**10. Selection**

Selection is an important method for breeding of varieties resistant to biotic stresses. It is an important means of isolating or identifying sources of disease resistance. Normally the sources of resistance are available in natural populations, wild species, introductions and spontaneous mutants. In the earlier periods selection was accomplished by sequestering the resistant survivors of natural epiphytotics. Now a days advanced artificial epiphytotics are being created and selection of resistant types are being done rather than escaped suscepts [34]. Selection of resistant plants from a commercial variety is the cheapest and quickest method of developing a resistant variety. IIHR Bangalore developed tomato varieties through pureline selection viz., Arka Alok and Arka Ahuti which showing resistant against bacterial wilt.

The production of garden pea is seriously limited by major diseases namely, wilt, powdery mildew and rust. *Fusarium* species cause root rot (*F. solani* f.sp. *pisi* and *F. avenaceum*) or wilt (*F. oxysporum* f.sp. *pisi*). Coyne *et al.* [35] developed three breeding lines (W6 26,740, W6 26,743, W6 26,745) having high level of resistance to Fusarium root rot caused by *F. solani* f.sp. *pisi* with acceptable agronomic traits. In melons the genotypes A19, A32, A30, JAB-11, JAB-20,JAB-3, JAB-7, C384, C67, JAB-9, JAB-18 were identified for powdery mildew resistance.

Pinheiro et al. [36] studied thirty seven pepper genotypes, *Capsicum chinense*, *C. annuum* and *C. frutescens*, were characterized for resistance to three root-knot nematode species (*Meloidogyne javanica, M. incognita* race 1 and *M. enterolobii*). Three experiments were carried out, in 2013, 2014 and 2016, in a greenhouse. Among the genotypes of *Capsicum frutescens* evaluated all were resistant or immune to *M. javanica* and *M. incognita* race 1. In *C. chinense* six accessions were susceptible. In the second experiment all genotypes of *C. chinense* and *C. annuum,* evaluated were resistant to *M. incognita.* In the third experiment, with *C. annuum* genotypes, most were susceptible to *M. incognita* race 1 while CNPH 30118 and CNPH 6144 were resistant to *M. enterolobii*, the most aggressive species. A greater degree of resistance was observed in few accessions of *C. chinense* and *C. frutescens.*

## **11. Hybridization**

The common method used for resistance breeding is hybridization. In this the resistance is transferred by two means. In the first, by backcross method the resistance is transferred from a wild species or a variety with undesirable horticultural attributes to a susceptible but otherwise a desirable variety. In pedigree method the resistance is combined with some desirable characters of one variety and superior characteristics of another variety.

Heterosis a complex biological phenomenon manifested in the superiority of hybrids over parental forms due to the rate of development of one or more complex traits. Heterosis values should be negative for getting tolerant hybrids through heterosis breeding. As far as pest infestation is concerned a hybrid with least incidence might be due to complementation of genes. With the genetic knowledge available on insect resistance in vegetable crops it would be necessary to employ hybridization to transfer single, multiple or additive genes into commercial varieties in the advanced filial generations. The major achievement is expected through inter specific hybridization. In order that fruit quality, yield and resistance to insect pest may preferrably be incorporated in one genotype, it may possibly involve number of parents to achieve desired results.

Inter-varietal and inter-specific crosses, followed by selection, have accounted for the development of resistant hybrids. The production of inter-specific hybrids is useful for the transfer of desirable genes from wild to cultivated species. The Source, mechanism, biochemical and genetical basis of resistance in squash, muskmelon, cucumber and watermelon were studied by several authors.

By utilizing interspecific breeding technique and recombination, borer-free brinjal can be developed to protect the high yield and satisfy the preference of consumers. The parental lines EP 65 and Pusa Uttam had recorded higher yield together with lower fruit and shoot borer incidence. Both of them were found to be good combiners for the above traits. So these parents can be involved in multiple crossing programmes to transfer the resistant genes and to isolate desirable hybrids with high yield and low fruit and shoot borer incidence.

Introgression breeding has been extensively used in the genetic improvement of potato and tomato. In potato twelve traits have been introgressed from wild species viz., *S.demissum, S. spegazzinii, S. stoloniferum, S. chacoense, S. acaule, S. vernei* and in tomato from the wild species *S. peruvianum, S. cheesmanii, S. pennellii* and *S. chilense.* In watermelon, the F 1 (between resistant and susceptible) showed pronounced resistance to fruit fly.

Kishaba *et al*. [37] studied resistance by using several melon aphid-susceptible (MAS) recurrent parents from an initial cross of 'PMR 45' with P1 414,723. Fifteen advanced melon breeding lines with different levels of melon aphid resistance (MAR), their recurrent parents and P1 414,723 were compared in a naturally infested field test for susceptibility to feeding damage by CB. None of the MAR entries were more susceptible than their recurrent parents for fruit damage by CB. P1 414,723 was found to have a low level of resistance to seedling damage, and a high level of resistance to fruit damage from feeding by Cucumber beetle.

Hybridization was undertaken in brinjal with *Solanum viarum* to combine the resistance trait with high yield. The derivatives of the inter specific cross of *Solanum melangena* and *Solanum viarum* EP 65 (accession of *Solanum melangena*) x *Solanum viarum* were assessed till F9 generation. As a result two recombinant progenies *viz.*, 7 and 9 were chosen and carried to the next generation as they had recorded high marketable yield and least infestation of shoot and fruit borer. Molecular confirmation with RAPD primers was done which depicted the introgression of the genes from donor parent *Solanum viarum* to brinjal and thus hybridity was confirmed.

**Backcross method:** This method is widely used for incorporation of resistant gene from a wild species or any variety with undesirable traits to a susceptible variety which is good in other agronomic attributes. The parents from which the resistant gene is to transferred is called as donor parent or non-recurrent parent.

*Breeding Approaches for Biotic Stress Resistance in Vegetables DOI: http://dx.doi.org/10.5772/intechopen.94983*

The susceptible parent in which the resistance gene is transmitted is known as the recurrent parent. The generation up to which the selection process has to be effected in a backcross program would differ according to the allelic relationship of the resistance genes *i.e.,* whether it is resistance or dominant to the allele. Normally the backcross method help in the recovery of recurrent parent phenotype along with the transfer of resistant genes.

[38] investigated hybrid progenies of late blight resistant potato somatic hybrids developed through hybridization with common potato varieties, and also linked ISSR markers with resistant parent/progenies. Potato somatic hybrids (*Solanum tuberosum* di haploid 'C-13' + *S. pinnatisectum*) were back-crossed with potato varieties (*S. tuberosum*) and true potato seed (TPS) were produced. TPS-raised seedlings were advanced to back-cross progenies clones (BC1-C1, BC1-C2, BC1- C3 and BC1-C4) during the five years based on tuber traits in field trials and field resistance to late blight. The BC1-C2 progenies were profiled by ISSR markers and alleles linked to late blight resistant somatic hybrid parent P8 and their progenies (P8 × Kufri Jyoti) were identified. Eight promising advanced hybrids for late blight resistance were identified. This is the first ever report in India towards widening the genetic base of potato by exploitation of interspecific somatic hybrids. The methodology followed was as follows (**Figure 1**).

Powdery mildew is a serious disease of pea and for which new good resistant donors are available, a typical backcross breeding approach as applicable to a character governed by a single recessive gene was outlined by Gritton [39].

**Pedigree method:** It is quite suited when the resistance is governed by horizontal or polygenic. In breeding for disease resistance, artificial disease epidemics are generally produced to help in selection for disease resistance. Pedigree breeding is mainly used when the resistance is governed by major genes and have higher heritability. If it is used for low heritability traits then the selection process will be time consuming as it takes several generations usually F5 or F6 to attain homozygosity

$$\begin{array}{llll} \text{Interpreific factor to simulate hybrid} & \text{Potato variables} & \text{Year I} \\ \text{Solar number between dihigh } \text{C-13}^{-1} & & \text{Hybridization} \\ (+) \text{ S- } p \text{-matrices} & & \\ & \text{TPS risking} & \\ & \text{J} & \text{C-local selection} & \text{Year 2} \\ & & \text{B} \text{C}\_{1} \text{-} \text{C}\_{1} \\ & & \text{J} \text{C-band selection} & \text{Year 3} \\ & & \text{B} \text{C}\_{1} \text{-} \text{C}\_{2} & \\ & & (\text{second back-cross clause}) \\ & & \text{J} \text{C-band selection} & \text{Year 4} \\ & & \text{B} \text{C}\_{1} \text{-} \text{C}\_{3} & \\ & & \text{B} \text{C}\_{1} \text{-} \text{C}\_{3} & \\ & & (\text{third back-cross clone}) & \\ & & \text{J} \text{C-band selection} & \text{Year 5} \\ & & \text{B} \text{C-d}\_{1} \text{-C}\_{4} & \\ & & \text{A} \text{V} \text{-} \text{C-d}\_{4} & \\ & & \text{A} \text{V} \text{-block bk-cross clone} & \\ & & & \text{Advanceed hybrid} & \\ \end{array}$$

**Figure 1.**

*A schematic presentation of development of interspecific potato somatic hybrids (*S. tuberosum + S. pinnatisectum*).*

[40, 41], implemented a quantitative trait locus (QTL) mapping approach to study the inheritance of anthracnose resistance in an F2 population derived from the pedigree of *C. annuum × C. chinense*.

The major crops in which break through research was done and the resistant varieties developed through introgression breeding methods is tabulated below.

