Role of Mycorrhizal Fungi in Tree Defense Against Fungal Pathogens of Roots

Abstract

The roots of most flowering plants are capable of entering into mycorrhizal symbiosis, which enhances mineral nutrition or in some species is essential for adequate nutrition. Zak (1964) suggested that ectomycorrhizal fungi, besides enhancing plant growth by other mechanisms, might also protect their feeder roots from attack by pathogens. Subsequent research led to evidence for root protection by these fungi (reviewed by Marx 1972, 1975) and by vesiculararbuscular (VA) mycorrhizae (reviewed by Schenck and Kellam 1978, Dehne 1982). The influence of mycorrhizal fungi on disease caused by root-pathogenic fungi (and nematodes) is now known to range from inhibitory to stimulatory (Schenck and Kellam 1978, Dehne 1982). The influence of mycorrhizal fungi on plant growth and disease development depends on the species and sometimes the strains of mycorrhizal and pathogenic fungi involved, as well as on the host cultivar, soil conditions, etc. Despite this complexity, interest in the practical application of mycorrhizal fungi for root protection and growth stimulation remains strong (Cordell et al. 1987, Caron 1989, Duchesne et al. 1989c, Marx et al. 1989. Marx and Ruehle 1989, Kropp and Langlois 1990). Although a high degree of host-symbiont specificity may be theoretically tolerable for the protection of roots of woody plants (which remain in place for many years), commercial application of mycorrhizal fungi for root protection may hinge on the development of fungal strains that can function broadly as root protectants. An understanding of the bases of specificity and the mechanisms of root protection, which are intimately related, is essential for the formulation of principles that can guide the selection and development of mycorrhizal fungi for root protection. Toward this understanding, we discuss the interactions of mycorrhizal fungi with roots, root pathogens, and rhizosphere microflora in light of contemporary interpretations of the bases of disease resistance in plants.