Challenges

Although the efficacy of an AMF species on plant pathogens has been assessed under controlled environments and usually in the absence of other AMF or other organisms (Budi et al. 1999; Kasuya et al. 1996; Li et al. 1997), research indicates that the potential of AMF for control of plant pathogens is high. Limitations in AMF research pertaining to biological control of plant diseases under field conditions are two fold, (a) production of large quantities of AMF inoculum is not feasible because of the obligate biotrophicity status of AMF, and (b) negative interactions between the introduced AMF and the indigenous AMF and microbial community after introduction into field. Challenges posed by interactions between AMF and indigenous microbial community and soil and environmental conditions often determine the success of AMF inoculation in disease control under field conditions. An appreciation of factors that influence AMF efficacy as biological control agents can further enhance their survival, competitiveness and efficacy. For example, it is known that intraradical proliferation of AMF within roots is a hostregulated event (Bever et al. 1996). Therefore, a highly mycotrophic host or host cultivar may be more favorable for AMF proliferation and reproduction than one that is not highly mycotrophic (Feldmann and Boyle 1998; Xavier 1999). In addition, nonconducive soil-environment combinations such as high soil P levels and soil disturbance can affect AMF colonization (Bever et al. 1996; Gazey et al. 1992; Stahl et al. 1988; Stutz and Morton 1996) and efficacy

(Graham et al. 1981; Menge et al. 1978; Ratnayake et al. 1978). Colonization of host roots by AMF is a crucial component in the AMF-mediated SIR response of host plants to plant pathogens as the expression of an SIR response requires a threshold level AMF presence within host roots (Cordier et al. 1996; 1998). The effect of phosphorus on AMF efficacy may be direct, wherein inoptimum P levels impair AMF activity and therefore its ability to effectively control pathogens. In contrast, soil disturbance has an indirect effect where AMF efficacy may be altered by a delay in mycelial network initiation and diversion of carbon for the synthesis or repair of the external mycelial network and not nutrient uptake. Zak et al. (1982) suggested that the ability of AMF to effectively re-establish their mycorrhizal association after disturbance might partially determine their success in a disturbed site. It is not known to what extent soil disturbance affects the biocontrol activities of AMF, but it disrupts the external mycelial network resulting in a severe reduction in mycorrhizal efficacy (Evans and Miller 1990; Stahl et al. 1988).

A richly diverse AMF community ensures qualitatively and quantitatively the presence of AMF species desired for specific activities such as biological control of plant pathogens. However, choice of host genotype and rotation (Bever et al. 1996; Johnson et al. 1992; Talukdar and Germida 1993), levels of fertilizer application (Baltruschat and Dehne 1982; Jasper et al. 1979; McGonigle and Miller 1993; 1996; Vivekanandan and Fixen 1991), tillage (Evans and Miller 1990; McGonigle and Miller 1993; Vivekanandan and Fixen 1991), pesticide application (Manjunath and Bagyaraj 1984; Schreiner and Bethlenfalvay 1997), and the effect of associated micro-organisms (Andrade et al. 1995; Xavier and Germida 2002) are some critical factors that can indirectly alter AMF diversity in soils. For example, continuous cropping selectively enhances the proliferation of parasitic AMF, which are relatively fast growing compared to beneficial AMF, leading to alterations in mycorrhizal biodiversity in the rhizosphere (Johnson et al. 1992). Similarly, one particular AMF host selects from an indigenous AMF pool resulting in the selective enrichment of certain AMF species over others (Xavier 1999). Studies assessing the significance of AMF biodiversity in AMF-mediated biocontrol are rare but critical. Given the importance of AMF to plant health and the complexity of the various microbial interactions, all the relevant factors have to be considered before AMF selected as biocontrol agents can effectively function in the field.

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