The microbial community in the rhizosphere plays a key role in plant growth and -health, either directly by influencing plant nutrient uptake and by causing disease, or indirectly via microbial interactions in the rhizosphere. The majority of field grown crops (70-80 %) naturally form symbiosis with Arbuscular Mycorrhizal Fungi (AMF); thus the relation between root pathogens and most plants under field conditions is an interaction between AM and pathogens. The AM symbiosis has functionally been characterised as the reciprocal exchange of nutrients between the symbionts: the fungus is obligate biotrophic, whereas the plant receives inorganic nutrients from the AMF. However, the antagonistic potential of AMF against a range of soil-borne pathogens has also been demonstrated, but the underlying mechanisms are unknown. Both direct competition for nutrients/space or antibiosis have been suggested, as indirect competition by AM fungi, changes of plant root structure, root-exudations, nutrient uptake and growth as well as AMF induction of plant defence have been investigated. Moreover, a bacterial community structure associated to AM structures has been demonstrated and some of these bacteria have shown antagonistic potential against pathogens. This raises the question whether it is the AMF or the associated bacteria, which control the pathogens. So far, a general mechanism for AMF control with soil-borne pathogens has not been identified, but investigations of more mechanisms and interplays between them might be the answer.
Proceedings of the Seventh Australasian Soilborne Diseases Symposium, 2012