Exploiting the biocontrol potential of plant-associated pseudomonads – A step towards pesticide-free agriculture?

Worldwide excessive use of chemical pesticides with harmful side effects to environment and humans has increased the call for a pesticide-free agriculture. Biological control of plant pathogens by antagonistic microorganisms may be an alternative. These should also include plant-associated pseudomonads. They are adapted to live on and inside plants due to their diverse metabolic and ecological abilities. Many Pseudomonas strains exhibit a diverse spectrum of antagonistic traits enabling them to suppress various phytopathogens by direct antibiosis and/or by triggering defence responses in the host plants. Although a certain proportion of strains within the indigenous populations of pseudomonads on and inside plants are able to do so, the spread of these bacteria in the field is rather restricted and individuals with antagonistic activity are unevenly and patchily distributed. Natural biological control of fungal and bacterial plant diseases by pseudomonads possibly supported by adapted management systems therefore, seems unlikely. However, selected Pseudomonas strains can be artificially introduced into the soil–plant ecosystem and have proven to be promising microbial antagonists. But, more substantial in-depth studies to improve the efficacy and survival of biocontrol agents are needed to use them in pesticide-free agriculture on a field scale. Potential biocontrol strains have to undergo a targeted selection process that also takes into account their mode of action. Their application in consortia of multiple strains or with other microbial taxa that are complementary in their action may have advantages in biocontrol efficiency compared to single strains. Biotechnological solutions have to ensure both efficient antagonistic performance and sufficient persistence of the introduced bacteria in the field. With today's knowledge, seed inoculation seems to be a practicable approach to exploit the biocontrol potential of pseudomonads in crop farming.