Considerations for Using Bacteriophages in Plant Pathosystems

Abstract

Lytic bacteriophages (phages) have the potential for controlling susceptible bacteria in the rhizosphere or phyllosphere. The success of phage application in plant disease control requires that high populations of both phage and bacterium exist in order to initiate a chain reaction of bacterial lysis. Physical factors in natural environments such as the presence of biofilms that trap phages, low soil pH which inactivates phages, low rates of diffusion of phages in soil that prevent contact with target bacteria, and inactivation of phages upon exposure to ultraviolet (UV) light and desiccation, all impact successful use of phages in control of plant pathogenic bacteria. Other considerations relate to the bacterial strains which exist in nature. The bacterial species may have a low or high degree of variation in sensitivity to phages. Therefore, phage selection for field use requires careful monitoring of the targeted bacterial strains in the field due to the potential for strain variation and the likelihood for development of resistance to the deployed phages. Application timing has also been shown to be an important factor in improving the efficacy of phages. For instance, UV light is deleterious to phages and upon exposure phage populations plummet; therefore, evening applications of phages result in persistence on leaf surfaces for longer periods of time and may result in improved disease control. Extending the period of time that phages persist in the phyllosphere has been a major hurdle. Formulations have been identified which improve the persistence of phages on leaf surfaces; however, there is a need to identify superior formulations that extend the life on leaf surfaces from hours to days. Another strategy for maintaining high populations of phages has been to use non-pathogenic bacterial strains that are sensitive to the phage(s) or a closely related organism that does not cause disease on the plant host. Finally, phages may have value as part of an integrated management strategy.