Multiline Varieties and Disease Control. III Combined Use of Overlapping and Disjoint Gene Sets

Abstract

Alternate procedures, based on the use of combinations of overlapping (of the type AB, AC, BC, etc.) or disjoint (of the type AB, CD, EF, etc.) subsets of genes, for developing multiline components carrying two or more major genes for disease resistance are described. Simple mathematical models are used to examine the effects of multilines blended from such components on pathogen evolution and long-term disease control. It is found, in agreement with earlier studies, that the use of more than one resistance gene per component, where the genes are combined in either overlapping subsets or disjoint subsets, will be justified in practice only if it can be demonstrated that unnecessary genes for virulence in pathogens act additively to reduce the fitness of their carriers. If unnecessary virulence genes in the pathogen reduce fitness multiplicatively then multilines composed of components carrying multigenic resistances have only limited advantages (in the case of disjoint subsets of genes) or are at a positive disadvantage (in the case of overlapping subsets of genes) in long-term disease control compared with multilines in which each component carries a single unique gene for resistance.