BACTERIAL ROOT ZONE COMMUNITIES, BENEFICIAL ALLELOPATHIES AND PLANT DISEASE CONTROL

Abstract

The release of root exudates from plants encourages the formation of beneficial bacterial communities in the root zone capable of generating secondary metabolites that improve plant health and crop yield. Metabolites with antibiotic or lytic action have been identified , while others are known to induce systemic disease resistance in the host plant, or interfere with the nutritional requirements of phytopathogens. However, despite existing positive relationships between bacterial communities and their plant hosts, man-made attempts at applying bacteria for biocontrol purposes have met with limited success. Inconsistent performance of biocontrol bacteria in the field may be due to the variable expression of genes involved in the biocontrol action, or simply the resistance of established soil communities to a sudden and inundative influx of adventive bacterial species or strains. Regardless of the inherent capacity of ‘naturally occurring’ soil microbial ecosystems to buffer anthropogenic interference, crop management systems are regularly used to distort agro-ecosystems through, for example, the use of tillage operations, alternate cropping systems, monoculture, crop rotation length, fertilizer and organic amendments, and various crop protection chemistries. The management of soil microbial communities for disease control appears to involve, in part, the creation of short term chaos in the microbial community through the application of such perturbation stresses. While hope remains that bacterial communities with biocontrol activity will one day be used as an adjunct to, or replacement for, agrichemical crop protectants, reliable biological controls that moderate pathogen attack remain elusive. In the interim, disease suppressive soils may be encouraged to form through the use of modest perturbation stresses that promote microflora species’ diversity and functionalities underpinning natural bioantagonism.