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Wolfe (1985) defined cultivar mixtures as "mixtures of cultivars that vary for many characters including disease resistance, but have sufficient similarity to be grown together." Cultivar mixtures do not cause major changes to the agricultural system, generally increase yield stability, and in some cases can reduce pesticide use. They are also quicker and cheaper to formulate and modify than “multilines,” which are defined as mixtures of genetically uniform lines of a crop species (near-isogenic lines) that differ only in a specific disease or pest resistance (Browning and Frey 1981). Cultivars used in the mixture must possess good agronomic characteristics and may be phenotypically similar for important traits including maturity, height, quality and grain type, depending on the agronomic practices and intended use. Cultivar mixtures in barley for the control of powdery mildew are an example of phenotypically similar mixtures, whereas red- and white-grained sorghum mixtures used in Africa are an example of phenotypically different mixtures. Mixtures of red-grained and white-grained sorghum (courtesy of Dr. Henry Ngugi, Pennsylvania State University) Another situation in which mixtures may be of economic interest is for the protection of susceptible host genotypes with superior agronomic characteristics. In that case, the deployment of the susceptible host in combination with an agronomically inferior, but disease resistant genotype, may be a solution (Garrett and Mundt 1999). |
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