Fusarium oxysporum f.sp. cubense
(Panama disease of banana)

F. oxysporum f.sp. cubense (Foc) is considered invasive because it can be distributed from location to location and from country to country with traditional planting material. Also, once established it can spread within plantations in runoff water and in soil on the tyres/wheels of farm machinery, feet of farm animals and shoes of farm workers. Once farm soil is contaminated, susceptible cultivars can only be grown with great difficulty and with much crop loss. Tropical race 4 (TR4) isolates of the pathogen threaten the production of Cavendish cultivars, which produce the bulk of export bananas (see Biology and Ecology).

F. oxysporum f.sp. cubense is believed to have evolved with wild and cultivated banana in South-East Asia because it is in this region that most genetic diversity of the pathogen occurs (Pegg et al., 1993, Bentley et al., 1995). Of all the locations in South-East Asia, the area encompassing Java, Sumatra, peninsular Malaysia and possibly Borneo would seem the most likely centre of origin and has been indicated as such in the above list. There is also some evidence for an independent origin in Malawi, Africa (see Biology and Ecology), but more research is needed to verify this theory.

F. oxysporum f.sp. cubense is probably present in most areas where bananas are grown in Asia, Africa and the Americas. Exceptions seem to be parts of the South Pacific and Melanesia, some countries bordering the Mediterranean and Somalia. A record for Israel in CABI/EPPO (1999) is based on specimens from 1952 which cannot be verified. In the absence of published reports and with information from the Israeli Plant Protection and Inspection Services, this record is now considered unreliable.

The latest locations to have recorded the pathogen are the islands of New Guinea (Shivas et al., 1996; Shivas and Philemon, 1996; Davis et al., 2000) and Yap in the Federated States of Micronesia (Smith et al., 2002). The progression of the disease across Wallace's line into Melanesia is significant (Ploetz and Pegg, 1997). It is likely that the disease has only very recently been introduced to these areas through the movement of infected planting material from South-East Asia. The replacement of traditional banana cultivars susceptible to black leaf streak by South-East Asian cultivars resistant to the disease has been noted in the Federated States of Micronesia (Watson, 1993). Introduced germplasm could have carried the Fusarium wilt pathogen to Yap. Circumstantial evidence indicates that migrants bringing planting material from Java may have introduced the pathogen to the island of New Guinea (Ploetz and Pegg, 2000).

The pathogen is commonly spread locally, nationally and internationally in infected rhizomes or suckers and attached soil. However, infected plant material may not exhibit symptoms. The fungus can also be spread in soil attached to farm machinery, implements and vehicles.

Although Fusarium wilt is found in most banana-growing regions, it has not been confirmed as present in many South Pacific countries. Banana material should only be introduced to this region as tissue cultures which would be free of the disease.

At least one pathotype of F. oxysporum f.sp. cubense, which is capable of attacking Cavendish cultivars (AAA genome) in the tropics, exists in South-East Asia, but is not found in the Latin American/Caribbean region where most Cavendish bananas for export are produced. Plantains (AAB genome), which are an important food resource in Africa and the Americas, have succumbed to subtropical race 4 in Australia. Care must be taken to ensure that strains that are now of limited distribution, but which have the capability of causing severe damage in other parts of the world, are not inadvertently moved to other countries. Movement of Musa germplasm by tissue culture is seen as essential to prevent dissemination of the more potent pathotypes (Jones and Diekmann, 2000).

F. oxysporum f.sp. cubense is one of around 100 formae speciales (special forms) of F. oxysporum which cause vascular wilts of flowering plants (Gerlach and Nirenberg, 1982). Hosts of the various formae speciales are usually restricted to a limited and related set of taxa. As currently defined, F. oxysporum f.sp. cubense affects the following species in the order Zingiberales: in the family Musaceae, Musa acuminata, M. balbisiana, M. schizocarpa and M. textilis; and in the family Heliconeaceae, Heliconia caribaea, H. chartacea, H. crassa, H. collinsiana, H. latispatha, H. mariae, H. rostrata and H. vellerigera (Stover, 1962; Waite, 1963). Additional hosts include hybrids between M. acuminata and M. balbisiana, and M. acuminata and M. schizocarpa.

F. oxysporum f.sp. cubense may survive as a parasite of non-host weed species. Three species of grass (Paspalum fasciculatum, Panicum purpurascens [Brachiaria mutica] and Ixophorus unisetus) and Commelina diffusa have been implicated (Waite and Dunlap, 1953).

Reproductive biology

Fusarium wilt is a lethal fungal disease of banana. The causal pathogen attacks the banana's vascular system. As diseased plants die, the fungus grows out of the xylem into surrounding tissues. Many chlamydospores are formed which are returned to the soil as the plant decays. Chlamydospores are stimulated to germinate and infect nearby banana roots. Following germination, mycelium is produced from which conidia form in 6-8 hours and chlamydospores in 2-3 days. Small secondary or tertiary roots are invaded. No banana cultivars are immune and the fungus is able to establish itself in the vascular system of the root. In susceptible plants, the fungus is not blocked by the host defence mechanisms and the infection becomes systemic through the vascular system of the corm, pseudostem and fruit stalk. In resistant cultivars, the fungus becomes blocked by vascular occluding responses of the host and cannot advance into the corm.

F. oxysporum f.sp. cubense can survive for up to 30 years as chlamydospores in infested plant debris or in the roots of alternative hosts (Stover, 1962).

Physiology and phenology

Pathogenic races in F. oxysporum f.sp. cubense are designated based on banana species and clones attacked. Previously, four races were recognized (Waite and Stover, 1960; Stover and Buddenhagen, 1986; Su et al., 1986; Stover and Simmonds, 1987; Stover, 1990; Ploetz and Pegg, 2000).

Race 1 attacks: Musa textilis (abacá), 'Gros Michel' (AAA genome), 'Maqueño' (AAB genome), 'Silk' (AAB genome), 'Pome' (AAB genome), Pisang Awak (ABB genome), 'I.C.2' (AAAA genome- bred hybrid).
Race 2 attacks: 'Bluggoe' (ABB genome), bred AAAA hybrids.
Race 3 attacks: Heliconia spp.
Race 4 attacks: Cultivars in the Cavendish subgroup (AAA genome), such as 'Dwarf Cavendish', 'Grand Nain' and 'Williams'. Plantains and  all cultivars that are susceptible to races 1 and 2.

Isolates found to attack cultivars in the Cavendish subgroup (AAA genome) in the subtropics only belong to F. oxysporum f.sp. cubense subtropical race 4. The susceptibility of Cavendish cultivars to subtropical race 4 has been attributed to abiotic stress conditions (cold winter temperatures) which weaken the plant’s resistance (Moore et al., 1993; Ploetz and Pegg, 2000).

Isolates that can attack Cavendish cultivars in tropical conditions belong to tropical race 4. This variant is also capable of attacking Cavendish in subtropical conditions (Buddenhaggen, 2009).

Seedlings of Musa balbisiana (BB genome) and 'Gros Michel' (AAA genome) have been reported to be slightly susceptible to race 3. However, as this race does not cause Fusarium wilt on banana it is no longer considered part of the F. oxysporum f.sp. cubense race structure (Ploetz, 2005; Fourie et al., 2011).

Given that host resistance is potentially the most effective means of Fusarium wilt management (see Prevention and Control), variability within F. oxysporum f.sp. cubense and the response of banana genotypes to pathogen variants must be defined. Field testing of isolates is, however, problematic as it is influenced by environmental factors, method of inoculation and other variables. It is also time consuming, expensive and the required set of differentials may not be available. The 'race' system of distinguishing different pathotypes of F. oxysporum f.sp. cubense has proved limiting as exceptions are known, which indicate that more races may exist. As described below, advances in methodology for assessing diversity have shown that the currently defined race structure does not adequately reflect the extent of genetic and phenotypic variability in F. oxysporum f.sp. cubense and needs to be redefined by testing a range of representative isolates on a variety of diverse banana genotypes (Ploetz and Pegg, 2000; Dita et al., 2010; Fourie et al., 2011).

In recent years, F. oxysporum f.sp. cubense isolates have been characterised on the basis of vegetative compatibility group (VCG), ability to produce volatile aldehydes and genetic variability as revealed by application of various molecular analytical techniques (see Biology and Ecology-Genetics).

As a phenotypic characteristic, vegetative compatibility may be subject to selection pressure and does not provide a measure of genetic relatedness. It does, however, provide a good means for assessing diversity within populations and identifying genetically isolated groups in F. oxysporum f.sp. cubense (Glass and Kuldau, 1992; Leslie, 1993; Visser et al., 2010; Fourie et al., 2011). The distribution and relationship between vegetative compatibility groups has provided important information on the dissemination and evolution of the pathogen. Twenty-four VCG have been recognized in F. oxysporum f.sp. cubense, which is a large number for a forma specialis of F. oxysporum and may be a result of the diverse banana host and the presumed old age of the pathosystem. Isolates in some VCG are cross compatible with those in other VCG, resulting in VCG complexes such as VCG 0120-01215, VCG 0124-0125-0128-01220 and VCG 01213-01216. The worldwide distribution of isolates observed for VCG 0120-01215 and VCG 0124-0125-0128-01220 is probably due to their frequent dispersal in banana rhizomes moved to new production areas. In contrast, VCG 0122, 01211, 01212, 01214, 01217 have only been found in Philippines, Australia, Tanzania, Malawi and Malaysia respectively. F. oxysporum f.sp. cubense subtropical race 4 isolates belong to VCG 0120, 0121, 0122, 0129 and 01211 and F. oxysporum f.sp. cubense tropical race 4 isolates to VCG 01213-01216. The majority of VCG are present in Asia, indicating that this is the main centre of origin (Correll et al., 1987; Ploetz and Corrrell, 1988; Moore et al., 1993; Bentley et al., 1995; Ploetz and Pegg, 1997; Ploetz and Pegg, 2000; Buddenhagen, 2009; Fourie et al., 2009; Visser et al., 2010; Fourie et al., 2011).

The use of VCG as a means of determining pathotypes in F. oxysporum f.sp. cubense is limited, as isolates of differing race may occur in one VCG and each race can comprise isolates of more than one VCG. The occurrence of race 1 and race 2 isolates in VCG 0124, for example, prevents the use of VCG to assign pathotypes in Australia (Gerlach et al., 2000).

Genetics

Molecular approaches have been widely used in recent years to analyse genetic variability within and between populations in F. oxysporum f.sp. cubense. They include analysis of microsatellites, simple sequence repeats (SSR), restriction fragment length polymorphisms (RFLP), amplified fragment length polymorphisms (AFLP), random amplified polymorphic DNA (RAPD) and DNA sequencing. These have helped to overcome the limitations of other, non-molecular methods and provided clarification as to the genetic structure and diversity of F. oxysporum f.sp. cubense and how this relates to phenotypic characteristics, including pathogenic races and VCG. Multi-gene phylogenetic analyses have provided a better understanding of the evolution of the fungus, while DNA sequencing has facilitated the development of population specific diagnostic tools (see Diagnosis) (Boehm et al., 1994; Bentley et al., 1995, 1998; Pegg et al., 1995; Koenig et al., 1997; O’Donnell et al., 1998; Ploetz and Pegg, 2000; Groenewald et al., 2006; Lin et al., 2009; Fourie et al., 2009, 2011; Dita et al., 2010).

Multi-gene phylogenetic studies have shown that at least eight distinct phylogenetic lineages exist in F. oxysporum f.sp. cubense, with most lineages consisting of isolates in closely related VCG, even where these are distributed across a broad geographic area. These relationships were supported by RAPD, RFLP and AFLP analyses and suggest a clonal reproductive strategy for the pathogen. As such, diversity in F. oxysporum f.sp. cubense may arise through mutation and/or parasexual recombination. However, analysis of RFLP haplotypes (Koenig et al., 1997) suggests that recombination may have occurred between two lineages (Taylor et al., 1999), while the recently reported occurrence of both mating type (MAT) idiomorph alleles (i.e. MAT-1 isolates and MAT-2 isolates) in some lineages suggests that these may have a sexual origin (Fourie et al. 2009, 2011). Some subtropical race 4 isolates were more closely related to isolates in other formae speciales of F. oxysporum than to those in F. oxysporum f.sp. cubense.

One lineage, comprising isolates from Malawi in VCG 0124, was distinct genetically from other F. oxysporum f.sp. cubense lineages. This may have arisen independently outside the main centre of origin of the pathogen and on a host other than banana (Koenig et al., 1997; Ploetz and Pegg, 2000; Fourie et al., 2011).

The research to date suggests that the ability of F. oxysporum f.sp. cubense to cause Fusarium wilt disease on some or all of the race differential cultivars has evolved convergently, as members of some races occur in different lineages. Various factors – including co-evolution with the host plant and horizontal gene transfer – may have shaped the pathogen’s evolution (Fourie et al., 2011).

Although attempts have been made to correlate genetic diversity in F. oxysporum f.sp. cubense with pathogenicity and the pathogenic races highlighted above, a relationship has not been established.

Environmental requirements

Soils that suppress the disease have been found in several locations (Stover, 1962; Alvarez et al., 1981; Chuang, 1988). In general, these disease-suppressive soils are recognized by the length of time high levels of production can be sustained in the presence of the pathogen. This may be associated with chemical and physical factors, such as a clay fraction of the montmorillanoid type (Ploetz and Pegg, 2000).

Plants grown from tissue-culture have been shown to be more susceptible to Fusarium wilt than conventional planting material (Smith et al., 1998).

Associations

The pathogen is able to colonise and persist in the roots of alternative hosts, including close relatives of banana, and three non-host species of grasses and a non-host weed (Commelinia diffusa), even though these plants remain symptomless under field conditions (Waite and Dunlop, 1953; Ploetz and Pegg, 2000).

Experiments have shown that there is a synergistic interaction between the nematode Meloidogyne incognita and F. oxysporum f.sp. cubense (Jonathan and Rajendran, 1998).

Diagnostic methods to identify F. oxysporum f.sp. cubense have been discussed elsewhere (see Symptoms, Similarities to Other Species/Conditions and Detection and Inspection). However, different races (pathotypes) of the fungus have been recognized (see Biology and Ecology-Physiology and phenology) and, more recently, emphasis has been placed on the identification of vegetative compatibility groups of the pathogen.

Races of F. oxysporum f.sp. cubense have been identified on the basis of host range. As a general rule, race 1 attacks cultivars in the 'Gros Michel' (AAA genome) and 'Pome' (AAB genome) subgroups and the 'Silk' (AAB genome) and 'Pisang Awak' (ABB genome) clones of banana; race 2 attacks 'Bluggoe' (ABB genome) and close relatives; race 3 attacks Heliconia spp. but not banana; race 4 attacks cultivars in the Cavendish subgroup (AAA genome) and hosts of races 1 and 2. The three races attacking bananas can be distinguished by pathogenicity tests using the cultivars 'Gros Michel', 'Bluggoe' and 'Dwarf Cavendish'. 'Gros Michel' is susceptible to races 1 and 4, 'Bluggoe' is susceptible to races 2 and 4 and 'Dwarf Cavendish' is only susceptible to race 4. However, it is not always possible to characterize populations of the fungus using pathogenicity tests because of interactions between plant and environment and because the current set of host differentials may not adequately assess the virulence, which exists among populations in the designated races.

A more thorough and reliable diagnosis can be made by vegetative compatibility group (VCG) analysis, which groups genetically identical or very similar isolates. It can also be used to assess the diversity of strains of F. oxysporum f.sp. cubense in a given region. Although strong correlation between VCG and pathogenicity has been observed in some regions, the occurrence of several races of the fungus within one VCG and more than one VCG in a particular race limits the potential for using VCG for pathotype determination. Currently, 24 VCGs have been described, and a pattern of distribution throughout the world is emerging (Pegg et al., 1994; Ploetz and Pegg, 2000; Fourie et al., 2011).

F. oxysporum f.sp. cubense can also be characterized by production of volatile odours produced when the fungus grows on rice cultures. In Australia, all race 4 isolates produced distinctive volatile aldehydes whereas race 1, race 2 and non-pathogenic isolates of F. oxysporum did not (Moore et al., 1991).

A variety of genetic markers have been used to assess diversity in F. oxysporum f.sp. cubense, such as arbitrarily primed PCR (e.g. RAPD type techniques) to generate DNA fingerprints (see Biology and Ecology-Genetics). Using DNA fingerprints, it is possible to determine the genetic relationships between different VCGs as well as isolates within each VCG (Bentley et al., 1995). Other molecular techniques have also been used (Boehm et al., 1994; Koenig et al., 1997). Information derived through application of such methods has also enabled the development of molecular markers to detect pathogen variants. Recently, Lin et al. (2009) used RAPD-PCR to develop a diagnostic for detection of race 4 isolates in Taiwan. Of significance, use of this technique indicated the presence of subtropical race 4 in Brazil, Costa Rica, Honduras and the USA, where it has not been officially reported. Dita et al. (2010) used polymorphisms observed in the intergenic spacer region (IGS) to develop a rapid and reliable PCR-based diagnostic tool to detect isolates of F. oxysporum f.sp. cubense, tropical race 4 (VCG 01213). Li et al. (2011) diagnosed isolates of race 4 using a PCR diagnosis specific to that race. Isolates were also subjected to amplified fragment length polymorphism (AFLP) analysis. Diagnostic tools such as these may be of major benefit for early detection and monitoring spread of the pathogen.

Pronounced yellowing symptoms of Fusarium wilt are easy to observe in the field. However,very early symptoms are more difficult for the untrained eye to detect. Proof of Fusarium wilt is vascular discoloration in the lower pseudostem or in the rhizome. In countries with Moko disease (Ralstonia solanacearum race 2), which also causes vascular discoloration, it is possible to confuse the two diseases. However, Fusarium wilt does not cause wilting and blackening of young suckers nor a dry rot in fruit as does Moko. In addition, the first symptoms of Moko on rapidly growing plants are the chlorosis, yellowing and collapse of the three youngest leaves, not the older leaves as with Fusarium wilt. Also, the vascular discoloration is concentrated near the centre of the pseudostem with Moko and is not found peripherally, which is common with Fusarium wilt.

Symptoms of Fusarium wilt of bananas can be mistaken for Moko disease caused by Ralstonia solanacearum race 2 and vice versa. The two can be distinguished if the plant has fruit. Fusarium wilt does not affect fruit, but Moko causes an internal dry rot symptom clearly visible if the fruit is cut (see also Detection and Inspection).

Symptoms similar to those caused by Fusarium wilt - such as leaf yellowing - may be caused by other biotic and abiotic factors, including water stress. Care should be taken to avoid attributing such yellowing to Fusarium wilt, by examining the plant for other external and internal symptoms, such as vascular dicolouration. Where possible, the presence of Fusarium oxysporum should be confirmed by isolation from plant tissues onto a suitable growth medium.

Many species of Fusarium look similar in culture to the untrained eye. However, F. oxysporum is readily identified and distinguished from other common species of Fusarium in banana tissue, such as F. pallidoroseum, F. moniliforme [Gibberella fujikuroi] and F. solani. Chlamydospores are lacking in F. moniliforme but present in F. oxysporum after 7-10 days in culture. Microconidia are usually absent in F. pallidoroseum, but present in F. oxysporum. The shape of the macroconidia of F. oxysporum is distinct from that of F. solani. By following well-illustrated keys such as those given by Nelson et al. (1983), F. oxysporum is readily identified (Stover, 1972). However, to determine whether the isolate is the pathogenic F. oxysporum f.sp. cubense and not a soil saprophyte or root inhabitant, it is necessary to carry out a pathogenicity test or use a molecular approach such as DNA analysis. Strains of F. oxysporum can also be classified on the basis of vegetative compatibility, genetic markers and other methods as described elsewhere in this datasheet.

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

There is no satisfactory method to control Fusarium wilt. Chemical control, flood fallowing, crop rotation and the use of organic amendments have not been effective in managing the disease (Ploetz and Pegg, 1999). As a soilborne pathogen producing chlamydospores, the fungus may persist in soil in the absence of the host for many years (Stover, 1962, 1990). Replacement of affected plants with susceptible varieties in infested soil, while it may provide temporary respite and yield benefit, would not be effective in controlling the disease.

Several factors influence the development of Fusarium wilt. The banana cultivar is of primary importance; drainage, environmental conditions and soil type also influence host-pathogen interactions. Suppressive soils, in which microbial populations suppress the pathogen population, were first described in the 1930s in Central America. Such soils have also been recorded in the Canary Islands, Australia and South Africa.

The only effective means of control is by host resistance. Natural sources of resistance exist in wild species and cultivars and in synthetic diploids developed by breeding programmes. Major, conventional breeding programmes are located in Honduras - Fundación Hondureño de Investigación Agricola (FHIA), Brazil - Empresa Brasileira de Pesquisa Agropecuária (EMBRAPA), Nigeria - International Institute of Tropical Agriculture (IITA), Guadeloupe - Centre de Coopération Internationale en Recherche Agronomique pour le Développement (CIRAD) and Cameroon - Centre Régional Bananiers et Plantains (CRBP). These programmes have concentrated on using resistance in 'Pisang Jari Buaya' (AA genome), 'Pisang Lilin' (AA genome) and Musa acuminata subsp. burmannicoides (accession Calcutta 4). Although it has not been possible to breed a 'Cavendish' replacement for Cavendish cultivars by conventional techniques because of fertility constraints, useful hybrids can be bred to replace AAB dessert and ABB cooking banana types. Biotechnology, mutation breeding and somaclonal variation techniques are also being used to produce resistant genotypes. The Taiwan Banana Research Institute (TBRI) have developed clones of Pei-Chiao (AAA, Cavendish subgroup) by somaclonal variation which are much more resistant to Fusarium wilt than the parent plants. These are now being adopted by the local banana industry (Jones, 2000). Efforts are also continuing to develop and evaluate genetically modified bananas with resistance to Fusarium wilt, including Cavendish cultivars resistant to F. oxysporum f.sp. cubense tropical race 4.

Quarantine and exclusion procedures are effective in controlling the disease by restricting the movement of corms, suckers and soil that could be carrying F. oxysporum f.sp. cubense from infested to clean areas.

The use of micropropagated planting material should be encouraged (Jones and Diekmann, 2000), as this, if managed correctly, should be free from contamination by the pathogen. Plants derived from tissue culture and planted in soil where bananas have not been previously grown should remain free of Fusarium wilt for a considerable period.

F. oxysporum f.sp. cubense tropical race 4 is the newest major threat to banana production and trade. Should the pathogen emerge or spread to the major banana producing areas of Latin America, the Caribbean or West Africa, extensive damage and yield loss is possible. Indeed, given the susceptibility of Cavendish cultivars, plantain cultivars and many other cooking and dessert bananas, it has been estimated that 80% of global production is under threat from tropical race 4 (Ploetz, 2005). If not contained, it has the potential to have a devastating impact not only on the export industry but also on the livelihood and food security of millions of people throughout the world. In the absence of resistant cultivars disease monitoring, and the identification and monitoring of the pathogen is critical.