Take-all decline: model system in the science of biological control and clue to the success of intensive cropping.

Take-all is a root disease of wheat and barley caused by the soil-borne ascomycete, Gaeumannomyces graminis var. tritici. The fungus causes serious limitations to grain yield when wheat is grown for 2 consecutive years in the same field. Crop rotation, preferably breaks of 2 or more years away from wheat or barley, was the only method recommended for reliable control. The discovery that this disease declines in severity and can all but disappear where wheat is grown in the same field over many years provided an exemplary model system for studying soil ecosystems that become 'disease suppressive'. This chapter summarizes the results of 40 years of research that led to the understanding of the key biological and biochemical components responsible for creating take-all suppressive soils and the resultant take-all decline. The chapter provides approaches to unravelling the complex microbial ecosystems in the rhizosphere and gives directions for management of root diseases in intensive cropping systems heretofore considered to be only controllable by broad-spectrum biocides.