In natural environments, where a particular host species occurs within mixed vegetation, the development and spread of disease is probably limited to some extent by the separation of individual plants within the area. This constraint does not apply in the crop situation, where localised infection and progression of disease within the homogeneous plant population can occur rapidly. In this artificial situation, where the natural balance between pathogen and host does not apply, special control measures often have to be adopted if the large scale occurrence of disease and consequent major crop loss are to be avoided. These measures fall into four main categories: chemical control, biological control, breeding of resistant cultivars and sanitary procedures.

Chemical control

Chemical control agents are of two main types: bactericides (synthetic organic and inorganic compounds) and antibiotics (naturally occurring microbial products). The use of these two types of control agent is considered in the first part of this section, with a final discussion on general aspects of chemical control.

Bactericides

The range of compounds used as bactericides has recently been reviewed by Sekizawa and Wakabayashi (1990) who divide these compounds into four main categories: synthetic bactericides formerly used for crop protection, currently used synthetic bactericides, traditional inorganic compounds and soil nitrification inhibitors.