The genetics of plant-pathogen interaction

Abstract

The attack and defence mechanisms described above are genetically determined in the pathogen and host plant, respectively. Genetical variation results in new races of pathogens, some of which have different pathogenic abilities and may be able to attack hitherto resistant plants. Similarly, new varieties of host plants arise, some of which are more resistant to pathogens. In both organisms the more effective races and varieties tend to survive, the less effective ones to disappear. Effective disease develops when a virulent race meets a susceptible variety but not when the pathogen is avirulent or the plant resistant. This may, however, be modified by environmental factors in that, for example, the normally effective resistance of a plant may be overcome under conditions highly favourable to the pathogen and/or unfavourable to the plant. The distinction between immunity and resistance, although theoretically valid and sometimes practically useful, can be of doubtful significance in the field, where high resistance is often equated with immunity (‘field immunity’). The outcome of plant/parasite interaction depends on the relative effectiveness of the plant’s resistance and the parasite’s pathogenicity as mediated by the prevailing environmental conditions, and a knowledge of the genetics of both resistance and pathogenicity is clearly necessary for intelligent disease control by plant breeding methods.