Most research on use of cultivar mixtures for disease suppression has focused on diseases caused by biotrophic, obligate pathogens that interact with their hosts on a gene-for-gene basis. The presence of differential, qualitative resistance to races of the pathogen in different host genotypes is commonly one of the most important criteria for selecting the cultivars that make up the mixture. With differential qualitative resistance, each host genotype potentially benefits from being in a mixed population; i.e., relative to a pure line stand of a genotype, the cultivar mixture will have a reduced proportion of tissue susceptible to the races that can infect it (Jeger et al. 1981a, 1981b; Garrett and Mundt, 1999).