Breeding for Resistance to Biotic Stresses: Disease Resistance
Disease is an abnormal condition in the plant produced by an organism of an environmental factor. More specially , disease may be defined as ‘ the series of invisible and visible responses of plant cells and tissues to a pathogenic microorganism of an environmental factor that result in adverse changes in form, function or integrity of plants and may lead to partial impairment or death of the plant or its parts. But in this chapter, we shall consider only such disease that are induced by organism and the abnormalities produced by the nonbiological environment or by a disease is known as host, while the organism that produces the disease is termed as pathogen. Diseases are produced by a variety of organisms from plant and animal kingdoms, viz. fungi, bacteria, viruses, nematodes and insects. Different crops are attacked to different degree by the different kinds of pathogens, but it may be emphasized that all the crop science are attacked by them. For example, cereals suffer from epidermises of air-borne fungi, most solanaceous crops are infected severely viruses, cotton, is damaged by many insects and so on. In the order of their importance, the pathogen may be listed as fungi>bacteria>viruses>nematodes= insects. Much of the breeding efforts have been directed against diseases caused by fungi, which may be greater than the effort against all other pathogens put together. Therefore, our discussion would be primarily based on the knowledge of fungal disease.
Mechanism of Disease Resistance:
A variety of mechanisms are involved in disease resistance. In some cases, the basis of resistance is better known than in others. The various mechanisms of disease resistance are as follows: 1) Mechanical, 2) Hypersensitivity and 3) Nutritional.
1) Mechanical:
Certain mechanical and or anatomical features of the host may prevent infection. For example, closed flowering habit of wheat and barley prevents infection by the spores of ovary infecting fungi.
2) Hypersensitivity:
In a large number of cases, immune reaction is due to the hypersensitive reaction of the host. This mechanism is found in case of biotrophic organism or obligate parasites.
3) Nutritional:
The reduction is growth and in spore production is generally supposed to be due to an unfavourable physiological conditions within the host. Most likely, a resistant host does not fulfil the nutritional requirements of the pathogen and thereby limits its growth and reproduction. However, more precise information is not available on this aspect.