A plant disease is an interaction between a plant host, a pathogen and the environment. Most plants are immune or completely resistant to almost all plant pathogens. When a virulent pathogen is dispersed onto a susceptible host and the environmental conditions are suitable then a plant disease develops and symptoms become evident. Disease control strategies must therefore focus on the host, the pathogen and/or the environment. 'Integrated Disease Management' involves the selection and application of a harmonious range of control strategies that minimise losses and maximise returns.