Exploitation of Vertical Resistance
1. Varieties with Individual Major Gene:
In this method an individual major gene is referred for the prevalent race of pest info well adapted cultivar through backcrossing. Resistant plants are selected from segregating population and backcrossed to the recurrent parent. Generally, 5-6 backcrosses are sufficient to retain the genotype of recurrent parent with added resistance to specific race of pathogen. This approach is widely used to control the prevalent race of pathogen. Transfer of major genes from one host to another is very simple. The main disadvantages of this approach is that major gene may sometimes become susceptible to the minor prevalent races.
2. Development of Multiline:
Multiline refers to seed mixture of isoline, related lines or unrelated lines. The seeds of genotypes with individual major gene are mixed together. This approach is used in self pollinated species. The resistant genes to different races of a pathogen are transferred into different genotypes by separate backcross programme. Suppose there are five major genes each for a different race of pathogen. These five major genes will be transferred in five different backgrounds. At the end, seed of five resistant genotypes thus developed will be mixed together to constitute multiline. Thus multiline provides protection against several races of pathogen. The new race of a pathogen whenever occurs will attack few genotypes of a multiline. All the component genotypes of a multiline will never become susceptible to new race of pathogen. The resistant components in a multiline will act as a barrier and delay the spread of the new race to susceptible plants. There are two main disadvantages of this method. First, several backcross programmes have to be launched to transfer each major resistant gene in a separate background. Second, the recurrent parent limits agronomic performance of the multiline. Moreover, it requires extensive testing against specific races.
3. Gene Pyramiding:
Gene pyramiding refers to incorporation of two or more major genes in the host for specific resistance to a pathogen. This is differs from multiline, where each major gene is incorporated into separate genotype and then resistant genotypes are mixed together. In pyramiding approach all major genes are incorporated in a single cultivar. Combination of several major genes for specific resistance provides protection against several new races that may develop in the pathogen. The new varieties can be developed with a contribution of 2,3 or more major genes. Such varieties have durable resistance. Gene pyramiding approach has been used in oat against crown rust in Canada.
4. Gene Deployment:
Another way of exploiting vertical resistance is gene deployment. Gene deployment refers to planned or strategic use of major genes in development of resistant cultivars for various geographical areas. This approach helps in preventing disease and maintaining the diversity for major genes in different geographical regions. This approach requires a number of genes with similar effectiveness for control of prevalent races. However, it is impossible to find out different genes with similar effectiveness. This is the main drawback of gene deployment approach. This approach was used for soybean breeding in Northern United State, but was not found effective.