Managing Soilborne Diseases by Managing Root Microbial Communities

Robert G. Linderman

Soilborne diseases have been managed in recent time by various methods, many of which were abandoned for reasons of cost or convenience in favor of the more direct approach of applying chemicals. Except for chemical fumigants or heat, no single treatment will eradicate soilborne pathogens effectively. However, many practices can go a long way toward suppressing soilborne diseases through biological/microbial means. Consistent control requires a thorough understanding of how the microbial communities associated with crop plant roots function to enhance plant growth and health.

The biological suppression of root diseases requires that the root-associated microbial community must contain sufficient antagonistic elements with potential to inhibit pathogens. In many crop cultural systems, we have greatly suppressed or eliminated those very microbial populations that we need to be there. We have killed these allies along with the pathogens that were targeted for elimination. In order to establish the microbial communities needed for stability, growth enhancement, and disease suppression, we must focus on the major players that normally colonize roots and that can most influence plant growth and health. For most plants on the earth, that means early establishment of mycorrhizae and their microbial associates.

When mycorrhizae form, great changes take place in the physiology of the roots and the whole plant, and in the soil surrounding the roots, now appropriately called the "mycorrhizosphere." In this paradigm, the mycorrhizal fungus is the quarterback, and the other associated microbes are the rest of the team. Due to specific changes in the microbial community resulting from altered root exudation plus the specific chemicals exuded by the fungal hyphae that have grown out into the soil, the "team" is ready to compete with pathogens, increase the availability of nutrients derived from organic substrates, help the plant acquire water and nutrients from well beyond the range of the roots themselves, and increase plant tolerance to other environmental stresses, such as toxicity from soil salinity. Mycorrhizae can affect root diseases by; (a) enhancing host plant nutrition, (b) competing with pathogens for host photosynthate and infection sites, (c) inducing morphological changes in the roots and root tissues that can block root invasion, (d) changing inhibitory chemical constituents of plant tissues, (e) reducing abiotic stresses, and (f) inducing changes in the populations of microbial antagonists in the mycorrhizosphere.

We have analyzed the mycorrhizosphere for microbial components that can influence the growth and health of plants. Specifically, we have attempted to quantify the "antagonistic potential" of plants with and without mycorrhizae. When mycorrhizae form, the antagonistic potential of the bacterial community in the mycorrhizosphere increases significantly compared to that of the rhizosphere soil of the non-mycorrhizal plant. Similarly, the diversity of the bacterial community from the background soil increases to levels of detection. But what happens if the diversity of the background soil or soilless growth medium is very low at the outset? Then the "team" of organisms will be weak. If soils have been steamed or fumigated, mycorrhizal fungi, and probably many other beneficial associates, have been eliminated or their populations greatly reduced. Those populations can only be restored by inoculation at the earliest opportunity in the production system.

Inoculum should include an effective microbial team, with a mycorrhizal fungus quarterback and its teammates of selected microbial associates. In order for the team to be in place when the pathogen and other stresses occur, seeds or transplants should be inoculated. This gives the microbial team the advantage of prior occupancy. This can be accomplished by including known antagonists in the inoculum that can function as the front line while mycorrhizae become established. Commercial biocontrol agents, whether fungal or bacterial, are fully compatible with mycorrhizae, and should be placed beneath the seeds or directly on the roots of transplants in order to become established in time to confront pathogens. After inoculation, cultural practices should be modified, by reducing fertilizer and pesticide applications, in order to favor and maintain the microbial team. There are few sources of such microbial teams, but growers should find them and adapt them to their production system. In the short run, this alternative disease control strategy might be less spectacular than using chemical fumigants, but in the long run will provide stable, sustainable disease and crop management.

Robert G. Linderman
USDA-ARS Horticultural Crops Research Laboratory
3420 Orchard
Corvallis, OR 97331
Tel: (541) 750-8760

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