A REVIEW OF PLANT-DISEASE, PATHOGEN INTERACTIONS AND MICROBIAL ANTAGONISM UNDER CONSERVATION TILLAGE IN TEMPERATE HUMID AGRICULTURE

The advent of conservation tillage presents a need for a greater under standing of plant disease and disease interactions in temperate humid agriculture, where excessive crop residues, continuous moist soil cond itions and soil compaction are potential constraints. In this review, biotic and abiotic factors, and aspects of microbial antagonism, which can influence plant disease development in the root zone, are charact erized in the context of conservation tillage in humid climates. Soil densification and reduction in macroporosity can aggravate abiotic roo t disease. Changes in soil aeration and permeability status can alter the quantitative and qualitative differences between soil rhizofloral populations, and survival and distribution of pathogen inoculum, Furth ermore, anaerobic soil conditions can result in root-pathogen interact ions leading to plant disease development. A good quality soil physica l environment is an important indicator for root health under conserva tion tillage in humid climates. Conservation tillage tends to concentr ate plant debris and consequently microbial biomass in the top 5 to 15 cm of soil, and thus promotes survival of pathogens. However, disease -causing microbes make up only a proportion of the rhizofloral populat ion. Relatively high soil microbial activity can lead to competition e ffects that may ameliorate pathogen activity and survival, and counter act a high pathogen inoculum pressure. Microbial antagonism in the roo t zone can lead to the formation of disease-suppressive soils. This ph enomenon, which is important for the adoption of conservation tillage in humid climates, can be influenced by soil and crop management pract ices, especially crop rotation.