Plant pathogenic fungi contribute significantly to crop damage and yield loss, followed by plant pathogenic bacteria and viruses. The potential of AMF to control various plant pathogenic fungi has been clearly demonstrated (Becker et al. 1999; Bodker et al. 2002; Boyetchko and Tewari 1996; Duchesne et al. 1989; Kapoor et al. 1998; Kasiamdari et al. 2002; Kegler and Gottwald 1998; Krishna and Bagyaraj 1983). In contrast, there are reports wherein AMF inoculation did not have any effect on disease severity (Guillon et al. 2002; Larsen and Bodker 2001; Wyss et al. 1991; Zambolin and Schenck 1983). In order for practical and routine use of
AMF as protectors of plants against plant pathogenic fungi, AMF performance must be consistent, specific, and effective.
Specificity of AMF for the control of crop diseases is crucial in order to mitigate any nontarget effects to beneficial micro-organisms. However, there are conflicting reports on the specificity of AMF. For example, inoculation of micropropagated banana with Glomus intraradices and a Glomus spp. isolate reduced rhizome necrosis and external disease symptoms caused by Fusarium oxysporum f. sp. cubense, but differences between the two AMF isolates were not noted, indicating that either both AMF species were equally effective against the pathogen or that they lacked specificity (Jaizme-Vega et al. 1998). In contrast, eggplant and cucumber seedlings transplanted into soils inoculated with G. versiforme and subsequently challenged with Verticillium dahliae and Pseudomonas lacrymans alleviated wilt symptoms caused by V. dahliae, but not G. mosseae, Glomus spp.-1, or Glomus spp.-2, indicating species-specific antagonistic symbiont-pathogen interactions (Li et al. 1997). Pozo et al. (1999) demonstrated the expression of two new basic glucanase isoforms, a phytoalexin elicitor-releasing factor between G. mosseae and G. intraradices used for the control of Phytophothora parasitica var. nicotianae. Because of the potential of AMF as bioprotectors against phytopathogens, this is an area that needs further study.
In order to enhance AMF efficacy, some researchers have used an AMF species mixture or a combination of microorganisms including AMF that act in concert to eliminate pathogens. For example, co-inoculation of groundnut with G. fasciculatum, Gigaspora margarita, Acaulospora laevis, and Sclerocystis dussii eliminated the damaging effects of Sclerotium rolfsii (Kulkarni et al. 1997). Also, tobacco inoculated with a mixture containing G. fasciculatum and Trichoderma harzianum effectively controlled damping-off caused by Pythium aphanidermatum and black shank disease caused by P. parasitica var. nicotianae (Sreeramulu et al. 1998). In some cases, microbial mixtures act synergistically with pesticides to result in effective control of plant diseases. A combination of wheat straw, carbendazim, G. fasciculatum, and T. viride protected safflower seedlings from the root rot pathogen Macrophomina phaseolina, resulting in 100% seedling survival (Prashanthi et al. 1997). Sharma et al. (1997) effectively managed ginger yellows disease caused by
F. oxysporum f. sp. zingiberi using a combination of Gi. margarita, pine needles, and T. harzianum.
The requirement of a fully established AM symbiosis for elicitation of bioprotective activity by AMF has been disputed. The invasion of phytopathogenic fungi is said to be prevented by an aggressively root colonizing AMF species, indicating that AMF root colonization was satisfactory for control of disease. For example, Feldmann and Boyle (1998) found an inverse correlation between G. etunicatum root colonization of begonia cultivars and susceptibility to the foliar pathogen caused by the powdery mildew fungus Erysiphe cichoracearum. However, it was not clear whether
G. etunicatum colonization preceded infection by E. cichoracearum or whether pathogen suppression was accompanied by other mechanisms of biocontrol. Using an in vitro system, Filion et al. (1999) demonstrated that extracts from the extraradical mycelium of G. intraradices reduced the conidial germination of F. oxysporum f. sp. chrysanthemi. Alternatively, alterations in the chemical equilibrium of the mycorrhizosphere may have resulted in pathogen control. In another study, pea mutants defective for mycorrhization and nodulation challenged with Aphano-myces euteiches required a fully established AMF symbiosis for protection against the pathogen (Slezack et al. 2000).
Several researchers have also demonstrated AMF-mediated reduction of root rot disease in cereal crops (Boyetchko and Tewari 1988; Grey et al. 1989; Rempel and Bernier 1990; Thompson and Wildermuth 1989) and take-all disease of wheat (Graham and Menge 1982). Phytophthora spp., which cause diseases in a variety of plants have been model systems for AMF-mediated plant disease control (Cordier et al. 1996; Guillemin et al. 1994; Mark and Cassells 1999; Norman and Hooker 2000; Pozo et al. 1996; Trotta et al. 1996). Using the AMF species G. intraradices and pathogen F. oxysporum f. sp. lycopersici on tomato, Caron and co-workers have shown that the growth medium used (Caron et al. 1985), the application of P (Caron et al. 1986a), and pretreatment of the growth medium with AMF (Caron et al. 1986c) can influence disease severity. Despite proof of AMF potential in controlling plant diseases, few published reports have successfully demonstrated biological control of plant pathogens by AMF in the field (Bodker et al. 2002; Newsham et al. 1995; Torres-Barragan et al. 1996). Newsham et al. (1995) showed that pre-inoculating the annual grass Vulpia ciliata var. ambigua with an indigenous Glomus sp. and re-introducing the grass into a natural grass population extended a favorable effect against an indigenous F. oxysporum. Onion pretreated with Glomus sp. Zac-19 delayed the development of onion white rot caused by S. cepivorum by two weeks in the field and protected onion plants for 11 weeks after transplanting in the field and resulted in a yield increase of 22% (Torres-Barragan et al. 1996). One of the first reports on the effect of indigenous AMF on the development of introduced A. euteiches infection and disease development on field-grown pea (Bodker et al. 2002) showed that there was no correlation between AMF root colonization and disease incidence or severity, and emphasized the importance of field evaluations for authenticating the use of AMF as biocontrol agents. Although the indigenous AMF community composition was not described, this study underscores the importance of a richly diverse indigenous AMF community to defend plants from plant pathogens. Thus, there appears to be tremendous potential for AMF control of plant pathogens and the need for more detailed and well-planned and executed studies that will address problems of inconsistent and unreliable results.
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