Peach Bacterial Spot, caused by Xanthomonas arboricola pv pruni (=X. Campestris pv pruni), is a serious disease of peaches in the Southeastern United States. The incidence of this disease increases during extended periods of leaf wetness caused by rain, dew, fog or high humidity. A mathematical model that includes growth, death and wash off functions describes population changes for the causal organism. Temperature within the water film where bacteria reside controls population changes. This temperature is dew point during condensation events or wet bulb when water is evaporating. The model mimics observed populations on peach fruit through the growing season. It shows that field sampling for bacterial populations must be done carefully in order to account for age of the leaves or fruit.