**3. Fusarium wilt pathogen**

*Fusarium oxysporum* is a pathogenic fungus commonly found around the world. It is a soil borne ascomycete causing Fusarium wilt, on many economically important crops. The pathogen comprises of over 120 known strains and each of which is specific to unique host plant in which it causes disease. *F. oxysporum* strains infect and kills many commercially harvested crops and legumes. The spores of *F. oxysporum* survive in a dormant stage in the soil for several years and are easily spread in water, it can infect vegetative cuttings, and can transmit to other individuals. Scientists around the world proposed developing *F. oxysporum* as a universal model for the understanding of fungal virulence [35]. *Fusarium oxysporum* infects its host by entering through the root and grows in the plant xylem. It blocks the vascular system and prevents transport of water and nutrients to the plant that causes wilting, discoloration, and eventually death of the plant.

**4. Pathogen diversity**

Compatibility Group [59].

The pathogen can interact with specific host which results in breakdown of plant resistance within very short duration of time [54]. Therefore, it is important to determine the pathogen genetic diversity in *Fol* population, which can be used by plant breeders for disease resistance and can also help in studying its epidemiology, taxonomy, and detection [55]. The amount of genetic variation can be evaluated by molecular markers techniques like Random Amplified Polymorphic DNA (RAPD), Amplified Fragment Length Polymorphism (AFLP), Simple

Fusarium Wilt: A Killer Disease of Lentil http://dx.doi.org/10.5772/intechopen.72508 123

The genetic diversity of *Fol* was studied by different researchers in many countries. Pouralibaba (2005) has studied the pathogenic diversity based on growth media (Czapecs Agar, PDA and Lentil Extract Agar) and a set of host differentials (ILL590, Gachsaran and Moghan lentil genotypes) using 13 isolates collected from Iran and Syria. The results showed that the difference among pathogen population was not related to different aggressiveness properties with no virulence patterns [56]. In other studies, Iranian isolates of *Fol* were grouped into 10 using RAPD markers and to six groups using ISSR markers [57]. In a recent study, 101 *Fol* isolates from five countries in Ilam provinces of western showed low level of genetic variability using Simple Sequence Repeat (SSR) markers [58]. In Algeria, all isolates were in one Vegetative

In India, 43 cultural and morphological groups were grouped into three clusters based on their aggressiveness of lentil genotypes [60]. Datta et al. (2011) showed varying degree of genetic diversity ranging from 54% in case of RAPD to up to 35% with ITS markers of *Fol* collected from different agro-ecologies in India where isolates from north region fall in same cluster, whereas isolates from north east regions and eastern region fall in different group [61]. In Syria, three major groups of *Fol* were identified using RAPD, SSR and ISSR markers [62].

In order to devise strategies for conferring resistance against disease, it is important to have knowledge of pathogen variability and prevalence of particular races in the target environment. The pathogen populations are primarily characterized by its virulence analysis on cultivars carrying differential resistance genes. Many researchers have studied the pathogen variability based on their grown on different solid media and on the basis of their pathogenicity [63, 64]. Later, Pandya et al. (1980) has evaluated the line (Pant-406) against seven races proposed by Kannaiyan and Nene [64], and found it immune to race 5, resistant against races

Belabid et al. (2004) has reported that all the 32 Algerian isolates of *Fol* under study represent a single race but differ in their aggressiveness on the susceptible line on the basis of virulence and vegetative compatibility [66]. In India, on the basis of disease reactions against seven lentil differentials, the isolates were grouped into three clusters [60]. In an another study based on genetic variability, the *Fol* isolates collected from north eastern Indo-Gangetic plains revealed two sub-populations groups [61]. Sallam and Abdel-Monaim (2012) have collected 10 isolates

Sequence Repeat (SSR) and Inter-Simple Sequence Repeat (ISSR).

**5. Race concept in** *Fusarium oxysporum* **f.sp.** *lentis*

3 and 6, and partially resistant against race 4 [65].

The pathogen Fol affecting lentil crop was first reported from Hungary [36] and later from many countries including India [37], USA [38], Czechoslovakia [39], USSR [40], Brazil [41], France [42], Argentina [43], Bangladesh [44], Turkey [45], Syria [26, 46], Myanmar and Pakistan [47], Nepal [48], Ethiopia [49], Egypt [50], Italy [51]. In India, Fusarium wilt is known to limit the production of lentil in the states of Uttar Pradesh, Madhya Pradesh, Himachal Pradesh, Bihar, West Bengal, Assam, Rajasthan, Haryana and Punjab [22].

Wilt appears in the field as patches at both seedling i.e. early wilting and adult stages i.e. late wilting. Early wilting is characterized by sudden drooping and drying of leaves and seedling death (**Figure 1**). The roots are healthy but having reduced proliferation and nodulation and no internal discoloration of the vascular system. Late wilting appears from flowering to late pod-filling stage and sudden drooping of top leaflets of the affected plant and dull green foliage followed by wilting of the whole plant or in the individual branches [45]. The pod filling stage of the plant is severely affected and eventually a huge yield loss occurs. The disease thrives at 22–25°C temperature, with warm and dry soil conditions [52].

A culture of *Fol* display hyaline, septate and much branched mycelium. On media the growth pattern varies from fluffy to appressed and also vary in color from no color to pink. The pathogen is known to produce three kinds of asexual spores; micro conidia, macro conidia and chlamydospores [53]. Microconidia are usually single celled, ovoid or kidney-shaped and hyaline. Macroconidia are usually two to seven celled, long with pointed apical cell and notched basal cell. Chlamydospores are single celled, oval or spherical shaped and thick walled, formed singly in macroconidia or apical or intercalary in the hyphae [53]. In laboratory, the culturing of infected plant tissue should be done with caution because other saprophytic *Fusarium* spp. may be present that appears similar to *Fol*.

**Figure 1.** Lentil wilt disease: (a) lentil plants infected by wilt disease in field; (b) cross section showing internal discoloration of tap root in wilted lentil plant; (c) Fusarium wilt symptoms on artificial inoculated lentil plants.
