Bacteria play an important, sometimes decisive r“le in relations between different organisms. The implications for man are frequently conflicting, involving health on the one hand, and ecosystem protection on the other. Attempts to cultivate the central Asian crops "kok-sagiz" and "tau-sagiz" in Ukraine in the fifties, for example, failed because of an epiphytotic of the opportunistic bacterium Erwinia herbicola. The same happened with cultivation of poplars in Europe and cotton around the Caspian Sea. Human pressure (the introduction of new plants, pesticides, industrial pollution etc.) also influences plant pathogenic bacteria. Within an ecosystem, change in status of different plant species in turn promotes change among the rather specialized plant pathogenic bacteria. New diseases and new aggresive strains can appear. At the same time, decline of plants under human pressure does not result in a decrease in resistance to plant pathogenic bacteria. Human factors promoting plant development can cause loss of resistance to plant pathogenic bacteria, for example, when nitrogen is added to soil. When they are in their so-called epiphytic phase, plant pathogenic bacteria in general do not survive without other living micro-organisms, insects, other associated plants or even their plant hosts. That is why the loss of any species, or its restriction in an ecosystem causes quantitative and qualitative changes in populations of plant pathogenic bacteria. How the aggression or loss of virulence in a bacterial population is influenced during its epiphytic phase or on associated plants remains unknown. To make conclusions about further development of plant pathogenic bacteria, their ecological niches and human pressure should be taken into account.