Method of Breeding for Disease Resistance

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Method of Breeding for Disease Resistance

The method of breeding for disease resistance is essentially the same as those for other agronomic characters. The following breeding methods have commonly used, 1) Selection, 2) Introduction, 3) Mutation, 4) Hybridization, 5) Somaclonal Variation, and 6) Genetic engineering.

1. Introduction:

This is easy and rapid method of developing disease resistant variety. The resistant variety may be introduced and after testing, if found suitable, can be released in the disease prone area. In 1860, the grape crop in France was completely destroyed by the attack of Phylloxera Vertifolia. Introduction of resistant root stocks to this pest from USA saved the grape crop from extinction in France.

2. Selection:

When the source of resistance is a cultivated variety, mass selection and pure lines selection in self pollinated crops, mass and recurrent selection in cross pollinated species, and clonal selection in the vegetatively propagated crops will be ideal for isolating disease resistant plants. The resistant plants may be multiplied, screened for disease resistance and released a variety.

3. Hybridization:

Hybridization is used when resistant genes are available either in the germplasm or in wild species of crop plants. After hybridization, the hybrid material is handled either by pedigree method or by backcross method. The pedigree method is used when the resistance is governed by polygene and the resistant variety is an adapted one which also contributes some desirable agronomic traits. The backcross method is used when resistance to governed by oligogenes.

Induced mutations are also use for disease resistance. Many disease resistant varieties have been developed in various crops through induced mutations.

4. Mutation:

We have already considered briefly the usefulness of spontaneous as well as induced.

5. Somaclonal Variation:

Disease resistant soma clonal variants can be obtained in the following two ways, firstly , plants regenerated from cultured cells or their progeny are subjected to disease test and resistant plants are isolated. Secondly, cultured cells are selected for resistance to the toxin or culture filterate produced by the pathogen and plants are regenerated from the selected cell. In most cases, these plants are also resistant to the disease in question. Cell selection strategy is most likely to be successful in cases where the toxin is involved in disease development.

6. Genetic Engineering:

Genes expected to confer disease resistance are isolated, cloned and transferred into the crop in question. In case of viral pathogens, several transgenes have been evaluated, viz, virus coat protein gene, DNA copy of viral satellite RNA, defective viral genome, antisense constructs of critical viral genes, and ribozymes. Viral coat protein gene approach seems to be the most successful. A virus transgenic variety of squash is in commercial cultivation in U.S.A.

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