Fusarium oxysporum f.sp. cubense
Encyclopedia
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.
have large, hard seeds, these have been bred out of the modern culinary banana which is seedless. Banana plants are therefore propagated asexually
from offshoots. Because these rhizomes are usually free of symptoms even when the plant is infected by F. oxysporum f. sp. cubense, they are a common means by which this pathogen is disseminated. It can also be spread in soil and running water, on farm implements or machinery.
Panama disease is one of the most destructive plant diseases of modern times. It is believed to have originated in Southeast Asia and was first reported in Australia in 1876. By 1950 it had spread to all the banana-producing regions of the world with the exception of some islands in the South Pacific, the Mediterranean, Melanesia and Somalia.
Panama disease affects a wide range of banana cultivars; however, it is best known for the damage it caused to a single cultivar in the early export plantations. Before 1960, a total reliance was put on the cultivar 'Gros Michel', and it supplied almost all the export trade. It proved susceptible to the disease and the use of infected rhizomes to establish new plantations caused widespread and severe losses. Some indication of the scale of the losses is demonstrated by the complete eradication of production on 30,000 hectares of plantation in the Ulua Valley of Honduras between 1940 and 1960. In Suriname, an entire operation of 4,000 hectares was out of business within eight years and in the Quepos area of Costa Rica, 6,000 hectares were destroyed in twelve years.
By the mid-1900s, resistant cultivars in the 'Cavendish
' subgroup were being used as a substitute for 'Gros Michel' in the export trade. These cultivars have proved resilient and grow well in the western tropics, remaining the clones on which the current export trades are based. Unfortunately, in several growing areas in the Eastern Hemisphere these cultivars are susceptible to the disease. It is considered inevitable that this susceptibility will spread to the Western Hemisphere, and this poses a significant threat to production because there are currently no acceptable replacement cultivars. To make things worse, this variant of the pathogen also affects plantain
s, Musa acuminata × balbisiana
, which are an important staple food
in tropical regions of the world. The average American eats 26.2 pounds of the Cavendish banana each year and the question is being asked as to whether this oft-consumed fruit is on course to extinction.
Apart from the export trade, 85% of banana production is for local consumption and many of the cultivars used for this purpose are also susceptible to infection.
is a common inhabitant of soil and produces three types of asexual
spore
s; 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 phialide
s on conidiophores by basipetal division. They are important in secondary infection.
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:
. This is followed by the formation of tylose
in the vascular
vessels which blocks the movement of water and nutrients to the upper parts of the plant. The tips of the feeder roots are the initial sites of infection which then moves on to the rhizome
. The signs of the disease are most noticeable as a dark stain where the stele
joins the cortex
. As the disease develops, large portions of the xylem
turn a reddish-brown colour. Externally, the oldest leaves start turning yellow and there is often a longitudinal splitting of the lower part of the outer leaf sheaths on the pseudostem. The leaves begin to wilt and may buckle at the base of the petiole. As the disease progresses, younger leaves are affected, turn yellow and crumple and the whole canopy begins to consist of dead or dying leaves.
s are largely ineffective. Chemical sterilisation of the soil with methyl bromide significantly reduced incidence of the disease but was found to be effective for only three years after which the pathogen had recolonised the fumigated areas. Injecting the host plants with carbendazim
and potassium phosphonate appears to provide some control but results have been inconclusive. Heat treatment of soil has also been tried in the Philippines but the pathogen is likely to reinvade the treated area. The greatest hope for managing this disease in infested soils is the development of genetic modification
s that will provide resistant cultivars.
Modified bananas developed in collaboration by Ugandan and Belgian scientists were reported in 2008 to be being grown experimentally in Uganda.
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.
Fungus
A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds , as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, and bacteria...
plant pathogen that causes Panama disease
Panama disease
Panama disease, a Fusarium wilt, is a banana plant disease caused by the fungus Fusarium oxysporum. The fungus attacks the roots of the banana plant. The disease is resistant to fungicide and cannot be controlled chemically.-History:...
of banana
Banana
Banana is the common name for herbaceous plants of the genus Musa and for the fruit they produce. Bananas come in a variety of sizes and colors when ripe, including yellow, purple, and red....
(Musa spp.), also known as fusarium wilt of banana.
Overview
Although fruit of the wild banana, Musa balbisianaMusa balbisiana
Musa balbisiana is a species of wild banana native to South Asia. It is one of the ancestors of modern cultivated bananas along with Musa acuminata. It grows lush leaves in clumps and grows with a more upright habit than most cultivated bananas. Flowers grow in inflorescences coloured red to...
have large, hard seeds, these have been bred out of the modern culinary banana which is seedless. Banana plants are therefore propagated asexually
Asexual reproduction
Asexual reproduction is a mode of reproduction by which offspring arise from a single parent, and inherit the genes of that parent only, it is reproduction which does not involve meiosis, ploidy reduction, or fertilization. A more stringent definition is agamogenesis which is reproduction without...
from offshoots. Because these rhizomes are usually free of symptoms even when the plant is infected by F. oxysporum f. sp. cubense, they are a common means by which this pathogen is disseminated. It can also be spread in soil and running water, on farm implements or machinery.
Panama disease is one of the most destructive plant diseases of modern times. It is believed to have originated in Southeast Asia and was first reported in Australia in 1876. By 1950 it had spread to all the banana-producing regions of the world with the exception of some islands in the South Pacific, the Mediterranean, Melanesia and Somalia.
Panama disease affects a wide range of banana cultivars; however, it is best known for the damage it caused to a single cultivar in the early export plantations. Before 1960, a total reliance was put on the cultivar 'Gros Michel', and it supplied almost all the export trade. It proved susceptible to the disease and the use of infected rhizomes to establish new plantations caused widespread and severe losses. Some indication of the scale of the losses is demonstrated by the complete eradication of production on 30,000 hectares of plantation in the Ulua Valley of Honduras between 1940 and 1960. In Suriname, an entire operation of 4,000 hectares was out of business within eight years and in the Quepos area of Costa Rica, 6,000 hectares were destroyed in twelve years.
By the mid-1900s, resistant cultivars in the 'Cavendish
Cavendish banana
The Dwarf Cavendish banana is a banana cultivar originally from Vietnam and China. It became the primary replacement for the Gros Michel banana in the 1950s after crops of the latter were devastated by the Panama disease. The name 'Dwarf Cavendish' is in reference to the height of the pseudostem,...
' subgroup were being used as a substitute for 'Gros Michel' in the export trade. These cultivars have proved resilient and grow well in the western tropics, remaining the clones on which the current export trades are based. Unfortunately, in several growing areas in the Eastern Hemisphere these cultivars are susceptible to the disease. It is considered inevitable that this susceptibility will spread to the Western Hemisphere, and this poses a significant threat to production because there are currently no acceptable replacement cultivars. To make things worse, this variant of the pathogen also affects plantain
Plantain
Plantain is the common name for herbaceous plants of the genus Musa. The fruit they produce is generally used for cooking, in contrast to the soft, sweet banana...
s, Musa acuminata × balbisiana
Plantain
Plantain is the common name for herbaceous plants of the genus Musa. The fruit they produce is generally used for cooking, in contrast to the soft, sweet banana...
, which are an important staple food
Staple food
A staple food is one that is eaten regularly and in such quantities that it constitutes a dominant portion of a diet, and that supplies a high proportion of energy and nutrient needs. Most people live on a diet based on one or more staples...
in tropical regions of the world. The average American eats 26.2 pounds of the Cavendish banana each year and the question is being asked as to whether this oft-consumed fruit is on course to extinction.
Apart from the export trade, 85% of banana production is for local consumption and many of the cultivars used for this purpose are also susceptible to infection.
Description
Fusarium oxysporumFusarium oxysporum
Fusarium oxysporum Schlecht. as emended by Snyder and Hansen comprises all the species, varieties and forms recognized by Wollenweber and Reinking within an infrageneric grouping called section Elegans...
is a common inhabitant of soil and produces three types of asexual
Asexuality
Asexuality , in its broadest sense, is the lack of sexual attraction and, in some cases, the lack of interest in sex. Sometimes, it is considered a lack of a sexual orientation...
spore
Spore
In biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions. Spores form part of the life cycles of many bacteria, plants, algae, fungi and some protozoa. According to scientist Dr...
s; macroconidia, microconidia and chlamydospores.
The macroconidia
Conidium
Conidia, sometimes termed conidiospores, are asexual, non-motile spores of a fungus and are named after the greek word for dust, konia. They are also called mitospores due to the way they are generated through the cellular process of mitosis...
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 phialide
Phialide
Phialide .*A flask-shaped projection from the vesicle of certain fungi.*Projects from the mycelium without increasing in length unless there is a subsequent increase in the formation of conidia....
s on conidiophores by basipetal division. They are important in secondary infection.
The microconidia
Conidium
Conidia, sometimes termed conidiospores, are asexual, non-motile spores of a fungus and are named after the greek word for dust, konia. They are also called mitospores due to the way they are generated through the cellular process of mitosis...
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
Conidium
Conidia, sometimes termed conidiospores, are asexual, non-motile spores of a fungus and are named after the greek word for dust, konia. They are also called mitospores due to the way they are generated through the cellular process of mitosis...
are globose with thick walls. They are either formed from hyphae
Hypha
A hypha is a long, branching filamentous structure of a fungus, and also of unrelated Actinobacteria. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium; yeasts are unicellular fungi that do not grow as hyphae.-Structure:A hypha consists of one or...
or by the modification of hyphal cells. They endure in soils for long periods and act as inocula
Inoculation
Inoculation is the placement of something that will grow or reproduce, and is most commonly used in respect of the introduction of a serum, vaccine, or antigenic substance into the body of a human or animal, especially to produce or boost immunity to a specific disease...
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
Teleomorph, anamorph and holomorph
The terms teleomorph, anamorph, and holomorph apply to portions of the life cycles of fungi in the phyla Ascomycota and Basidiomycota.*Teleomorph: the sexual reproductive stage , typically a fruiting body....
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 Musa (AAB group) 'Pome' and its subgroups, Musa (AAB group) 'Silk' and Musa (ABB group) 'Pisang Awak'.
- Race 2 attacks Musa (ABB group) 'Bluggoe' and its close relatives.
- Race 3 attacks HeliconiaHeliconiaHeliconia, derived from the Greek word helikonios, is a genus of about 100 to 200 species of flowering plants native to the tropical Americas and the Pacific Ocean islands west to Indonesia. Many species of Heliconia are found in rainforests or tropical wet forests of these regions...
spp. - Race 4 attacks Musa (AAA group) 'Dwarf Cavendish' as well as the hosts of races 1 and 2.
Symptoms
Infection by Fusarium oxysporum f.sp. cubense triggers the self-defense mechanisms of the host plant causing the secretion of a gelGel
A gel is a solid, jelly-like material that can have properties ranging from soft and weak to hard and tough. Gels are defined as a substantially dilute cross-linked system, which exhibits no flow when in the steady-state...
. This is followed by the formation of tylose
Tylose
Tyloses are outgrowths on parenchyma cells of xylem vessels . When the plant is stressed by drought or infection, tyloses will fall from the sides of the cells and "dam" up the vascular tissue to prevent further damage to the plant.Tyloses can aid in the process of making sapwood into heartwood in...
in the vascular
Vascular
Vascular in zoology and medicine means "related to blood vessels", which are part of the circulatory system. An organ or tissue that is vascularized is heavily endowed with blood vessels and thus richly supplied with blood....
vessels which blocks the movement of water and nutrients to the upper parts of the plant. The tips of the feeder roots are the initial sites of infection which then moves on to the rhizome
Rhizome
In botany and dendrology, a rhizome is a characteristically horizontal stem of a plant that is usually found underground, often sending out roots and shoots from its nodes...
. The signs of the disease are most noticeable as a dark stain where the stele
Stele (biology)
In a vascular plant, the stele is the central part of the root or stem containing the tissues derived from the procambium. These include vascular tissue, in some cases ground tissue and a pericycle, which, if present, defines the outermost boundary of the stele...
joins the cortex
Cortex (anatomy)
In anatomy and zoology the cortex is the outermost layer of an organ. Organs with well-defined cortical layers include kidneys, adrenal glands, ovaries, the thymus, and portions of the brain, including the cerebral cortex, the most well-known of all cortices.The cerebellar cortex is the thin gray...
. As the disease develops, large portions of the xylem
Xylem
Xylem is one of the two types of transport tissue in vascular plants. . The word xylem is derived from the Classical Greek word ξυλον , meaning "wood"; the best-known xylem tissue is wood, though it is found throughout the plant...
turn a reddish-brown colour. Externally, the oldest leaves start turning yellow and there is often a longitudinal splitting of the lower part of the outer leaf sheaths on the pseudostem. The leaves begin to wilt and may buckle at the base of the petiole. As the disease progresses, younger leaves are affected, turn yellow and crumple and the whole canopy begins to consist of dead or dying leaves.
Management
There are few effective options for managing Panama disease as fungicideFungicide
Fungicides are chemical compounds or biological organisms used to kill or inhibit fungi or fungal spores. Fungi can cause serious damage in agriculture, resulting in critical losses of yield, quality and profit. Fungicides are used both in agriculture and to fight fungal infections in animals...
s are largely ineffective. Chemical sterilisation of the soil with methyl bromide significantly reduced incidence of the disease but was found to be effective for only three years after which the pathogen had recolonised the fumigated areas. Injecting the host plants with carbendazim
Carbendazim
Carbendazim is a widely used broad-spectrum benzimidazole fungicide. A 4.7% solution of carbendazim hydrochloride is sold as Eertavas, an effective treatment for Dutch elm disease....
and potassium phosphonate appears to provide some control but results have been inconclusive. Heat treatment of soil has also been tried in the Philippines but the pathogen is likely to reinvade the treated area. The greatest hope for managing this disease in infested soils is the development of genetic modification
Genetic engineering
Genetic engineering, also called genetic modification, is the direct human manipulation of an organism's genome using modern DNA technology. It involves the introduction of foreign DNA or synthetic genes into the organism of interest...
s that will provide resistant cultivars.
Modified bananas developed in collaboration by Ugandan and Belgian scientists were reported in 2008 to be being grown experimentally in Uganda.
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. The resistance of different banana cultivars to the pathogen is under scrutiny.Research into the phylogenetic
Phylogenetics
In biology, phylogenetics is the study of evolutionary relatedness among groups of organisms , which is discovered through molecular sequencing data and morphological data matrices...
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
Monophyly
In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...
and appears to have multiple evolutionary
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
origins.