Summary Bacterial spot caused by Xanthomonas perforans ( Xp ) is an economically important disease in tomato. Previous studies have shown that the recently isolated Xp strains have acquired and retained the effector gene, xopJ2 , which has been reported to increase fitness of the pathogen in the field. To elucidate the fitness benefit of xopJ2 , we quantified the effect of xopJ2 on the dispersal and evolution of Xp populations...