Abstract
Plant pathologists have traditionally viewed soil as a hostile environment, harbouring pathogens that have adverse effects on plant health. The emerging concept of soil quality sees pathogens as components of total soil biological diversity, with disease resulting from disturbances to the balance between functional groups in soil. Experimental support for this concept is beginning to be accumulated. Molecular methods based on polymerase chain reaction (PCR) of DNA extracted from soil allow rapid assessment of genetic diversity and will increasingly be used to measure functional diversity as well. This will enable the relationships between diversity and disease suppression to be characterized. Suppressiveness of soils to disease is biological in nature, although modified by abiotic factors, and is of two types. General suppression depends on overall diversity and activity of the soil biota and acts against a broad range of pathogens. Specific suppression is due to particular antagonists or functional groups, and acts against single pathogens. Studies on the effects of management practices on disease suppression are still limited in scope, and are often difficult to interpret because most practices have direct effects on pathogen populations as well as on suppressiveness. Continuous cropping of a plant species selects for microflora adapted to its rhizosphere, which may suppress the activities of some pathogens. Rotations and reduced tillage should increase microbial diversity and increase suppressiveness, but evidence for these causal links is hard to find. Treatments that increase soil organic matter, such as residue retention and application of manure, may increase general suppression, and certain types of manures may also increase specific suppression. Soil biodiversity, disease suppression and management practices are yet to be fully synthesized in models that have general applicability. This means that management thresholds remain speculative in most instances.