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''Fusarium oxysporum'' f. sp. ''cubense'' is a fungal plant pathogen that causes Panama disease of banana ('' Musa'' spp.), also known as fusarium wilt of banana. The fungi and the related disease are responsible for widespread pressure on banana growing regions, destroying the economic viability of several commercially important banana varieties.

Description

''Fusarium oxysporum'' is a common inhabitant of soil and produces three types of asexual spores: macroconidia, microconidia and chlamydospores.

The macroconidia are nearly straight, slender and thin-walled. They usually have three or four septa, a foot-shaped basal cell and a curved and tapered apical cell. They are generally produced from phialides on conidiophores by basipetal division. They are important in secondary infection.Couteaudier, Y. and C. Alabouvette, 1990 Survival and inoculum potential of conidia and chlamydospores of ''Fusarium oxysporum'' f. sp. ''lini'' in soil. Can. J. Microbiol. 36:551-556

The microconidia are ellipsoidal and have either a single septum or none at all. They are formed from phialides in false heads by basipetal division. They are important in secondary infection.

The chlamydospores are globose with thick walls. They are either formed from hyphae or by the modification of hyphal cells. They endure in soils for long periods and act as inocula in primary infection.

The macroconidia and chlamydospores are normally only formed on dead or dying host plants. Chlamydospores are the most significant survival structures of this pathogen.

The teleomorph or sexual reproductive stage of ''F. oxysporum'' is unknown.

Four races of this pathogen have been described which attack different banana cultivars:
* Race 1 attacks cultivars in the ''Musa'' (AAA group) 'Gros Michel' and caused the 20th century epidemic. It also attacks 'Ducasse', ' Lady Finger', ''Musa'' (AAB group) 'Pome' and its subgroups, Musa (AAB group) 'Silk' and ''Musa'' (ABB group) 'Pisang Awak'. (See .)
* Race 2 attacks ''Musa'' (ABB group) 'Bluggoe' and its close relatives. (See .)
* Race 3 attacks ''Heliconia'' spp. (See .)
* Race 4 attacks ''Musa'' (AAA group) 'Dwarf Cavendish' as well as the hosts of races 1 and 2.Crop Protection Compendium 2005 Edition. ''Fusarium oxysporum'' f.sp. ''cubense'' (Panama disease of banana). (CAB International: Wallingford, UK). (See .)

Taxonomy

A ribosomal intergenic spacer analysis by Kurtz and Schouten 2009 failed to distinguish some ''F. oxysporum'' isolates merely endophytic on ''Musa'' from pathogenic ''Foc'' strains.

Dispersal

Splash by rainfall, movement of contaminated soil, and movement of contaminated propagation materials are the major means of dispersal of ''Foc''. Dispersal by wind alone remains unproven and while animals can test positive for ''Foc'' on their outer surfaces, it remains unproven whether they can be effective vectors. Although it is a soil-borne pathogen, it does not compete well against other soil microbes for growth on dead buried tissue. It is nonetheless able to produce infection in living ''Musa'' hosts after a complete absence of hosts for 20 years - despite a population decline of 97% within the first three years. This is thought to be due to durable chlamydospores and due to persistence as an asymptomatic infection.

Infection process

Anigorufone is a phytoalexin produced by ''Musa''. It is a nematicidal compound and so infection with ''Foc'' induces an anti-nematode defense. Anigorufone is the only nematicidal or nematistatic compound known among the phytoalexins. ''Foc'' rapidly invades cortical cells as do many other ''Fo f.sp.''.

Reproduction

There is some debate as to whether ''Foc'' is sexual and this is investigated by studying its history of recombination - or lack of it. One study of the linkage disequilibrium of gametes showed relatively high disequilibrium and another a high degree of correlation between independent genetic markers, both of which are diagnostic for a lack of recombination and thus a clonal population. Other post-sequencing data analysis performed by the disequilibrium study also ''failed to reject'' recombination however this could be consistent with horizontal transfer. Horizontal transfer has been experimentally induced and appears to have been proven in ''Foc''s past and so seems the more likely explanation. Both ''Fo'' mating types have been observed in ''Foc'' and protoperithecia-like structures are produced, but not the sexual structures. This does not necessarily mean that the sexual process has degenerated however, instead this may be a defect of the experiment.

Spores germinate at a higher rate in the presence of ''Musa'' root secondary metabolites from susceptible cultivars than those from resistant cultivars. This suggests that inhibition of germination is an important part of host resistance.

Tropical Race 1/TR1

/ is also found in '' Paspalum fasciculatum'', ''Panicum purpurescens'', ''Ixophorus unisetus'', and ''Commelina diffusa'' in Central America. These weeds may be acting as an inoculum source.

Tropical Race 2/TR2

Tropical Race 3/TR3

/ is a pest of ''Heliconia'' ornamental flowers. Formerly reported to be a lesser pest of ''Musa balbisiana'' seedlings and of Gros Michel, but that is no longer thought to be true. Now renamed ''Fusarium oxysporum'' f. sp. ''heliconiae''.

Race 4

Tropical Race 4/TR4

/ belongs to vegetative compatibility group 01213/16. All cultivars which are susceptible to Race 1 and Race 2 are susceptible to TR4 (see and ). Starting in 2019 some authorities are following Maryani ''et al.'', 2019 in regarding this strain as '.

Subtropical Race 4/STR4

/ is a subtropical race and does not become symptomatic on Cavendish until the trees are stressed by cold. Also found in '' Paspalum'' spp. and ''Amaranthus'' spp. in Australia. These weeds may be acting as sources of inoculum.

Research

Much research is being undertaken because of the urgency in formulating effective control methods for Panama disease and breeding resistant banana cultivars. Researchers at University Sains Malaysia are examining variability in the genome of the pathogen and its genetic variability is being studied, as are the evolutionary relationships within vegetative compatibility groups of the pathogen.

Research into the phylogenetic relationships among the different strains of ''F. oxysporum'' that cause wilt of banana has been undertaken to determine whether the strains that are specific to the banana have descended from a common ancestor or have developed independently. Results of this study show that it is not monophyletic and appears to have multiple evolutionary origins. The largest lineages of ''F. oxysporum ''f. sp.'' cubense'' ( and ) are genetically distinct from a lineage originating from East Africa () and developed pathogenicity for bananas independently from one another.

Identification, differentiation, and usage of vegetative compatibility groups is useful and valid within ''Foc'' because there are relatively few VCGs.

See also

* List of banana and plantain diseases

References

Further reading

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External links

Information on Fusarium wilt on Musapedia
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{{Taxonbar, from=Q5509901

Fungal plant pathogens and diseases
Banana diseases
oxysporum f.sp. cubense
Forma specialis taxa
Banana Wars