Arbuscular Mycorrhizal Fungi in Plant Disease Control

Abstract

Biological control of plant pathogens presents a compellingmethod of increasing plant yields by suppressing ordestroying pathogens, enhancing the ability of plants toresist pathogens, and/or protecting plants against pathogens.Micro-organisms antagonistic to plant pathogens may bederived from the resident microbial community or may be offoreign origin. Although there are concerns towards therelease of an organism of foreign extraction, in general,biological control presents a myriad of benefits such as beinga component of the environment, resistant to development ofchemical pesticide resistance, being relatively safe and riskfree, and by being compatible with sustainable agriculture.Arbuscular mycorrhizal fungi (AMF) form one such group oforganisms that can act as bioprotectors of plants. Thesezygomycetous fungi that form specialized structures such asarbuscules and/or vesicles are obligate biotrophs and utilizehost photosynthates for their growth. They are ubiquitousand co-exist with over 80% of terrestrial plants includingagricultural or horticultural crops. Their interactions withrhizosphere flora and fauna influence the growth and fitnessof the associated plants (Azcon-Aguilar and Barea 1992;Fitter and Sanders 1992). An incompatible associationbetween the host plant and the indigenous AMF communitycan lead to serious losses in crop yields, indicating thesignificance of AMF in crop production. In contrast, acompatible association can result in enhanced plantproductivity, through enhanced host P nutrition (Ravnskovand Jakobsen 1995), prevention or control of plant diseasescaused by soil-borne pathogens (Caron 1989a; St-Arnaudet al. 1995), and/or enhancement of plant hormonal activity(Frankenberger and Arshad 1995).The rhizosphere, a zone of soil loosely surrounding theroots, is a dynamic environment wherein complex chemicaland microbiological activities occur (Lynch 1990). Themycorrhizosphere is the region of the rhizosphere that issubjectedtomodificationsfollowingAMFcolonizationofthehostplant(Linderman1988).Inducedbiochemicalchangesinthe plant as a result of AMF root colonization is collectivelytermed the “mycorrhizosphere effect.” The mycorrhizosphereeffect typically results in a transient or permanent shift in theresident microbial community that may favor the eliminationorproliferationofpathogens(Edwardsetal. 1998;MeyerandLinderman 1986; Nemec 1994; Paulitz andLinderman 1989).In general, these changes are mediated by modifications inhost root membrane permeability that subsequently leads tomodifications in root exudate composition (Graham et al.1981; Ratnayake et al. 1978). Meticulous management of themycorrhizosphere may serve as an effective, safe, andenvironmentally friendly alternative to conventional methodsof plant disease control.