Breeding Crops for Durable Resistance to Disease

Durable resistance to disease is a common component of plant defense systems in natural ecosystems and can be found in virtually all cultivated species. Agricultural production practices increase crop vulnerability to most diseases, so higher levels of resistance than occur in natural ecosystems may be needed. Monogenic resistance to highly specialized pathogens is often highly effective when first developed but is generally race-specific and nondurable, especially when characterized by hypersensitive host reactions. Partial resistance conditioned by several to many genes with additive effects is usually durable, particularly when it involves morphological or developmental changes in the plant. Exceptions to the general rule of nondurable monogenic resistance and durable polygenic resistance are presented. Recurrent selection is commonly used to develop cultivars with durable resistance, but durable resistance has also been achieved through pedigree breeding in small grains. Success in accumulating polygenic durable resistance to multiple diseases of maize and to leaf rust and stripe rust of wheat are described as the mixed record of success in breeding for late blight resistance in potato. The mlo gene for resistance to powdery mildew in barley offers an intriguing case of highly effective monogenic resistance that appears to be durable. Modern molecular genetic approaches offer promise not only for marker-assisted selection of partial resistance genes but also for creation of novel forms of resistance to plant diseases. Nevertheless, traditional breeding and field tests will still have an essential role in developing commercial cultivars.