5.1.5S
BIOLOGICAL SYSTEM MANAGEMENT: STRENGTHENING THE ANTAGONISTIC POTENTIAL IN SOIL ECOSYSTEMS FOR THE CONTROL OF COMPLEX DISEASE SYNDROMES CAUSED BY PATHOGENS AND NEMATODES

RA SIKORA

Soil-Ecosystem Phytopathology, lnstitut fur Pflanzenkrankheiten, Universitat Bonn, Nussallee 9, D-53115 Bonn, Germany

Background and objectives
The root systems of all crop plants are negatively affected by a broad spectrum of soilborne fungal pathogens and plant-parasitic nematodes. Root health syndromes caused by the activity of these organisms, acting additively or synergistically, cause severe reductions in root growth and ultimately crop yield. Such complex disease inter-relationships are even more important in production systems affected by the loss of broad-spectrum soil fumigants. Fungal pathogens and plant-parasitic nematodes are constantly confronted by a variety of microbial antagonists in the rhizosphere. The inter-relationships between these two groups of organisms influence disease development and thereby crop loss. The level of microbial activity in the soil, in particular the activity of plant-specific microbial communities in the rhizosphere, are important in regulating damage intensity and the formation of complex disease.

Results and conclusions
Cropping systems can be used to manage microbial antagonists in the rhizosphere in order to improve the health of the root system. All aspects of IPM, for example rotation, organic amendments, resistant and/or tolerant cultivars, not only directly affect diseases and nematodes, but can strongly influence microbial community structure. These components of IPM can be utilized to effectively manage certain segments of the antagonistic potential in the rhizosphere. Cropping pattern and cultural aspects of production can be used to support microbial antagonists in the rhizosphere as well as those active endophytically in the root system. Mycorrhizal fungi, plant health-promoting rhizobacteria, mutualistic bacterial and fungal endophytes, as well as pathogenic and parasitic antagonists of fungi and nematodes, can be positively manipulated so that they further support overall IPM system effectiveness. Furthermore, commercially available biological control agents that are used in inundative approaches can be supported by proper biological system management procedures.

Examples are presented of management systems that can be used to improve IPM practices and reduce soilborne diseases and nematodes by stimulating the antagonistic potential in both high-input agriculture and low-input sustainable production systems. Methods used to measure and characterize the antagonistic potential in soils will be discussed. Examples of biological enhancement techniques used to treat seed and/or planting material in both annual and perennial crops in tropical and temperate cropping systems will be shown.

References
1. Sikora RA, 1992. Annual Review of Phytopathology 30, 245-270.
2. Sikora RA, 1997. Med. Fac. Landbouww. Univ. Gent 62/2a, 105-112.
3. Vilich V, Sikora RA, 1997. Plant-Microbe Interactions and Biological Control. Marcel Dekker, New York, pp. 1-14.