. The wind was believed to be responsible for the transport of bacteria to distances 32 m from the focus. Initially in both seasons, the epiphytic populations occurred as distinct gradients from the focal sources of diseased plants. These gradients flattened over time and the disease incidence increased to near 100%. The increase in the epiphytic populations of the pathogen to > 3.0 log10 (cfu cm-2) on healthy plants away from the foci preceded disease appearance by several weeks. Applications of cupric hydroxide plus mancozeb significantly reduced the epiphytic population of X. campestris pv. vesicatoria on pepper leaves and slowed the spread of disease in the plots.