Biological control of hairy root disease on hydroponically grown tomatoes using Pseudomonas

Hairy root disease (HRD) caused by Agrobacterium rhizogenes is a problematic disease leading to substantial losses in marketable yield in hydroponically grown tomatoes (Solanum lycopersicum). Growers use strict hygiene protocols and sanitation, relying on hydrogen peroxide and other sanitizers to clean hydroponic systems after disease outbreaks, however, this is 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 by employing the use of natural enemies. In this work we have 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, 14 out of 52 different Pseudomonas strains were able to inhibit pathogen growth. In the in planta experiments, we identified three different Pseudomonas strains (1B1, 48G9 and 93G8) that were able to reduce disease incidence up to 95% on Kalanchoe and soybean. Pseudomonas treatments were able to reduce A. rhizogenes numbers by nearly 1000-fold in soybean and 100-fold in tomato. On hydroponically grown tomatoes, Pseudomonas strains 1B1 and 93G8 were able to reduce disease incidence by 80%, compared with the non-pseudomonad control while strain 48G9 was able to reduce disease incidence by 50%. In yield trials, we showed that HRD reduced fruit numbers and fruit weight, while treatment with our Pseudomonas do not negatively impact yield (i.e. fruit number and fruit weight).  Our results suggest that certain Pseudomonas strains can inhibit A. rhizogenes growth and disease development under hydroponic conditions and can be a potentially useful tool for growers in mitigating disease that arise in hydroponic systems.