Horizontal resistance is the resistance that invariably remains after a matching allo-infection has occurred. To postulate that horizontal resistance does not occur would be to postulate an absolute susceptibility. Such a level of susceptibility is experimentally unproved, and is theoretically impossible. Horizontal resistance is polygenically inherited and it can be exhibited at any level between its minimum and its maximum. Its maximum level should provide a virtually complete control of a parasite under conditions of maximum epidemiological competence. Breeding for comprehensive horizontal resistance will require simultaneous quantitative improvements and will eventually control all the parasites that have epidemiological competence in a particular agro-ecosystem. (5). However, because epidemiological competence is so variable, a cultivar that is in balance with one agro-ecosystem, is likely to be unbalanced in another agro-ecosystem, having too much resistance to some parasites and too little to others.