Multiline varieties and disease control : I. The "dirty crop" approach with each component carrying a unique single resistance gene.

The effects of the widespread use of "dirty crop" or "partially resistant" multilines on the racial composition of a pathogen population were investigated using simple theoretical models. It was found that the evolutionary changes in the pathogen attacking multiline varieties depend critically on two factors - the level of selection against unnecessary genes for virulence(s) and the number of lines in the multiline (n): (i) If s>0.5, then multilines will stabilize the racial composition of the pathogen population and simple races, carrying a single gene for virulence, will be the predominant biotypes. (ii) If s< 1/2 (n - 1) when unnecessary genes for virulence are additive in their effects in reducing pathogen fitness, or s< 1/n when unnecessary virulence genes act multiplicatively to reduce pathogen fitness, then the use of a multiline will lead to the development of a superrace which can simultaneously attack all the component lines. (iii) If 1/2> s>1/2 (n-1) for the additive model, or 1/2> s>1/n for the multiplicative model, the use of multiline varieties will stabilize the pathogen population, but with complex races, carrying two or more virulence genes, predominant. These findings are discussed in relation to the potential of multiline varieties as a means of achieving stable, long-term control of plant diseases. It is concluded that "dirty crop" and "partially resistant" multilines will provide stable disease control in crop plants only in limited and relatively rare circumstances.