**5. Epidemiology**

*Genetic Variation*

of the fruit [16].

(**Figure 3H**) [5, 6, 9].

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

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

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

*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* 

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**Figure 3.**

*rib with dark brown discoloration (H).*

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 plantations [25].

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 soil [26].

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 disease [6].

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 seedlings [6].

Foc has the ability to spread in the soil, which indirectly contaminates in and around plantations, but unfortunately it is also used in nurseries for the propagation of seedlings used for field establishment [25]. Surface waters are easily polluted and use for irrigation of polluted river or pond water is highly risky. In addition, Foc is spread by contaminated tools (shovels, machetes, hoes, etc.), agricultural machinery, clothing and footwear [9, 28]. Any or all of these ways can facilitate the spread of Foc in and around a plantation, and may be possible through other means [6, 28].

Studies carried out in Australia detected TR4 spores in the exoskeletons of the banana weevil (*Cosmopolites sordidus*) and suggested that the insect could be a predisposing agent as a vector of the disease [29].

The recent transcontinental disseminations of TR4, suggest that something other than vegetative material (suckers) was responsible for these long-distance disseminations. Although these outbreaks may have been the result of something as simple as workers' boots impregnated with soil contaminated by Foc spores from plantations in Southeast Asia, or some other means could be responsible such as the entry of machinery from affected areas. Better knowledge is needed to understand the long-distance spread of this pathogen [6].
