Biological system management in the rhizosphere: an inside-out/outside-in perspective

Root health is a major factor in crop production worldwide due to the absence of stable resistance in many plants to soil pests and diseases and to the lack or loss of effective soil pesticides. In some crops severe root-health syndromes exist, caused by the interaction between multiple pathogens and parasites and by the additive impact of many organisms acting alone. Damage is increased further by interacting abiotic factors. Integrated Pest Management, which has relied heavily on pesticides, is no longer applicable in many cases due to the lack of reliable control alternatives. Therefore, an alternative approach or research philosophy is needed that stresses the biological components of root health. Biological System Management is proposed as an alternative approach. The term is defined as an approach to improve root health that places stress on resistance/tolerance defense mechanisms of the plant, the use of antagonists for biological control and targeted disruption of sensitive developmental stages of a pest through cultural practices to improve root. Attempts to optimize disease control in the rhizosphere requires a broad biological system management approach due to the complex nature of the system. The terms inside-out and outside-in pertain to the site of biological control activity emitted by microorganisms which are either intimately associated with die root tissue or are present in the rhizosphere. It sets itself aside from inundative control approaches in which microorganisms are applied to the non-rhizosphere soil in a field. To be effective in root health management, biological control must be streamlined so that control activity occurs in or on the root tissue where early root infection and damage is initiated. Microorganisms occupying this niche and those that have definite root health promoting affects have been identified: arbuscular mycorrhizae, rhizobacteria, fungal pathogens or parasites and more recently mutualistic microbial endophytes. Such biological control agents can he applied to seed, transplants or tissue culture produced plantlets before the host is planted into infested fields to insure targeted placement, higher levels of activity, reduced production costs, simplified formulation and application, reduce extension costs and minimal impact on the environment. These organisms, acting singly or as communities, have been shown to elicit new and unique modes-of-action that are outside the realm of normal parasitism and pathogenicity. Rhizosphere competent microorganisms have been shown to influence nematodes and diseases through mechanisms that are often closely coupled with the plant's own defense mechanisms: alteration of root exudates, induced resistance, increased plant tolerance and stimulation of allelopathic substances. These new and unique control alternatives are just some of the new avenues open to pathologist working with Biological System Management of soil-borne nematodes and diseases.