**2. Pathogens (***Fusarium* **spp.)**

Fusarium species are widely distributed in soil, aerial plant parts, plant debris, and other organic substrates. A genus *Fusarium* is a large group of hyaline filamentous fungi [19]. Fusaria are common soil saprophytes and are also known as phytopathogens [36]. The genus Fusarium currently contains over 20 species [37]. Some Fusarium species have a teleomorphic state [38]. The commonest species include *Fusarium solani*, *F. oxysporum*, *F. equisetti* and *F. chlamydosporum* [39]. Two

### *Current Status of* Fusarium *and Their Management Strategies DOI: http://dx.doi.org/10.5772/intechopen.100608*

*Fusarium* species were recently included in the list of the top ten plant pathogenic fungi with both economic and scientific importance [11]. This genus interacst with plants as endophytic root colonizers [40]. they may be responsible for a wide range of human infections [41]. *Fusarium* genus has more than 1500 species and several strains occur on plants/animals producing mycotoxins. *Fusarium* belongs to Phylum—Ascomycota, Order—Hypocreales and Family—Nectriaceae. *Fusarium* species are complex includes plant pathogens, human pathogens, and non-pathogens. Pathogenic strains are morphologically indistinguishable from nonpathogenic strains. *Fusarium* pathogens are persisting in the soil as chlamydospores, cause infect through the feeder rootlets and then colonize the vascular system, leading to severe wilting and death of plants. Important species are *Fusarium oxysporum*, *F. solani*, *F. fujikuroi* and *F. graminearum* well known plant-pathogens. It's characterized by fast-growing colonies with floccose aerial mycelium, colony pigmentation from pale, rose, burgundy to bluish violet depending on species and growth conditions. *Fusarium* usually produces pale violet to the dark magenta pigment in agar media (some do not produce). Conidia are often produced in sporodochia which are slimy dots in the culture, macroconidia are fusiform, multi-celled by transverse septa and characteristic foot-shaped basal cell pointed apical cell. Some species also produce microconidia are mostly single-celled, in some cases three to five celled and vary from globose, oval and fusiform. A few species produce microconidia in chains and others in slimy. *Fusarium* characteristics by morphological conidia in size and shape of macroconidia, the presence or absence of microconidia and chlamydospores, colony color and conidiophore structure [42]. Macroscopic and microscopic features, such as the color of the colony, length and shape of the macroconidia, the number, shape and arrangement of microconidia, and presence or absence of chlamydospores are key features for the differentiation of Fusarium species [43]. The *Fusarium oxysporum* is a soil borne fungi found in cultivated and uncultivated soils worldwide [7]. *F. oxysporum* have high functional and genetic diversity [6]. *F. oxysporum* can affect perennial and annual plants, including aquatic plants (lotus), cause wilts and crown rot on field crops, garden, ornamental crops and weeds (broomrape and witchweed). Strains with the same host range are grouped into *forma specialis*. In some *formae speciales* are subdivided into races by cultivar specialization [44]. Based on size and shape of macroconidia, presence or absence of microconidia and chlamydospores, colony color, and conidiophore Fusaria classified [42]. Morphological pictures of plant pathogenic, saprophytic and bio-control strains of *F. oxysporum* are indistinguishable. Based on taxonomic fusaria recognizing more than 100 species [23–25]. Pathogenic strain is very host specific, attacking only one or a few species and certain cultivars and designated as *formae speciales* and race of the pathogen. Proposed a system of classification of *F. oxysporum* strains, on basis of vegetative compatibility group (VCG), but not a universal tool of identify *formae speciales* or non-pathogenic isolates [45]. Nitrate reductase and phosphate permease have been used successfully to distinguish *Fusarium* species [46]. The presence or absence of microconidia is a primary character in *Fusarium* taxonomy. *Fusarium* teleomorphs have been described, classified into several different genera (*Gibberella*, *Nectria*) (**Figure 1**) [47].

## **2.1 Identification**

There are three basic concepts for identification of *Fusarium* sp., by morphology can differentiate species, biological as sexual viable and phylogenetic as the common origin of the same species. Colonies character on potato dextrose agar of *Fusarium* species. *F. oxysporum* and *F. solani* can establish in suppressiveness soil than other species. *Fusarium* microconidia are oval to kidney-shaped, generally

#### **Figure 1.**

*(a) macroconidia, (b) microconidia, (c) chlamydospores, (d) conidia and conidiophores of* Fusarium *spp.*

one-celled produce on short conidiophores on aerial mycelia and enter into the sap stream transported upward, macroconidia are fusiform having three to five cells and produced large numbers on sporodochia and chlamydospores are usually two types one within the macroconidium and other within the mycelium, formed singly/in pairs or chains with thick-walled, survive in the soil for a long time. A system of classification of strains of *F. oxysporum*, based on their vegetative compatibility as a described method based on pairing nitrate non-utilizing mutants to determine the vegetative compatibility group (VCG) of each strain and use of various molecular tools that group together genetically similarity in strains [45]. VCG cannot be used as a universal tool to identify *formae speciales* or nonpathogenic isolates only molecular tools can provide information for a taxonomic framework for species identification to relationships among species. Sequences of the β-tubulin region have been useful to distinguish some Fusaria [48]. Use nuclear restriction fragnment length polymorphism (RFLP) and VCG to determine *F. oxysporum* f. sp. *radicis-lycopersici* [49]. Use random amplified fragment length polymorphisms (RAPD) to differentiate races of *Fusarium oxysporum* f. sp. *vasinfectum* on cotton [50]. DNA sequences of the ITS regions are very useful in distinguishing species in many eukaryotic organisms, but not is very informative for Fusarium [51]. Random amplified polymorphic DNA identify sequence-characterized amplified region (SCAR) markers. Many *formae speciales* are known to be polyphyletic, making it difficult to identify specific molecular markers [52, 53]. Molecular methods, such as 28S rRNA gene sequencing, may be used for rapid identification of Fusarium strains to species and subspecies levels [54]. Polymerase chain reaction (PCR) based rDNA detection method [55] and detection of protein banding patterns by SDS-PAGE and esterase isozyme electrophoresis [56]. Cultures of Fusarium species grown on Sabouraud Dextrose Agar at 25°C produce wooly, cottony, flat or spreading colonies [57]. *F. oxysporum* are responsible for severe damage on many economically important plant species and show a high level of host specificity, based on infection of the plant species and plant cultivars they are classified into more than 120 *formae speciales* and races [58]. Molecular tools are providing species identification as well as evolutionary relationships among species.
