Agrobacterium biovar 1 is a plant pathogen with the ability to colonize the irrigation system of tomato greenhouses, causing hairy root disease (HRD). Management focuses on hydrogen peroxide to disinfect the nutrient solution, but due to the emergence of resistant strains, its efficacy and sustainability are questioned.

Using a relevant collection of pathogenic Agrobacterium, consisting of >10 species, a diverse set of lytic phages, named OLIVR, were isolated from infected greenhouses. A combination of three can infect 85% of the complete collection. All phages remained stable under greenhouse-relevant conditions (pH, peroxide, temperature, nutrient solution). To assess the efficacy of the phages, their ability to disinfect greenhouse nutrient solution inoculated with agrobacteria was tested. Each phage infected their host, but their ability to decrease the bacterial concentration and limit phage resistance development differed. Phage resistant mutants were tested for reduced virulence.

Based on these data, a phage cocktail was selected and tested in small-scale plant experiments (tomato plants in pots) and in a commercial greenhouse setup (240 tomato plants divided over three objects: bacteria only, negative control, phage and bacteria treatment). In the latter setup, the plants were treated four times at the start of the production season with bacteria and/or phages and the plants were followed during the whole growth season (9 months), after which they were scored for hairy root disease symptoms.

The cocktail was shown to significantly reduce disease symptom development. Together, these data show the potential of these phages as tool to tackle HRD.