Types of genetic resistance

The two types of genetic resistance to pathogen in plants are monogenic and polygenic. While monogenic characters are stable over a wide range of conditions, polygenic resistance is highly variable and influenced by environmental conditions. Polygenic resistance is also influenced by host nutrition, while monogenic resistance is completely stable. Repeated selections through breeding or resistance in a crop result in accumulation of more genes for resistance in the new varieties, which is the case in most of the varieties of crop plants which are widely cultivated over the world.

Another recent finding is that in a crop variety there may be a gene which inhibits completely or partially the expression of resistance to a diseases. Similarly, an inhibitor gene may suppress the expression of susceptibility in some varieties. It has been shown in some varieties that resistance may fluctuate within certain limits. There are also reports showing that some crop varieties are resistant to a pathogen at certain temperature, but May completely breakdown and become susceptible at a different temperature. If the pathogen is capable of producing many races, the monogenic resistance in the host may result in its becoming susceptible to one or more of the newer races to the gungus. This is what happens in wheat varieties which breakdown due to infection by newer rust races. If large numbers of genes for resistance are accumulated in one variety by repeated breeding and selection, then the possibilities of its sustained resistances to most races of rust are improved. Host resistance may inherited as a qualitative character, and its expression is influenced by environment by environment. In polygenic a virulent recombination in the pathogen, durability of resistance is prolonged.

A thorough knowledge of the variability and genetic characters of the pathogen as well as the inheritance and expression of resistance of susceptibility in the host is essential in a breeding programme for diseases resistance. Plant improvements in agriculture have been achieved through several means, the chief ones being introduction of new varieties of crops into a locality, hybridization and selection, and induced mutation. A variety resistant to a given diseases can be obtained either by selecting lines which already have genes for resistance or by hybridization to combine resistance. To obtain the desired variety through selection, we must have (i) resistant lines or biotypes in the population we are working with, (ii) reliable technique for screening the varieties for resistance, and (iii) the selected variety should combine other desirable properties, including good agronomic qualities. The variety must be tested under optimum condition for resistance to the diseases, to avoid diseases escape leading to erroneous conclusions.

The breeding programme is a continuing process. Plant breeders, pathologists and agronomists should work hand in hand to reach the goal. Worldwide search for genetic material in different crops plants has been going on and extensive collection of germ-plasma are available at many institutions. These wild and cultivated plants carrying resistance genes for various plant diseases. It is for the plant breeders, agronomists and pathologists to make best use of them and evolve resistant varieties with combined agronomic qualities.