A wide range of fungal, bacterial and viral pathogens, affect the yield of cultivated crop species, especially in tropical climates. Crop losses can often be significant, up to 20-30 per cent, or sometimes even total.
In this situation, breeding and development of cultivars resistant to disease enhances food production. This also helps reduce the dependence on use of fungicides and bacteriocides. Resistance of the host plant is the ability to prevent the pathogen from causing disease and is determined by the genetic constitution of the host plant.
Before breeding is undertaken, it is important to know about the causative organism and the mode of transmission. Some of the diseases caused by fungi are rusts, e.g., brown rust of wheat, red rot of sugarcane and late blight of potato; by bacteria– black rot of crucifers; and by viruses – tobacco mosaic, turnip mosaic, etc.
Methods of breeding for disease resistance: Breeding is carried out by the conventional breeding techniques (described earlier) or by mutation breeding. The conventional method of breeding for disease resistance is that of hybridisation and selection.
It’s steps are essentially identical to those for breeding for any other agronomic characters such as high yield. The various sequential steps are : screening germplasm for resistance sources, hybridisation of selected parents, selection and evaluation of the hybrids and testing and release of new varieties.
Some crop varieties bred by hybridisation and selection, for disease resistance to fungi, bacteria and viral diseases are released.
Conventional breeding is often constrained by the availability of limited number of disease resistance genes that are present and identified in various crop varieties or wild relatives.
Inducing mutations in plants through diverse means and then screening the plant materials for resistance sometimes leads to desirable genes being identified. Plants having these desirable characters can then be either multiplied directly or can be used in breeding.
Other breeding methods that are used are selection amongst somaclonal variants and genetic engineering. Mutation is the process by which genetic variations are created through changes in the base sequence within genes (see Chapter 5) resulting in the creation of a new character or trait not found in the parental type.
It is possible to induce mutations artificially through use of chemicals or radiations (like gamma radiations), and selecting and using the plants that have the desirable character as a source in breeding – this process is called mutation breeding.
In mung bean, resistance to yellow mosaic virus and powdery mildew were induced by mutations. Several wild relatives of different cultivated species of plants have been shown to have certain resistant characters but have very low yield.
Hence, there is a need to introduce the resistant genes into the high-yielding cultivated varieties. Resistance to yellow mosaic virus in bhindi (Abelmoschus esculentus) was transferred from a wild species and resulted in a new variety of A. esculentus called Parbhani kranti.
All the above examples involve sources of resistance genes that are in the same crop species, which has to be bred for disease resistance, or in a related wild species. Transfer of resistance genes is achieved by sexual hybridisation between the target and the source plant followed by selection