Bacterial spot caused by Xanthomonas perforans (Xp) is an economically important disease in tomato. Previous studies have shown that more than 70% of recently isolated Xp strains have acquired and retained an effector gene avrBsT, which has been reported as a fitness factor. In order to elucidate how avrBsT confers fitness to Xp, we conducted a study with the objectives of quantifying pathogen dispersal and predicting the prevalence and movement of Xp strains in relation to the avrBsT gene. We compared movement of two wild-type Xp strains, GEV872 and GEV1001, which carry avrBsT, with their respective avrBsT- mutants when co-inoculated in the field. We developed a model to quantify the spread of Xp with or without avrBsT over spatial and temporal dimensions. The model was obtained using binomial distribution for the absence or presence of each strain in samples collected over the tomato growing season. Weather variables and the lag period were accounted for in the model. According to the model, wild-type strains spread faster than mutant strains (~4 vs. ~1.25 meters/week). Preliminary results show that plants infected with wild-type strains exhibit higher disease severity than those with mutant strains. Based on the model, we developed an application to predict and visualize the spatio-temporal distribution of Xp with and without the avrBsT gene. Our results show that the presence of a single gene can affect pathogen movement and significantly alter bacterial population dynamics.