Biological control of hairy root disease using beneficial Pseudomonas strains

•Agrobacteium rhizogenes causes crazy root disease in hydroponically grown tomatoes.•Pseudomonas spp. can exhibit bacterial inhibiting properties.•Pseudomonas strains can inhibit A. rhizogenes under in vitro conditions.•Application of Pseudomonas to hydroponic systems reduces disease development. Hairy root disease (HRD) caused by Agrobacterium rhizogenes is a problematic disease leading to substantial losses in marketable yield in hydroponically grown crops including cucumbers (Cucumis sativus) and tomatoes (Solanum lycopersicum). Growers use strict hygiene protocols and sanitation, relying on hydrogen peroxide and other chemical-based sanitizers to clean the hydroponic system after disease outbreaks, however, this can be an expensive and time-consuming process. Biological control is emerging as a possible solution to this troubling problem. Biological control is an environmentally sound and effective means of reducing or mitigating pests and diseases using natural enemies. In this work we tested and identified numerous strains of Pseudomonas that can inhibit the growth of A. rhizogenes under in vitro and in planta conditions. In our in vitro experiments, 13 out of 49 different Pseudomonas strains tested were able to inhibit pathogen growth. Under in planta conditions we identified three different Pseudomonas strains (P. protegens 1B1, P. clororaphis 48G9, and P. brassicacearum 93G8) from the 13 in vitro-active strains that were able to reduce disease incidence up to 95% on Kalanchoe, tomato, and soybean. Pseudomonas treatments were able to reduce Agrobacterium rhizogenes numbers by nearly 1,000-fold in soybean and 100-fold in tomato. On hydroponically grown tomatoes, strains 1B1 and 93G8 were able to reduce disease incidence by 80%, and strain 48G9 was able to reduce disease incidence by 50% as compared with the control. We also screened the remaining collection on tomato (potting media) aiming to identify strains that may have been missed in the in vitro study. From this study we identified five additional strains that were effective in reducing disease incidence on potting media. We further tested strains 93F8, Wood3, and 90F12-1 in our hydroponic system and found that strain 93F8 reduced disease by 62%, Wood3 by 50%, and 90F12-1 by 55%. Our results suggest that certain Pseudomonas strains can inhibit A. rhizogenes growth and disease development under hydroponic conditions and may be suitable for use as biocontrol agent(s) for the control of HRD in hyd