Global climate warming and involvement of new regions with endemic populations of microorganisms in commercial seed production have led to an increase in the diversity of phytopathogenic bacteria that are affecting major crops, including the fruit trees. As a rule, emergence of new pathogens is associated with importation of infected seeds and planting material, cultivation of new species and varieties of plants, and expansion of agricultural trade with foreign countries. One of the leaders in diversity among phytopathogens is the genus Xanthomonas bacteria, affecting more than 400 plant species. Among the characteristic signs of xanthomonads is the high frequency of horizontal gene transfer both within the genus and between phylogenetically removed bacterial taxa - up to 25% of the genes are of this origin. In this paper, we consider another source of increasing the number of phytopathogenic species - by the epiphytic populations. These bacteria are the likely ancestral form of the phytopathogenic bacteria of the genus Xanthomonas.