The backcross is the breeding method for developing multilines

The backcross is the breeding method for developing multilines. The agronomically superior line is the recurrent parent, while the source of disease resistance constitutes the donor parent. To develop multilines by isolines, the first step is to derive a series of backcross-derived isolines or near-isogenic lines. A method for developing multilines is illustrated in Figure 16.8. The results of the procedure are two cultivars that contrast only in a specific feature. For disease resistance, each isoline should contribute resistance to a different physiological race  of the disease.

The component lines of multilines are screened for disease resistance at multilocations. The breeder then selects resistant lines that are phenotypically uniform for selected traits of importance to the crop cultivar. The selected components are also evaluated for performance, quality, and competing ability. Mixtures are composited annually based on disease patterns. It is suggested that at least 60% of the mixture comprise is olines resistant to the prevalent disease races at the time. The proportion of the component lines are determined by taking into account the seed analysis.

Multiline cultivars consist of one genetic background but different genes for the trait of interest. A multiline is hence spatially differentiated, plant-to-plant. When planted, the cultivar creates a mosaic of genotypes in the field to provide a buffering against the rapid development of disease. Two basic mechanisms are used by multiline cultivars to control disease – stabilization of the patterns of virulence genes and population resistance. By stabilizing the patterns of virulence genes in the pathogen, it is supposed that genes for resistance would retain their value in protecting the cultivar for an extended period. The concept of population resistance is the delay in the buildup of the pathogen in the multiline cultivar. Spore trapping has also been proposed to explain disease buildup in the population of a multiline by reducing the effective inoculation load in each generation. Following the primary inoculation, spores that land on resistant genotypes will not germinate. Similarly, progeny spores from susceptible genotypes landing on resistant genotypes will not germinate. The sum effect of these events is a reduction in the inoculum load in each generation.

Multilines have certain key advantages and disadvantages.

Advantages

_ A multiline would provide protection to a broad spectrum of races of a disease-producing pathogen.

_ The cultivar is phenotypically uniform.

_ Multilines provide greater yield stability.

_ A multiline can be readily modified  by replacing a component line that becomes susceptible to the pathogen, with new disease resistant line.

Disadvantages

_ It takes a long time to develop all the isolines to be used in a multiline, making it laborious and expensive to produce.

_ Multilines are most effective in areas where there is a specialized disease pathogen that causes frequent severe damage to plants.

_ Maintaining the isolines is labor intensive.

Cultivars can be created with different genetic backgrounds. When different genetic lines are combined, the mixture is a composite called a variety blend. Blends are less uniform in appearance than a pure line cultivar. They provide buffering effect against genotype_environment interactions.