Bacteriophages are a promising method of combating phytopathogenic bacteria, the mechanism of action of which has three components

Growing agricultural crops requires enormous resources—energy, human resources, and more. Meanwhile, approximately a third of what's grown ends up in landfills. Some spoils during storage and sale, some is thrown away by consumers, but a significant portion of waste comes from harvest losses due to pests, including phytopathogenic bacteria.

Bacterial diseases are a problem for many agricultural crops worldwide, including tomatoes, potatoes, peanuts, and tobacco. They are caused by gram-negative bacteria, which quickly develop resistance to pesticides, just as human pathogens do to antibiotics. As with human pathogens, scientists have turned to natural enemies of bacteria—bacteriophages (phages)—to combat plant pathogens. Unlike pesticides and antibiotics, phages are highly specific—they kill only a specific species or even strain of bacteria, leaving beneficial microflora—in the gut, in the soil, and elsewhere—behind.

Currently, there is limited data on the effectiveness of phages in combating plant diseases, but this area is gradually beginning to develop. For example, the results of a study by Chinese scientists were recently published: they evaluated* the potential of phage therapy against bacterial blight caused by Ralstonia solanacearum in tomatoes. The results were quite satisfactory, and the phage effect was threefold.

Scientists prepared a cocktail of four bacteriophages that demonstrated the greatest effectiveness against R. solanacearum and tested it in both greenhouse and open-field conditions. In both settings, phage therapy reduced the number of plant pathogens and, consequently, the incidence of tomato diseases by 80% (the first component of phage therapy). Bacteria that were not destroyed, i.e., those that acquired resistance to the bacteriophages used, turned out to "pay" a high price for this advantage: they began to reproduce too slowly and were unable to form a population sufficient to cause disease in the plant (the second component of phage therapy). Furthermore, the dramatic reduction in the R. solanacearum population in the soil, both literally and ecologically, freed up space for a variety of beneficial bacteria. The proliferation of such bacteria in the soil, which often have mechanisms for actively resisting phytopathogenic relatives, is another factor in protecting plants from diseases (the third component of phage therapy).

The authors of the study plan to determine whether phage-resistant bacteria regain their ability to rapidly replicate over time. The authors believe the use of phages in plant growing is extremely promising.

*Wang X, Wei Zh, Yang Keming et al. Phage combination bacterial therapies for wilt disease in tomato // Nature Biotechnology, 2019, 37: 1513–1520. DOI: 10.1038/s41587-019-0328-3