Genetic Background of Durable Resistance

Abstract

Durable disease resistance is defined as resistance that has remained effective while a cultivar possessing it has been widely cultivated in an environment favoring the disease. This characteristic of resistance is recognised retrospectively, as are all other characteristics of interactions between hosts and pathogens. Resistance that has been shown to be durable should be considered as a valuable resource for further breeding and for investigation of the causes of durability. Durable resistance is often achieved against diseases that show little or no specialisation into races pathogenic to particular cultivars. For such diseases, the control of resistance may be by single genes or by several genes, each of small effect, and the resistance may be either complete or incomplete (quantitative). For pathogens that possess pathogenicity specific for particular cultivars, failures of resistance can often be shown to be due to race-specificity of resistance genes and the evolution and spread of pathogen races with matching pathogenicity. These race-specific genes may have large (major) or small (minor) effects, giving either complete or quantitative resistance, and may operate at any stage of host development. Despite failures of resistance due to race-specificity in some cultivars, other cultivars remain resistant for relatively long periods, though this does not prove that their resistance will be permanent. For some diseases such durable resistance may be controlled by one or few genes of large effect. More usually a number of genes, not necessarily a large number, giving cumulative and often quantitative resistance, may appear to control durable resistance. Breeding programs should be modified to increase the chance of incorporating genetic components that appear to be related to durable resistance. However, breeding involves the selection of new genotypes, and the achievement of durable resistance in new cultivars, whatever the breeding method,. will only be proved by a widespread and prolonged test.