**3. Fusarium wilt caused by the fungus** *Fusarium oxysporum* **f. sp.** *cubense* **(Foc)**

*Fusarium* is a genus comprises several species of filamentous ascomycetes, including pathogenic and non-pathogenic species for agricultural crops. One of the best known is *F. oxysporum*, this causes vascular wilt and root rot in more than 100 plant species [12].

In the *Fusarium* system, Foc belongs to the *Fusarium oxysporum* species complex (FOSC), four clades have been identified from this, using the translational elongation factor 1-alpha (tef1) and the rDNA of the mitochondrial subunit (mtssu), in Foc isolates, which were grouped as baseline lineage [13].

The pathogenic isolates of *F. oxysporum* have been classified in more than 100 special forms. Members of a special form usually cause disease in a particular range of host species, with some special forms capable of colonizing a wider range of plants [14]. A special form can be subdivided into races based on characteristic virulence patterns in differential host cultivars [15].

Taxonomic classification: Domain: Eukaryota Kingdom: Fungi Phylum: Ascomycota Class: Ascomycetes Subclass: Sordariomycetidae Order: Hypocreales

One of the most devastating special forms is responsible for Fusarium wilt of bananas and plantains [9], which is caused by *Fusarium oxysporum* Schlect. f. sp. *cubense* (E.F. Smith) Snyder & Hansen, who lives in the soil. The sexual phase (teleomorphic) of the fungus is unknown and cannot be distinguished morphologically between different strains. This pathogen produces three types of asexual spores, these are macroconidia, microconidia and chlamydospores, which function as mechanisms of dispersal, reproduction and survival [16].

The microconidia (5–16 × 2.4–3.5 μm) are oval in shape and consist of a single cell, generally without septa, may be oval, elliptical to reniform, and develop abundantly on false heads on short monophialides. While macroconidia (27–55 × 3.3– 5.5 μm) are abundant, slightly curved, and relatively thin, they have 4–8 cells, with 3–5 septa (generally 3 septa) see **Figure 1A**. The apical cell is attenuated or hookshaped in some isolates. The basal cells are shaped like a foot. Macroconidia develop into single hyphal fialids (**Figure 1B**). Micro and macroconidia occur on branched or unbranched monophial cuts [17, 18].

Chlamydospores (7–11 μm in diameter) are generally globose and form individually or in pairs, they are abundantly formed in hyphae or conidia, single or in chains, generally in pairs, this type of spores constitutes resistance structures of the fungus, These have thick cell walls, and their production is abundant on infected tissues in advanced stages of the disease [4]. They can be interspersed or in the terminal part of the hyphae [17].

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system [20, 21].

*Genetic Diversity of Fusarium Wilt Disease of Banana DOI: http://dx.doi.org/10.5772/intechopen.94158*

*In vitro* development of the pathogen onto potato dextrose agar (PDA) culture

*Colony morphotypes of* Fusarium oxysporum *f. sp.* cubense *isolated from bananas and plantains in different* 

medium, has a variable morphology, its growth is 4 to 7 mm per day at 24°C, forming colonies with abundant aerial mycelium and variable color pigmentation from white, salmon to pale violet. In general, the *F. oxysporum* strains cannot be morphologically distinguished between different races or groups of vegetative compatibility (VCGs). The fungus *F. oxysporum* generally produces black to violet sclerotia, while the pigmentation of the colonies is pale violet to dark red

Some isolates rapidly mutate from pionnotal (with abundant fatty or shiny aggregates of conidia) to a flat, moist pale yellowish-white to peach mycelium

The pathogen *Fusarium oxysporum* f.sp. *cubense* causes a typical wilt syndrome on infected plants, it has a saprophytic and parasitic phase in its life cycle. It begins as a saprophyte in the soil as chlamydospores, which are dormant and immobile until plant exudates stimulate their germination to spread towards the roots [9]. These germinated chlamydospores develop a thallus that produces conidia after 6–8 hours. The conidia germinate and adhere to the roots of the host plant where they penetrate the epidermal cells and then invade and colonize the vascular

on PDA culture media [9, 19], as shown on **Figure 2**.

grown on a PDA culture [9, 19].

**4. Symptomatology**

**Figure 2.**

*states of Mexico.*

*Genetic Variation*

The pathogenic isolates of *F. oxysporum* have been classified in more than 100 special forms. Members of a special form usually cause disease in a particular range of host species, with some special forms capable of colonizing a wider range of plants [14]. A special form can be subdivided into races based on characteristic

One of the most devastating special forms is responsible for Fusarium wilt of bananas and plantains [9], which is caused by *Fusarium oxysporum* Schlect. f. sp. *cubense* (E.F. Smith) Snyder & Hansen, who lives in the soil. The sexual phase (teleomorphic) of the fungus is unknown and cannot be distinguished morphologically between different strains. This pathogen produces three types of asexual spores, these are macroconidia, microconidia and chlamydospores, which function

The microconidia (5–16 × 2.4–3.5 μm) are oval in shape and consist of a single cell, generally without septa, may be oval, elliptical to reniform, and develop abundantly on false heads on short monophialides. While macroconidia (27–55 × 3.3– 5.5 μm) are abundant, slightly curved, and relatively thin, they have 4–8 cells, with 3–5 septa (generally 3 septa) see **Figure 1A**. The apical cell is attenuated or hookshaped in some isolates. The basal cells are shaped like a foot. Macroconidia develop into single hyphal fialids (**Figure 1B**). Micro and macroconidia occur on branched

Chlamydospores (7–11 μm in diameter) are generally globose and form individually or in pairs, they are abundantly formed in hyphae or conidia, single or in chains, generally in pairs, this type of spores constitutes resistance structures of the fungus, These have thick cell walls, and their production is abundant on infected tissues in advanced stages of the disease [4]. They can be interspersed or in the

*Reproductive structure of* Fusarium oxysporum *f.sp.* cubense *(A) Microconidia y (B) Macroconidia.*

virulence patterns in differential host cultivars [15].

as mechanisms of dispersal, reproduction and survival [16].

Taxonomic classification: Domain: Eukaryota Kingdom: Fungi Phylum: Ascomycota Class: Ascomycetes

Subclass: Sordariomycetidae

or unbranched monophial cuts [17, 18].

terminal part of the hyphae [17].

Order: Hypocreales

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**Figure 1.**

**Figure 2.** *Colony morphotypes of* Fusarium oxysporum *f. sp.* cubense *isolated from bananas and plantains in different states of Mexico.*

*In vitro* development of the pathogen onto potato dextrose agar (PDA) culture medium, has a variable morphology, its growth is 4 to 7 mm per day at 24°C, forming colonies with abundant aerial mycelium and variable color pigmentation from white, salmon to pale violet. In general, the *F. oxysporum* strains cannot be morphologically distinguished between different races or groups of vegetative compatibility (VCGs). The fungus *F. oxysporum* generally produces black to violet sclerotia, while the pigmentation of the colonies is pale violet to dark red on PDA culture media [9, 19], as shown on **Figure 2**.

Some isolates rapidly mutate from pionnotal (with abundant fatty or shiny aggregates of conidia) to a flat, moist pale yellowish-white to peach mycelium grown on a PDA culture [9, 19].
