**4. Symptomatology**

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 system [20, 21].

#### *Genetic Variation*

After successfully infecting the roots, the pathogen grows towards the rhizome and pseudostem, causing a deficiency in the absorption of water and consequently an eventual wilting of the leaves and finally causing the death of the plant [9, 16]. This pathogen has the ability to invade all the organs of the plant with the exception of the fruit [16].

Externally, the first signs of the disease are usually wilting and yellowing of the older leaves around the margins (**Figure 3A**), the older chlorotic leaves collapse (**Figure 3B**), the old leaves hang down and dry forming a skirt (**Figure 3C**), the suckers are shown asymptomatic (**Figure 3D**), while internally the vascular bundles of the pseudostem turn reddish brown (**Figure 3E**), the corm shows an abnormal dark brown discoloration (**Figure 3F**), the base of the pseudostem shows fissures (**Figure 3G**) and the midrib of the leaves shows a dark brown discoloration (**Figure 3H**) [5, 6, 9].

To better understand the process of the Foc-banana interaction, some investigations have emerged using isolates transformed from Foc with the gene for the

#### **Figure 3.**

*External and internal symptoms caused by* Fusarium oxysporum *f.sp. cubense in banana and plantain plants (*Musa *spp.). Chlorosis in older leaves around the margins (A). Older leaves collapsed (B). Hanging and dried leaves forming a skirt (C). Asymptomatic children (D). Reddish-brown vascular bundles of the pseudostem (E). Corm with abnormal dark brown discoloration (F). Fissures at the base of the pseudostem (G). Central rib with dark brown discoloration (H).*

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

inoculation.

**5. Epidemiology**

plantations [25].

soil [26].

disease [6].

seedlings [6].

green fluorescent pigment (GFP), with the aim of studying the movement of the

Recently [24], using GFP they demonstrated the movement of the pathogen before the appearance of external symptoms, as well as the presence of inoculum on the external surface of the veins of senescent or decomposing leaves, followed by the substantial production of macroconidia and chlamydospores, these results demonstrate that there may be serious implications regarding the spread of the pathogen. In addition, chlamydospore production occurs inside and outside the veins of the leaves, which increases the risk of spores returning to the ground through leaf removal. Also, it was possible to identify the progress of the pathogen in the pseudostem before the development of external symptoms. The authors suggest that future studies are required on the possible wind-borne spread of inoculum and the potential of the pathogen to infect a healthy plant through aerial

Fusarium wilt is a "polycyclic" disease. However, several cycles of infection can occur in affected banana plantations. Losses can eventually develop, even when very small amounts of the pathogen inoculum manage to infest fields and the disease is initially of little concern to growers [6]. For example, the first outbreaks of TR4 reported in China and the Philippines were not taken with great importance; this resulted in devastation and uncontrollable problems in the affected

In addition to prevention, early recognition and rapid containment of a disease outbreak is necessary to prevent epidemic development. A good understanding of the key factors responsible for the development of the disease is required when designing practical protocols for the destruction of infected plants, the treatment of the surrounding infested soil, and the reduction of inoculum in plant residues and

Foc was shown to have the ability to survive for decades in infested soil, as "Gros Michel" production was generally impossible in plantations previously affected by Foc [9]. Chlamydospores of Foc in dead host material play a role in their survival, but their persistence for long periods is probably due to their ability to infect weed species [6]. For example, in studies in tropical America and Australia, Foc was isolated from the roots of various weed species (*Chloris inflata, Euphorbia heterophylla Tridax procumbens, Cyanthillium cinereum, Commelina diffusa*, *Ixophorus unisetus*, *Panicum purpurascens*, *Cyperus luzulae*, *Paspalum fasciculatum*), present in banana plantations that were affected by R1 and TR4 [27]; however, these are asymptomatic and their presence in banana fields could be of high risk and therefore it is important to carry out a targeted control to reduce their presence. Foc's ability to survive in the absence of its host is an important factor in the management of this

Foc has been shown to spread in various ways, with infected suckers being the most efficient, since they are the most used as vegetative material for new plantations [9]. In many cases, the suckers are washed and treated with fungicides. However, infected suckers were the main material before tissue culture seedlings were available [6], being practically impossible to establish plantations free of the pathogen. However, even after it was possible to produce tissue culture material, secondary contamination of plantations by Foc was common. For example, TR4-affected Cavendish plantations were routinely established with tissue culture

pathogen from the soil towards the roots and rhizome [22, 23].

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

green fluorescent pigment (GFP), with the aim of studying the movement of the pathogen from the soil towards the roots and rhizome [22, 23].

Recently [24], using GFP they demonstrated the movement of the pathogen before the appearance of external symptoms, as well as the presence of inoculum on the external surface of the veins of senescent or decomposing leaves, followed by the substantial production of macroconidia and chlamydospores, these results demonstrate that there may be serious implications regarding the spread of the pathogen. In addition, chlamydospore production occurs inside and outside the veins of the leaves, which increases the risk of spores returning to the ground through leaf removal. Also, it was possible to identify the progress of the pathogen in the pseudostem before the development of external symptoms. The authors suggest that future studies are required on the possible wind-borne spread of inoculum and the potential of the pathogen to infect a healthy plant through aerial inoculation.
