The success of biological control also depends on the ecological fitness of the BCAs, especially when they target soil-borne plant pathogens. To be effective, a BCA must establish in soil and express its protective capacities. Failure of biological control has often been attributed to a lack of fitness of the BCA, the population of which does not reach the threshold level needed to be effective. After introduction into the soil, the BCA must cope with the physico-chemical properties and with the biological characteristics of the soil. In a recent study (Edel-Hermann et al., 2009) the population dynamics of Fo47 was followed in two soils of contrasting physico-chemical characteristics. In the disinfested soils, in the absence of the native biota, Fo47 established at similar high levels (close to 106 colony-forming units per gram of soil) in an acidic sand (pH = 4.3) and a clay-loam (pH = 8.2), whatever the initial concentration at which it was introduced. Under such conditions, the temperature and water potential of the soils did not influence the population dynamics. In the presence of the native microflora, Fo47 established in both soils at densities lower than those at which it was introduced. These results demonstrate that Fo47 is more susceptible to the biotic than to the physico-chemical properties of the soils. The concept of soil suppressiveness to diseases (Cook & Baker, 1983), which describes the capacity of the soil to control the establishment of the pathogen and the expression of its virulence, also applies to biological control. In other words, some soils are more favorable than others to the establishment and expression of the protective capacity of a given BCA.
Ecological fitness depends also on the production and formulation processes. The propagules of the BCA, industrially produced on rich media and under aseptic conditions, are probably less well adapted to the soil environment than the native propagules of the pathogen. Therefore, one can speculate that, in soil, competitive interactions among the native microflora, the pathogen and the BCA will favor the pathogen. Several approaches have been proposed to produce and formulate BCAs at low cost, and solid-state fermentation is usually seen as a method of producing propagules with higher fitness than propagules produced in submerged fermentation. Moreover, in the case of solid-state fermentation, the growth substrate also constitutes the ready-to-use formulation (McQuilken et al., 1997). To give a competitive advantage to the BCA, it has been proposed that a food base be added to promote its growth and activity after release. Already some biological control products are made of fungal conidia ‘coated’ with glucose or saccharose. It would be preferable, however, to select a food base required more specifically by the BCA than sugars, which can be consumed by most soil microorganisms.
Finally, the success of biological control depends on the mode and dose of application. In soilless cultures it is quite easy to apply the BCA evenly either by mixing it with the growth substrate before planting or by using the drip irrigation system. This delivery system has been used to apply Fo47 to control Fusarium wilts in soilless tomato crops (C. Olivain& C. Alabouvette, unpublished data). In open fields it is usually not economically feasible to apply the BCA evenly at the dose needed for efficacy.