Fusarium spp – Definition, Structure, Pathogenesis

Kingdom:	Fungi
Division:	Ascomycota
Class:	Sordariomycetes
Order:	Hypocreales
Family:	Nectriaceae
Genus:	Fusarium

Fusarium is a large genus of filamentous fungi belonging to the hyphomycetes group, which is extensively distributed in soil and closely associated with plants. The majority of species are saprobes and are relatively abundant constituents of the soil microbiome. Certain species generate mycotoxins in cereal crops that, if they enter the food chain, can be harmful to human and animal health. The principal toxins produced by these Fusarium species are trichothecenes and fumonisins. Some Fusarium species and subspecific groups are among the most significant fungal pathogens of plants and animals, despite the fact that the majority of species appear innocuous (some exist on the skin as commensal skin flora). Fusarium is derived from the Latin word fusus, which means filament.

Classification of Fusarium spp

The complex taxonomy of the genus. Various schemes have been utilized, and up to one thousand species have at times been identified, with approaches varying between broad and limited concepts of speciation (lumpers and splitters). Phylogenetic studies indicate that the genus contains seven main clades.

There is a widely supported proposal among specialists that would encompass the genus as it currently exists, including all agriculturally significant Fusaria. There is a counterproposal (unrelated to Watanabe 2011) that proposes seven wholly new genera in the opposite direction.

Information	Subdivision
Subgenera and Sections	Various schemes have subdivided the genus into subgenera and sections. There is a poor correlation between sections and phylogenetic clades.
Sections	Arachnites, Arthrosporiella, Discolour, Elegans, Eupionnotes, Gibbosum, Lateritium, Liseola, Martiella, Ventricosum, Roseum, Spicarioides, Sporotrichiella
Selected Species	Fusarium acaciae, Fusarium fujikuroi, Fusarium acaciae-mearnsii, Fusarium acutatum, Fusarium aderholdii, Fusarium acremoniopsis, Fusarium affine, Fusarium arthrosporioides, Fusarium avenaceum, Fusarium bubigeum, Fusarium circinatum, Fusarium crookwellense, Fusarium culmorum, Fusarium graminearum, Fusarium incarnatum, Fusarium langsethiae, Fusarium mangiferae, Fusarium merismoides, Fusarium oxysporum, Fusarium pallidoroseum, Fusarium poae, Fusarium proliferatum, Fusarium pseudograminearum, Fusarium redolens, Fusarium sacchari, Fusarium solani, Fusarium sporotrichioides, Fusarium sterilihyphosum, Fusarium subglutinans, Fusarium sulphureum, Fusarium tricinctum, Fusarium udum, Fusarium venenatum, Fusarium verticillioides, Fusarium virguliforme, Fusarium xyrophilum
Etymology	The name of Fusarium comes from Latin fusus, meaning a spindle.

Characteristics

• Foodborne Fusarium species are characterized by colonies with floccose, velvety aerial mycelium that proliferate rapidly.
• Depending on the species and growth conditions, colonies can range in color from mild, rose, and burgundy to violet-blue.
• Conidia, which are asexual spores, are frequently produced by sporodochia, where they manifest as slimy spots on the culture.
• Some cultures may have sporodochia so pervasive that it fuses into a thicker layer of slime.
• The typical Fusarium conidium (macroconidium) is fusiform (spindle-shaped), multicelled by transverse septa, and is comprised of a foot-shaped basal cell and a pointed to a whiplike apical cell.
• Some species may produce minor conidia (microconidia), which are mostly unicellular and can be globose, elliptical, reniform, or fusiform in shape.
• A few species produce microconidia in chains, whereas the majority produce them singly or in slimy caps.
• Fusarium species thrive in humid environments with a water activity greater than 0.86 and temperatures ranging from 0 to 37 degrees Celsius.
• No species of Fusarium are thermophilic.
• Consult Table 1 for information regarding common foodborne Fusarium species.

Fusarium species	Habitat	Mycotoxinsa	Colony diameterb
F. avenaceum	World-wide: cereals, peaches, apples, pears, potatoes, peanuts, peas, asparagus, tomatoes	2-Amino-14,16-dimethyloctadecan-3-ol, Acuminatopyrone, Antibiotic Y, Aurofusarin, Beauvericin, Chlamydosporol, Chrysogine, Enniatins, Fusarin C, Moniliformin	PDA: 30–59 mm, TAN: 3–20 mm
F. cerealis	Worldwide: cereals, potatoes	Aurofusarin, Butenolide, Chrysogine, Culmorin, Fusarin C, Nivalenol, Zearalenone	PDA: 75–90 mm, TAN: <2 mm
F. culmorum	Mainly temperate region: cereals, potatoes, apples, sugar beet	Aurofusarin, Butenolide, Chrysogine, Culmorin, Deoxynivalenol, Fusarin C, Nivalenol, Zearalenone	PDA: 75–90 mm, TAN: <2 mm
F. equiseti	Worldwide: cereals and fruits contaminated with soil, vegetables, nuts, spices, UHT processed juices	Chrysogine, Diacetoxyscirpenol, Equisetin, Fusarochromanone, Nivalenol, Zearalenone	PDA: 45–69 mm, TAN: 3–30 mm
F. graminearum	Worldwide: cereals and grasses	Aurofusarin, Butenolide, Chrysogine, Culmorin, Deoxynivalenol, Fusarin C, Nivalenol, Zearalenone	PDA: 75–90 mm, TAN: <2 mm
F. incarnatum	Warmer to tropical regions: nuts, bananas, citrus, potatoes, melons, tomatoes, spices	Beauvericin, Equisetin, Fusapyrone, Zearalenone	PDA: 45–69 mm, TAN: 5–15 mm
F. oxysporum	Worldwide: cereals, peas, beans, nuts, bananas, onions, potatoes, citrus fruits, apples, UHT processed juices, spices, cheese	Beauvericin, Fusaric acid, Moniliformin, Naphthoquinone pigments	PDA: 30–55 mm, TAN: 5–40 mm

Morphology of Fusarium spp

• Fusarium spp. are filamentous fungus that have a complicated morphological structure. Mycelium is a network of threadlike structures called hyphae, and it ranges in color from white to pinkish or reddish. Under a microscope, the hyphae appear as a web-like network of filaments that grow in a branching pattern.
• Spore-making structures called conidiophores develop from the mycelium. Conidiophore structure differs amongst Fusarium species. In many Fusarium species, the conidiophores, known as phialides, have only a single opening through which the endoconidia are released. Poly-phialides, which have two or more apertures or holes through which the endoconidia are driven out, are also present in some species.
• Large, multicellular spores called macroconidia are developed in a sporodochium. The macroconidia are supported by the sporodochium, which is an erumpent, dense cluster of conidiophores growing from the stroma. Macroconidia can also form on the aerial mycelium of certain Fusarium species. Macroconidia range greatly amongst species in terms of size, form, and even color.
• Microconidia are tiny, unicellular spores that develop on mono-phialides or poly-phialides in the form of false heads or false chains. When the conidiophore is exposed to moisture, false heads form, which eventually fill with endoconidia. Some microconidia are generated in chains, and these have a truncate base.
• Chlamydospores are thick-walled, asexual spores that are filled with lipid-like substance. Most Fusarium species produce them, and they’re widespread in soil. The outer wall of chlamydospores can be smooth or rough, and they can cluster, pair off, or even form chains. Overwintering Fusarium spp. in soil and other substrates relies heavily on them.
• Different species of Fusarium can be distinguished from one another by a wide range of morphological traits.

Habitat of Fusarium spp

• Fusarium species are found in a wide range of habitats including soil, plants, water, and air. Some species are soil-borne pathogens that infect crops such as wheat, corn, and tomatoes, causing significant economic losses.
• Others are associated with plant debris or can grow as saprophytes on decaying organic matter. Some Fusarium species are also human and animal pathogens, causing infections in immunocompromised individuals or in farm animals. Overall, Fusarium species are ubiquitous and can be found in diverse environments.

Cultural characteristics of Fusarium spp

• Growth rate: Fusarium spp are generally fast-growing fungi with colonies that range in size from moderate to rapid growth. The growth rate of Fusarium spp is usually dependent on the species and the composition of the medium used.
• Colony color: Fusarium colonies are usually white, pink, orange, yellow, or purple in color. The color of the colony may vary depending on the species, age, and composition of the medium.
• Texture: The texture of Fusarium colonies is usually cottony or woolly. However, some species may exhibit a more powdery or granular texture.
• Reverse side color: The reverse side of Fusarium colonies is usually yellowish or reddish-brown in color. Again, the color may vary depending on the species and composition of the medium.
• Pigmentation: Some species of Fusarium produce pigments such as carotenoids, which are responsible for the yellow and orange coloration of the colonies.
• Odor: Fusarium colonies are usually odorless, although some species may produce a musty or earthy smell.
• Sporulation: Fusarium spp produce asexual spores, and the type and amount of spores produced can vary depending on the species and composition of the medium. The sporulation pattern of Fusarium spp is typically in a circular or fan-shaped pattern.

It is important to note that the cultural characteristics of Fusarium spp may vary depending on the growth conditions, such as temperature, humidity, and the composition of the medium. Therefore, it is essential to use standardized procedures and media to accurately identify and differentiate Fusarium spp based on their cultural characteristics.

Culture Media of Fusarium spp

• Carnation leaf agar:
  • Promotes sporulation and inhibits mycelial expansion.
  • Large quantities of conidia and conidiophores are produced, and the spores have distinct morphologies.
  • Low in carbohydrates and rich in complex substances that provide an environment conducive to Fusarium growth.
• Potato dextrose agar:
  • The most valuable medium for the growth of Fusarium, which produces a grotesque morphological appearance and colored colonies.
  • This medium’s high carbohydrate content promotes sporulation, but growth in this medium is slower.
  • Produced conidia are misshapen and atypical.
• KCl medium:
  • Used to observe the species-specific formation of microconidia chains.
  • On this medium, the species that form chains of microconidia form more numerous and longer chains.
  • Because there is less moisture on the surface of the agar and fewer moisture droplets in the aerial mycelium, the chains are simpler to observe.
• Soil agar:
  • Promotes the rapid formation of chlamydospores in a variety of Fusarium species.
  • Large inoculum containing actively growing Fusarium inoculates produces chlamydospores in 3 to 4 days, whereas secondary inoculum produces chlamydospores in 30 days.

Pathogenesis of Fusarium spp

• Little is known about the host’s defenses against Fusarium species.
• Many characteristics of invasive fusariosis are shared with invasive aspergillosis and other invasive fungal infections.
• Patients receiving high doses of corticosteroids and those with protracted and severe neutropenia are susceptible.
• In vitro and in vivo experimental studies, the unique susceptibility of severely immunocompromised patients to disseminated fusariosis, and the strong correlation between immune reconstitution and outcome all support the importance of immunity in the pathogenesis of fusariosis.
• Mold infections are defended against in large part by innate immunity.
• Macrophages and neutrophils injure fusarial hyphae, and their effect is stimulated by gamma interferon, G-CSF, GM-CSF, and interleukin-15.
• Toll-like receptors have been identified in the recognition of fungi by the innate immune system, and this system is likely also important in invasive fusariosis.
• To investigate the pathogenicity of Fusarium species, animal models of fusariosis have been developed.
• Mortality was proportional to inoculum size.
• The infection was characterized by necrotizing abscesses with hyphae, hemorrhage, and neutrophil and macrophage infiltration in nonneutropenic rodents.
• Neutropenic rodents lacked an inflammatory cellular response and had a notably greater fungal burden.
• The occurrence of disseminated fusariosis in nonneutropenic recipients of hematopoietic stem cell transplant (HSCT) demonstrates the significance of T-cell defenses against Fusarium.
• Patients suffering from severe T-cell immunodeficiency due to multiple therapies for their underlying disease and graft-versus-host disease (GvHD).
• The significant effect of corticosteroid therapy on the prognosis of fusariosis, as demonstrated by the significantly higher mortality rate among patients who received corticosteroid therapy compared to those who did not.

Virulence Factors

• The virulence factors of Fusarium species include production of mycotoxin, adhesion to prosthetic material, and production of proteases and collagenases.
• Fusarium solani is the most virulent species, causing mortality in all immunocompetent animals tested in a murine model of fusariosis.
• Recently, genetic virulence determinants of Fusarium oxysporum were investigated in immunocompromised rodents.
• In this model, the mitogen-activated protein kinase gene, which is essential for virulence in fungal plant pathogens, was not required for F. oxysporum virulence.
• Animal virulence required the pH response transcription factor but not plant virulence.
• The chitin synthase knockout mutant isolates caused severe lung injury, leading to respiratory insufficiency and rapid death in mice, most likely due to the physical obstruction of lung interstitial capillaries by numerous large mutant conidia.

Plant pathologies caused by Fusarium species

Fusarium species are known to cause a range of plant diseases, including:

1. Fusarium wilt: This disease affects many crops, including tomatoes, bananas, cotton, and cucurbits. It causes wilting, yellowing, and stunting of the plant, and can lead to significant yield loss.
2. Fusarium head blight: This disease affects cereal crops, including wheat, barley, and maize. It can cause significant yield loss and reduction in grain quality.
3. Root rots: Fusarium species can cause root rots in many plants, including beans, peas, and soybeans. This can lead to reduced plant growth and yield.
4. Leaf spots: Fusarium species can cause leaf spots in a variety of plants, including tomatoes, cucumbers, and ornamentals. The spots may be yellow or brown and can lead to defoliation and reduced plant vigor.
5. Stem rot: Fusarium species can cause stem rot in many crops, including soybeans and cotton. It can lead to lodging and yield loss.
6. Fruit rot: Fusarium species can cause fruit rot in many fruits, including strawberries, melons, and grapes. This can lead to reduced yield and quality.

Clinical Features

• The clinical manifestations of Fusarium infections depend on the site of infection and the underlying immune status of the host.
• Cutaneous infections caused by Fusarium spp can present as nodules, ulcers, or cellulitis, and are often associated with trauma or an existing skin lesion.
• Disseminated infections can present as fever, sepsis, and organ dysfunction, and are associated with a high mortality rate.
• Pulmonary infections caused by Fusarium spp can present as cough, dyspnea, and hemoptysis.
• Ocular infections caused by Fusarium spp can lead to endophthalmitis, keratitis, and vision loss.
• Infections caused by Fusarium spp are often difficult to diagnose and may require a combination of clinical, radiological, and laboratory findings.
• Antifungal therapy is the mainstay of treatment for Fusarium infections, although the choice of antifungal agent and duration of therapy may vary depending on the site and severity of infection.

Methods of Detection of Fusarium species

• Fusarium species are a major concern in the food and agriculture industries. Conventional methods based on agar substrates are commonly used for the detection of Fusarium in food and agricultural products.
• Three selective media, including Czapek-Dox iprodione dichloran (CZID) agar, dichloran chloramphenicol peptone agar (DCPA), and pentachlornitrobenzene (PCNB) agar, can be used for direct or dilution plating.
• CZID is preferred as it can differentiate Fusarium species based on cultural appearance. In situ detection by molecular methods, such as real-time polymerase chain reaction, has also been developed.
• However, these methods require an array of probes to cover the relevant Fusarium species, in contrast to conventional isolation methods that require purification and culturing before identification.

Methods of Identification

Methods of identification for Fusarium species include both conventional and molecular techniques. Conventional methods are based on agar substrates, and three Fusarium selective media are commonly used: Czapek-Dox iprodione dichloran (CZID) agar, dichloran chloramphenicol peptone agar (DCPA), and malachite green agar (MG2.5). These media can be used for direct plating or dilution plating of subsamples, and plates are incubated at 25°C for 5-7 days. Purification and culturing are required before identification using conventional methods. Molecular methods, such as real-time polymerase chain reaction, are increasingly being used for specific detection and quantification of Fusarium species in cereals. Immunological techniques and molecular biology are fast and useful but require an array of probes to cover the relevant Fusarium species. Care should be taken when using selective media, as other fungal genera may be able to grow if just a tiny amount of the sample is present, thus eliminating the inhibition provided by the Fusarium selective media.

Treatment and Management of Fusarium

• Fusarium infections in humans are usually treated with antifungal medications, including voriconazole, amphotericin B, and posaconazole. Surgical removal of infected tissue may also be necessary in severe cases.
• In agriculture, the management of Fusarium involves using cultural and chemical control methods, including crop rotation, planting of resistant cultivars, and application of fungicides. Biologically-based control methods, such as the use of beneficial microbes, are also being explored. Proper storage and handling of harvested crops can also help prevent contamination by Fusarium.
• Prevention is key in both human and agricultural settings. In the case of human infections, those with weakened immune systems or who are at high risk for infection should avoid environments where Fusarium is likely to be present. In agriculture, preventative measures include proper storage of seed, avoiding planting in fields with a history of Fusarium infection, and maintaining optimal growing conditions for crops. Regular monitoring and early detection of Fusarium are also important in order to prevent the spread of the infection.

Prevention and Control of Fusarium

Fusarium is a fungal genus that includes many species that can cause plant diseases, including fusarium wilt, root rot, and head blight. These diseases can be devastating to crops and can lead to significant economic losses for farmers. Here are some strategies for preventing and controlling Fusarium:

• Crop rotation: Fusarium can survive in soil for several years, so rotating crops can help reduce the buildup of the fungus in the soil.
• Resistant varieties: Use plant varieties that are resistant to Fusarium if possible.
• Sanitation: Practice good sanitation practices, such as removing infected plant debris and cleaning equipment to prevent the spread of the fungus.
• Fungicides: In some cases, fungicides can be effective in controlling Fusarium, but they should be used as part of an integrated pest management approach.
• Soil solarization: This involves covering the soil with a clear plastic tarp and allowing the sun’s heat to kill the Fusarium fungus in the soil.
• Biological control: Certain microorganisms, such as Trichoderma species, have been shown to be effective in controlling Fusarium.
• Avoiding stress: Stressed plants are more susceptible to Fusarium infection, so it is important to provide proper irrigation, nutrition, and other care to maintain plant health.

Overall, preventing Fusarium requires a multifaceted approach that includes cultural, chemical, and biological control strategies. Regular monitoring of crops for signs of Fusarium infection and prompt action to control the fungus can help reduce the impact of these diseases.

FAQ

References

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