Chemo-biological suppression of root-infecting zoosporic pathogens in recirculating hydroponic systems

Zoosporic root-infecting pathogens are the most destructive organisms in enclosed environment agricultural systems which recycle irrigation water. Numerous strategies have been proposed for reducing or eliminating these pathogens from recycled irrigation water. These strategies include the following: filtration, sedimentation, chlorination, ozonation, heat, ultraviolet light, application of antimicrobial chemicals, suppressive potting substrates, and biological control agents. The latter strategy has been the focus of numerous investigations but consistency in performance in disease abatement following their application has not been realized. This has been attributed, in part, to the inability of these potential biocontrol bacteria to maintain a critical threshold population necessary for sustained biocontrol activity. That threshold population has been estimated at not less than 10⁶ CFU g^?1 of soil or root. Recently, we demonstrated that amending recycled irrigation water with specific carbon substrates (i.e., N-Serve® or Truban®) resulted in the selective enhancement of the indigenous fluorescent pseudomonad populations to levels at or above the presumed threshold levels necessary for disease control. In our current study, we verified the ubiquity of that response in different cultural systems involving two different host plants (i.e., cucumber and pepper) and, most significantly, we document, for the first time, significant and sustained disease abatement. Additionally, we demonstrated that nitrapyrin, the active ingredients in N-Serve® and Truban®, exerted direct antifungal activity whereas the inert ingredients had an indirect role in disease suppression. Specifically, the inert ingredients were responsible for the increase in the fluorescent pseudomonad population. Amending the recirculating nutrient solution with a representative fluorescent pseudomonad isolate verified and substantiated their role in disease control. Cumulatively, our results support the hypothesis that it is possible to modify the environment to make it more conducive to the multiplication and survival of indigenous biocontrol bacteria. The resident microflora in many, if not most, habitats already contain prospective biological control members who, if enhanced, could function as antagonists to other microorganisms in the same habitat.