Effects of Mycorrhizas on Plant Tolerance to Diseases

Abstract

The mycorrhizal association with the roots of land plants has existed for hundreds of millions of years and logically includes associations with other functional groups of soil microbes. Disease is rare in undisturbed ecosystems compared to disturbed agroecosystems where disease can often cause serious economic loss. It is logical to surmise that mycorrhizas played a significant role in somehow protecting plants against pathogen invasion, but the demonstration of that phenomenon experimentally has been difficult and conclusions from published work are not possible largely because of the differences in experimental procedures, pathogens, and diseases. Many reviews on the subject have focused on the mechanisms of interaction such as (a) enhanced nutrition, (b) competition for nutrients and infection sites, (c) morphological changes, (d) changes in chemical constituents in plant tissues, (e) alleviation of abiotic stress, and (f) microbial changes in the mycorrhizosphere. Depending on the disease and the environmental situation, any or all mechanisms could be involved, but changes in microbial populations in the mycorrhizosphere seems to be the best explanation, yet the least studied. This chapter focuses on this mechanism by defining the mycorrhizosphere paradigm where soil microbes are influenced by both rhizodeposition of substrates from roots as well as exudation from arbuscular mycorrhizal (AM) fungal hyphae. The microbial populations in the mycorrhizosphere can change dynamically over time and are influenced by what microbes are present in the soil or substrate, the formation of the AM association intraradically as well as extraradically, and the process of selective enrichment of specific functional groups of microbes, including those that can contribute to the antagonistic potential against root pathogens. Documentation is provided to demonstrate that the numbers and proportion of bacterial antagonists against several root pathogens increase when the AM association is established. The antagonistic potential index derived from in vitro antagonism tests is greater than that of the non-AM plant rhizosphere soil as a result of selective enrichment of bacterial antagonists from the bulk soil by root and hyphal exudation. The data suggest that early establishment of the AM association by inoculation of transplants or in-furrow application that can enrich for antagonists is a management strategy that can contribute to disease suppression under agricultural conditions.