Diseases Resistance varieties of plants

In many cases, growing resistant crop varieties id the only method to control diseases, and perhaps is the ideal one. In an ecological set-up equilibrium is maintained. If the equilibrium is altered in any one respect then several changes take place. If the plant has been growing wild in a locality, it is exposed to plant pathogens. Due to continued exposure to pathogenic invasions, the host might develop resistance to infection. Likewise the plant may alter several of its morphological and physiological characters. The Darwinian principles of variation, struggle for existence and survival of the fittest play under these circumstance and we get a resistant plant containing to grow and perpetuate while the susceptible ones may disappear from the scene. This has been going on for hundreds of thousands of years. When man cultivates the plant in bulk in field, he is upsetting the natural ecological balance and exposing the plants to a newer environment. Under the influence of cultural practices and other factors it may become susceptible to the diseases. Hence, he has to select and choose crop varieties which are resistant or tolerant to higher doses of fertilizers, high quality of the grains or other plant products, and resistance to pets and diseases. The pets and diseases of important or major crops plants like wheat, rice, sorghum and other millets are many. To obtained a variety resistant to all the diseases of a crops and to combine in it the best of agronomic qualities is an impossible task. Added to this, the pathogens are constantly changing, evolving new races of pathogens arise to survive. Hence, it is a never-ending struggle for man to go on evolving newer crops varieties not only to secure better agronomic qualities and increases production, but to combat newer and more virulent pathogens and pests. In a way, the breeder encourages the development of new races!

With the rediscovery of Gregor Mendel’s finding by de Vries in Holland, Correns in Germany and Tschermak in Austria almost simultaneously in 1900, the science of genetics had a rebirth. Though there are earlier reports on the possibilities of obtaining varieties resistant to diseases, systematic studies to select varieties of disease resistance started only during 1900, and the credit for this goes to W.A. Orton of the United States Department of Agriculture, who selected cotton varieties resistance to Fusarium wilt. The breeding work for resistant varieties against bunt and smut of wheat started in Australia by the turn of the century and later various workers including Stakman and his associates worked on resistance to rust and smuts. They evolved several techniques for testing crop varieties for disease resistance and also to differentiate the varieties and races of fungal species, based on host reactions under a given set of environment conditions. These results revealed that new resistant varieties should not be expected to solve the problem permanently, since races arise in nature by more than one method. Along with this information, data on the heterothallic nature and genetics of other fungi, such as Neuospora, were worked out. Starting from 1946, the basis for the occurrence of newer races among fungi came to be better understood. Expansion of our knowledge on the physiology and biochemistry of different living systems has helped us to understand many aspects of host-pathogen relationships, as also the genetic basis of diseases resistance in plants.