Crop Rotation, Monoculture and Soil Ecology1

Abstract Crop rotation and monocultural systems are discussed in relation to disease management and control. The ecological basis of both systems is examined. In crop rotation the leading principle is that in the absence of host‐plants parasite populations invariably decline below threshold levels, mostly within a few years. Knowledge of the behaviour of parasite populations in the soil is becoming vast, especially in the case of plant parasitic nematodes. Analysis of soil samples for nematodes and fungi often helps the farmer to plan rotation schemes. From the ecological point of view, crop rotation is interpreted as a system of interrupted monocultures. Monocultural practices cannot be applied so universally as crop rotation, but in a number of cases monoculture is feasible, e.g. with cereals. In the equilibrium situation between host‐plant (e.g. cereals), parasite (Gaeumannomyces graminis [Sacc.] v. Arx & Olivier) and the antagonists of the parasite, growth of the crop is satisfactory. These cases illustrate the role of antagonists, and it is proposed to investigate more systematically monocultures in relation to antagonisms. This may lead to more opportunities for biological control of soil‐borne diseases.