Breeding of Fruit and Plantation Crops 2 + 1

Breeding methods for biotic /abiotic stress resistance

Introduction

• An introduced variety resistant to the concerned insect pest and diseases or abiotic stresses may be released for cultivation if it performs well in the new environment and is agronomically desirable. Thus, it is the quickest and perhaps, the earliest method of developing a biotic stress resistant variety. e.g. introduction of Phylloxera vertifoliae resistant grape rootstock from USA to France. Sometimes, the introduced variety may not perform well in the new environment and it may be susceptible to the biotypes of the concerned pest prevalent in the area or to a new insect pests and/or diseases of the area.

Selection

• Biotic/abiotic stress resistant variants may be found in an existing variety of a crop. In such case, selection for insect and disease resistance is practised to isolate biotic stress resistant variety. Screening large number of germplasm for resistance at field level and further confirmation through artificial testing will help in selection of a resistant line which may be directly used as variety or used as donor for developing a hybrid

Hybridization

• When the desired biotic/abiotic stress resistance is present in an agronomically inferior variety of the crop or in a related wild species, hybridization is the only course of action for the breeder e.g. breeding for fruit fly resistant variety in Ber (Vashishtha et al.,1997) However breeding in ber is difficult due to incompatibility, low fruit set etc.

Backcross Method of Breeding

• The backcross is a form of recurrent hybridization by which one or two desirable characteristics are added on to a superior variety, wherein the hybrids and the progenies in the subsequent generations are repeatedly back crossed to one of their parents. The object of back crossing is to transfer one or two desirable characteristics from an inferior variety to a superior variety, disturbing the genotype of the superior variety as little as possible in the process. Backcrossing is particularly well suited for the transfer of one or two simply inherited and easily recognized characters to a variety that excels in most of its characters.
• In a back cross breeding programme, the genetic consequences of repeated back crossing must be understood. Repeated back crossing leads to rapid increase in homozygosity and in the frequency of homozygote’s as that of selfing. The steps involved in back cross breeding depend upon the genetic nature of the gene to be transferred.
• *This resistant material is thus followed to next F3,F4,F5 generation till the desired homozygoisty is obtained.
• Normally, in the sixth back cross progeny (BC6F1) more than 98 per cent of plants would have attained the genotypes of recurrent parent and by 10th back cross (BC10F1) or with BC6F6 almost 99.95 per cent progenies would have become completely homozygous.
• If a recessive gene is to be transferred, the step involved in the backcross breeding programme is different.
• Back cross method of breeding has been generally employed for
a. Inter-varietal transfer of simply inherited characters which is controlled by one or two major genes (e.g. disease resistance, seed colour, plant height)
b. Inter-specific transfer of simply inherited characters – especially to transfer the disease resistance gene from a wild species (e.g. Yellow Vein mosaic resistance in okra).
c. To transfer cytoplasm from one variety or species to another (e.g. Onion)


(iv) Mutation

• Generally, it has not been used to produce a successful biotic stress resistant crop. The reason for this is difficulty in screening of suitable mutations, the failure of such mutagenesis to generate positive changes to the genome and large number of progeny that must be handled.Production of disease resistant plant by non-conventional breeding