Genetically resistant cultivars have been the mainstay for the economic control of plant diseases of many food, fiber, and forage crops around the world. Disease resistance is the primary means of control for the major diseases of almost all field crops as well as many fruits and vegetables. The southern corn leaf blight epidemic of 1970 served as a reminder of the potential of pathogens to continually adapt to the plants we breed, and the problems of genetic uniformity in our crops. At the risk of oversimplification, it is often easiest to classify types of disease resistance by their mode of inheritance. Resistance can be classified as either oligogenic in inheritance, or as polygenically inherited. Breeding for disease resistance differs from breeding for other traits in that expression of resistance is dependent not only on variability in the host plant but also in the pathogen.