Biological control by indigenous and introduced micro-organisms

Disease-suppressive soils provide some of the best examples of indigenous micro-organisms protecting plants’ roots against plant pathogens (Weller et al., 2002). “Suppressive soils are soils in which the pathogen does not establish or persist, establishes but causes little or no damage, or establishes and causes disease for awhile but thereafter the disease is less important, although the pathogen may persist in the soil” (Baker and Cook, 1974). In contrast, conducive (non-suppressive) soils are soils in which disease readily occurs. Suppressive soils occur globally and are known for many different pathogens (Weller et al., 2002).

Instances of natural pathogen and insect suppression have been rich sources of micro-organisms for development into BCAs. For example, crown gall caused by Agrobacterium tumefaciens is a disease of a wide variety of plant species, but it is especially serious in deciduous fruit nurseries. The observation four decades ago by

Allen Kerr (New and Kerr, 1972) that the incidence of crown gall on almond correlated with the ratio of pathogenic to nonpathogenic agrobacteria suggested the potential for biocontrol by bacterization with nonpathogenic strains. Agrobacterium radiobacter strain K84 (isolated from soil around a peach gall) applied to seeds or roots resulted in dramatic control of crown gall (Kerr, 1980). K84 and its transfer-deficient mutant (K1026, see below) (Jones et al., 1988) are used worldwide for crown gall control.

During the last four decades, thousands of putative BCAs have been isolated and then tested on hundreds of diseases, insects, weeds and other pests. Although the use of biocontrol technology remains only a small fraction of that of chemical pesticides, the number of new BCAs, their performance and acceptance by growers continues to increase steadily. Bacillus and Trichoderma spp. have been the micro-organisms of choice for development into commercial BCAs of plant diseases (Harman et al., 2010; Kloepper et al., 2004; McSpadden Gardener and Driks, 2004), and Bacillus, Beauveria and Metarhizium spp. have been the microbes of choice for development as insect BCAs.1 These micro-organisms are appealing because they are easily mass produced and formulated. Interestingly, Pseudomonas spp. have been the microbes of choice for fundamental studies of biocontrol mechanisms because they are easily genetically modified and engineered. Although they are easily mass produced, they are harder to formulate because they do not produce a dormant spore like Bacillus spp. do.

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