**2. Biotic stress resistance in wheat**

#### **2.1 Types of disease resistance**

There are basically two types of genetic resistances as described by Vander Plank [6] for the different diseases in wheat i.e. Qualitative/Vertical resistance and Quantitative/Horizontal resistance.

#### *2.1.1 Qualitative (vertical) resistance*

It is specified to pathogen races controlled by a single or few genes i.e. monogenic or oligogenic. Race-specific is used to describe resistance that interacts differentially with different pathogenic races i.e. it is applied both to complete resistance and components of incomplete resistance that so interact [7]. This kind of resistance is easily detectable with specific pathogenic races or pathotypes which are controlled by genes with major effects. In wheat rust pathosystems, these resistances are recognized by characteristic low infection types. Most of these genes can be detected in seedling evaluations using specific pathotypes. For every resistance gene in the host plant, there is a corresponding virulent gene in the pathogen as stated by gene for gene hypothesis. However the ability of a virulent gene to mutate to avirulent gene, no longer recognizable by the corresponding resistance gene, implies a type of resistance termed race-specific resistance.

#### *2.1.2 Quantitative (horizontal) resistance*

This kind of resistance varies in continuous way among the different phenotypes of the host population, ranging from almost imperceptible to quite strong resistance response. The resistance expression depends upon the genotype and environment, where pathogen is the part of that environment. The environment can considerably

*Breeding Wheat for Biotic Stress Resistance: Achievements, Challenges and Prospects DOI: http://dx.doi.org/10.5772/intechopen.97359*

affect its durability also [7]. Partial resistance is supposed to be under polygenic control and such resistance will be race-nonspecific. Being controlled by minor genes, the quantitative resistance has complex genetic basis which operates against all the pathotypes/races of that specific pathogen. Race-nonspecific resistance is mainly effective at the post-seedling and adult plant stages and adult plant resistance (APR) is often detected as field resistance [8]. The best known APR genes in wheat are *Sr2* (stem rust resistance gene) and *Lr34*, a gene that provides resistance to leaf and stripe rust and powdery mildew. These genes have been used in commercial wheat varieties for almost 100 years. *Sr2* and *Lr34* have provided partial resistance for decades over large areas and under prolonged disease pressure in the field, proving their durability. Adding to complexity, Ug99 had a very wide spectrum of virulence towards most of the commonly used R genes and rapidly evolved virulence to the important R genes (*Sr24* and *Sr36*) which has impeded the initial emergency breeding response to incorporate resistance to this strain [9].
