Exposure dose, light distribution and wavelength affect the fate of introduced bacterial biological control agents in the phyllosphere of greenhouse grown tomato

Societal Impact Statement The use of chemical plant protection products must be reduced to promote sustainability in food production. One possible alternative is biological control agents (BCAs), but their efficacy under commercial conditions does not always reach the standard of chemical control agents. Previously, light has been found to induce mechanisms in bacterial BCAs that can affect their distribution and establishment. This could promote BCA efficacy. We looked into how monochromatic and polychromatic (which is what growers use) light treatments affected the occurrence of three BCAs post‐application. By combining two non‐chemical methods: a biological (BCA) and a cultural (light) control method, this offers a new integrated pest control strategy. Summary The dynamics and functionality of beneficial and non‐beneficial, non‐phototrophic bacteria can be influenced by light quality. We investigated if light could aid the survival of three bacterial biological control agents (BCAs; Bacillus amyloliquefaciens DSM7, Pseudomonas chlororaphis 50083 and Streptomyces griseoviridis CBS904.68) in the canopy of greenhouse‐grown tomatoes at four light treatments. Tomato plants were exposed to 50 μmol m−2 s−1 of either polychromatic light (white) or monochromatic light (blue: 420 nm, green: 530 nm and red: 660 nm) using DYNA LED lamps for a total of 48 h post foliar application of the BCAs. Leaves were harvested from two levels in the canopy at the top and middle of each plant at 0, 2, 4, 8, 12, 24 and 48 h post inoculation. The occurrences of the BCAs were quantified by plate count and droplet digital PCR (ddPCR). S. griseoviridis persisted under most treatments, whereas P. chlororaphis and B. amyloliquefaciens preferred the polychromatic and green light treatments as depicted by the viable count analyses. Significant differences between the DNA and cDNA concentrations were only noted for P. chlororaphis, with prominent wavelength effects. Light exposure dose, placement in the canopy and wavelength were found to be decisive factors for BCA re‐isolation, indicating that they have different optimal light environments.