A major problem in agriculture is the difficulty in achieving control of root pathogens. Even when chemical pesticides are available, their efficiency in nurseries or fields can be unpredictable. In the future, most agricultural systems will tend to less intensive crop production with reduced inputs both of fertilizers and pesticides. Arbuscular mycorrhizal (AM) fungi, which form root symbioses with most agricultural, horticultural and hardwood crop species, are widespread potential biocontrol agents. It has been shown, using tomato as a model plant, that colonization by AM fungi renders root systems resistant/tolerant to attack by Phytophthora, which causes considerable losses in nursery stock and forestry. Necrosis of adventitious and lateral roots caused by P. parasitica is reduced in AM plants and viability of apical and subapical tissues greatly increases, so diminishing root damage. The degree of protection conferred depends on the AM fungus involved and on the extent of AM development. The bioprotective effect of AM results from local and induced systemic resistance (ISR) in mycorrhizal and non mycorrhizal root parts, respectively. Mycorrhiza-elicited signals produced in mycorrhizal parts of root systems must be involved in ISR to P. parasitica in nonmycorrhizal parts. ISR in nonmycorrhizal parts of mycorrhizal root systems is associated with plant wall defense responses. Pectin and PR-1 protein-containing wall thickenings are formed in cells in contact with intercellular pathogen hyphae, and callose-rich, phenolic-containing papilla structures develop around hyphae penetrating host cells. No wall defense reactions occur in nonmycorrhizal P. parasitica-infected root systems where disease causes host cell death. Furthermore, root exudates are modified and antagonistic microorganisms develop in the mycorrhizosphere, both of which interfere with the pathogen's life cycle, in particular with zoospore germination, the primary infective agent of P. parasitica. Therefore AM fungi can be exploited not only as biofertilizers but also as biological control agents.