About a third of the food that we grow, along with all of the effort and energy and labor and resources put into growing it, goes to waste. Much of it is thrown out by consumers or rots on shelves. But a substantial fraction of it is attacked by pests while still in the field.

Bacterial wilt infects a number of crops throughout the world, including tomatoes, potatoes, peanuts, and tobacco. It is caused by Gram-negative bacteria. As with human antibiotics, treating agricultural pathogens suffers from problems with destructive, broad-spectrum, and increasingly ineffective pesticides. And just as in humans, people have suggested using viruses to attack the bacterial pests.

Phages are viruses that infect bacteria. They are highly selective, disabling only the bacterial species they specifically target and leaving neighboring bacteria alone. Since undesirable pathogens are often mired within a diverse bacterial community containing species that we want·(both in our guts or in the soil), this specificity is usually preferable to antibiotics and pesticides that indiscriminately kill every microbe they encounter.

While the concept sounds good, evidence that phages are effective has been lacking. New work has just assessed if phage therapy can control bacterial blight in tomato plants. Results show that it does so using three related tactics.

First, the phages (the researchers used a combination of four, which was more effective than any of them individually or any combination of fewer) reduces the number of pathogens, which reduces disease. This was true both in an experimental greenhouse and out in the field.

The remaining pathogens develop resistance to the phages, but this comprises another way in which the phages combat the pathogen: through distraction. There is an evolutionary cost to developing phage resistance, so the pathogens that survive grow more slowly. The virulence of this particular blight is dependent on its population size—it can’t cause disease until it crosses a critical population density threshold. So having phage-resistant-but-slower-growing bacteria is a second way that the phages stop bacterial blight.

By knocking down the number of pathogens with this one-two punch, the phages also open up a literal and ecologic space in the soil in which new, beneficial bacteria can thrive. The phages thus promote a more diverse bacterial community that even further protects the plants from disease, since it includes bacterial species that actively help fight off the pathogenic one.

The authors note that more work needs to be done to ascertain if the phage-resistant pathogenic bacteria get their competitive advantage back over time; the experiments linking resistance to slow growth were only done in the greenhouse, and only over twenty-four hours. Still, they conclude that “using phages for biocontrol in agriculture shows promise.”

It should be interesting to see how the buying public responds to a label conveying that on their heirloom tomatoes.

Nature Biotechnology, 2019. DOI: 10.1038/s41587-019-0328-3 (About DOIs).