Genetic resistance refers to those heritable features of a host plant that suppress or retard development of a pathogen or insect. In other words, genetic resistance is the ability of some genotypes to give higher yields of good quality than other varieties at the same initial level of pest infestation under similar environmental conditions, thus resistance is defined in relation to susceptible varieties. Genetic resistances are considered as a major form of biological control of biotic stresses. Main features of genetic resistances are given below:
1. Genetic resistance is governed by nuclear genes or cytoplasmic genes or both in other words; genetic resistance is an inbuilt mechanism or inherent property.
2. Genetic resistance is measured in relation to susceptible varieties or genotypes.
3. Breeding of resistant cultivars takes into account the genetic variability of both pest and host plant.
4. The resistant variety may become susceptible after few years due to formation of new races of pathogen or new biotypes of an insect.
5. Breeding for disease and insect resistance differs from breeding for higher yield. There is triangular interaction is between genotype and environment only.
Types of Genetic Resistance:
1. Vertical or Specific Resistance:
Specific resistance of a host to the particular race of a pathogen is known as vertical resistance. This type of resistance is governed by one or few genes and, therefore , is referred as oliogenic resistance. When the resistance is controlled by single gene, it is called monogenic resistance. Since vertical resistance controls only one race of a pathogen, it is also termed as specific resistance. Because of its simple inheritance, it is known as major gene resistance. As the controlling genes have distinct effect, it is also known as major gene resistance. The host with vertical resistance controls only one race; therefore, it is also known as non-uniform resistance. Main features of vertical resistance are given below:
1) Vertical resistance displays discontinuous variation among genotypes and classification of genotypes into resistances and susceptible classes is possible.
2) Transfer of oliogenic resistance from one host genotype to another is simple.
3) Oligogenic resistance is usually short lived or less durable. The resistance can easily break down when new race of a pathogen is formed.
4) Vertical resistance provides protection only from one race of a pathogen.
5) It has high heritability and can be easily identified in the breeding programmes.
6) Vertical resistance applies to host pathogen gene for gene relationships.
2. Horizontal or General Resistance:
The resistance of a host to all the races of a pathogen is called horizontal resistance. This type of resistance is called by various names as per reasons given below:
1. General Resistance:
The host plant provides protection from all the prevailing races of a pathogen.
2. Polygenic Resistance:
The resistance is controlled by a number of genes, also called quantitative resistance.
3. Minor Gene Resistance:
Each gene involved in the resistance has small effect which is not visible.
4. Non-specific Resistance:
The host resistance is not for a specific race of a pathogen. The resistance is similar to all the races of a pathogen; hence also resistance is similar to all the races of a pathogen, hence also referred as uniform resistance. Main features of horizontal resistance are given below:
a) Horizontal resistance exhibits continuous variation among genotype into different distinct classes is not possible.
b) It has low heritability and, therefore, identification of resistant types is difficult.
c) It provides protection from several races of a pest.
d) The resistance cannot be easily overcome by new races of a pathogen due to polygenic control.
e) It is difficult to transfer polygene resistance from one genotype to another, because individual allele in a parent can not be identified.
f) General resistance is not applicable to gene for gene relationships.