**3. Major diseases of wheat**

There are many diseases found in wheat caused by different microorganisms from fungi to bacteria and viruses. But only a few of them caused by pathogenic fungi are economically important with global implications. The major diseases in wheat (**Table 1**) are stripe rust, leaf rust, stem rust, powdery mildew, loose smut, Fusarium head blight (FHB) and more recently wheat blast (WB) also. Besides Stem rust, which is under control to some extent, Leaf rust and yellow rust have the potential to affect production levels up to 60 and 43 million hectares respectively in Asia if susceptible cultivars were grown [10]. Though fungicidal applications offer control, their use is an added cost to farmers besides being unsafe environmentally. Hence growing resistant cultivars is the most effective and efficient control strategy


**Table 1.**

*Major diseases of wheat with their respective behavior and number of resistance genes identified for each disease (up to 2020).*

[11]. The rusts and mildew diseases are caused by biotrophic fungi (survive by obtaining nutrients from living plant tissues). Among these, *Puccinia* rusts continue to affect and threaten the world's wheat production [12], although powdery mildew has also emerged as an economically important disease. In case of stem rust, the emergence of *Ug99* group of stem rust races placed it among one of the most significant threats to global wheat production [13].

The other diseases like FHB and WB are caused by necrotrophic fungi (facultative parasites feeding on dead tissue during unavailability of living plants). Wheat blast was first identified in Parana, Brazil in 1985 [14]. It is also of utmost significance as WB outbreaks in Bangladesh [15] and more recently in Africa [16] have attracted immediate global attention from the wheat scientists.

Another economically significant disease, Karnal Bunt (KB) of wheat was first reported in Karnal, India [17]**,** soon extended to Northern and Central India. Later, KB was found to occur in Nepal, Pakistan, Iraq, Afghanistan, South Africa, Mexico and USA [18]. The pathogen is seed, soil and airborne in nature, therefore difficult to control after it is introduced and then established over a region. Although host plant resistance is the most effective and economic method of its management but development of KB resistance varieties is difficult task owing to limited genetic variability in hexaploid wheat [19], quantitative inheritance and considerable impact of environment on KB resistance screening [20].

#### **4. Major insect-pests of wheat**

Various insect pests delimit the yields of wheat crop in different agro-climatic zones. Some of these insect pests are foliar aphid complex in irrigated wheat, root aphids in loose soils, pink stem borers in fields having rice stubbles, cut worms in residues, termites in raised beds and brown mites in rainfed conditions [21]. Six different species of aphids are reported to attack cereals. Out of these, Russian wheat aphid and bird cherry-oat aphid are important pests of wheat. The Russian wheat aphid (*Diuraphic noxia*) is a sucking pest of wheat. Aphid attack is characterized

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

by leaf rolling which is the result of toxic injection by the aphid. The rolled leaves serve as a protection site for the insects. Yield losses up to 40% have been reported in case of aphid infection [22]. The bird cherry oat aphid (*Rhopalosiphum padi*) has been reported to affect wheats all over the world. Feeding symptoms are almost absent. Yield losses due to *R. padi* dependent upon the crop stage at which insect attacks. High yield losses upto 24–65% have been reported in case the attack occurs at seedling stage. Losses decrease if attack occurs at later stages [23]. The aphid is also reported to cause significant indirect losses as it is a vector of Barley Yellow Dwarf Virus (BYDV), which is the most important viral disease in cereals. Greenbug (*Schizaphis graminum*) is another sucking pest of the wheat aphid complex. The green bug feeds on wheat leaves and stems, extracting sap from the phloem. Injection of toxins concomitant with feeding further reduces the chlorophyll content thereby inversely affecting the carbon assimilation and overall plant development [23–25].

*Cephus* spp., the wheat stem sawfly has also been reported to cause major losses in wheat. The adult females oviposit into the young stems of wheat. Upon hatching within the stem, the larvae feed voraciously moving up and down in the stem. When the plant attains maturity, larvae migrate to the basal portion of the stem and build a hibernaculum. The stem above the hibernaculum weakens and breaks [26]. The Hessian fly (*Mayetiola destructor*) is another major pest of wheat crop. Larvae damage stems of plants, thereby preventing internode elongation and disrupting nutrient transport. Significant losses (upto 40%) have been reported upon sawfly attack [27].
