**4. Pathogen diversity**

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

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,

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

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.

**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.

nutrients to the plant that causes wilting, discoloration, and eventually death of the plant.

122 Fusarium - Plant Diseases, Pathogen Diversity, Genetic Diversity, Resistance and Molecular Markers

Bihar, West Bengal, Assam, Rajasthan, Haryana and Punjab [22].

thrives at 22–25°C temperature, with warm and dry soil conditions [52].

may be present that appears similar to *Fol*.

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

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 Compatibility Group [59].

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].
