Fusarium spp. Mycotoxin Production, Diseases and their Management: An Overview

| In total, more than 1.5 million fungal species exist in the world, amongst them pathogenic species can attack plants at different stages causing considerable damage amounting to millions of rupees. One of the plant pathogenic fungi is Fusarium spp. Fusarium species are very well-known soil-inhabiting fungi that cause many economically important diseases of crops.Many species are included in the Fusarium genus, which are not only pathogenic to plants but also cause different diseases in humans and livestock.Apart from diseases, one of the most dangerous characteristics of this fungus is the ability to produce dangerous secondary toxic metabolites, which are commonly known as mycotoxins. Some of the important toxins produced by different species of Fusarium are fumonisins and trichothecenes. Fusarium species are present around the world and have a very wide host range including many economically important species of crops and plants. Most of the plant diseases are caused by F. solani, F. oxysporum and F. graminearum. Fusarium species can infect grains in storage, but they are more prevalent in the field where they cause infection in crops and then may invade grains and cause infection in storage. Different methods including chemical, cultural, and biological control strategies are employed to control this fungus. In this review, mycotoxin production, characterization, identification, and different economically important diseases associated with Fusarium species as well as their control are discussed in detail.

and humans then it may cause mycotoxicosis in them (Kosmidis and Denning, 2017). Fusarium spp. may also act as important biodegrading agents and it can sustain in the soil for up to 16 years without any host as well as in dead and decaying plant material (Early, 2009). Fusarium species have a very broad host range. They cause economic losses in all cereal crops in North America and western Europe,cotton, wheat and barley in China, rice plants in Taiwan, Thailand and Japan, in all important crops in the tropics and worldwide in timber trees in the forest (Voigt, 2002).
All species of Fusarium can produce different secondary metabolites whose functions are still unknown or not properly understood. These secondary metabolites are different toxins that cause virulence during the development of diseases in plants. When mycotoxin contaminated grains are consumed by humans and livestock it may cause great health impact (Bakker et al., 2018). Symptoms produced by mycotoxins may vary depending upon the type as well as the concentration of mycotoxin (Bennett and Klich, 2003). The mycotoxins produced by different Fusarium species are trichothecenes, fumonisins and zearalenone (Table 1). Trichothecenes are the mycotoxins produced by 24 different species of Fusarium. These 24 species that produce trichothecenes are Fusarium acuminatum, F. oxysporum, Fusarium avenaceum, F. poae, Fusarium camptoceras, F. proliferatum, F. chlamydosporium, F. sambucinum, F. compactum, F. scirpi, F. crookwellense, F. semitectum, F. culmorum, F. solani, F. equesiti, F. sporotrichioides, F. graminearum, F. subglutinans, F. moniliforme, F. tricinctum, F. nivale, F. tumidum, F. nygamai, F. venenatu (Bullerman, 2007;Mulè et al., 1997;Pitt and Hocking, 1997;Sweeney and Dobson, 1998). Zearalenone is the mycotoxins of Fusarium graminearum and some Fusarium sambucinum related species but they are not associated with the disease on wheat (Munkvold, 2017). Zearalenone is a beneficial mycotoxin as it is being used to increase the growth of cattle (Yu et al., 2004). Fumonisins mycotoxins are produced by the F. verticilloides (Desjardins and Plattner, 2000) and some other species of Fusarium like Fusarium moniliforme, F. proliferatum (Keller and Sullivan, 1996), Fusarium napiforme (Nelson et al., 1992) and Fusarium nygamai (Thiel et al., 1991). They are related to ear rot in corn, but they are not needed for disease-causing in corn (Desjardins and Plattner, 2000). Their adverse effects on the health of livestock as well as humans have been reported. They are very toxigenic for kidney and liver as well as they are also carcinogenic in nature (Stockmann-Juvala and Savolainen, 2008). All the mycotoxins that are produced by different species of Fusarium and their effects are summarized in Table 1.

Control of mycotoxins produced by different Fusarium species
Mycotoxins produced by different fungal species can be detoxified by using chemicals, but it is not a commonly used method as crops subjected to these chemical treatments may become unsuitable for human consumption. However, different chemical treatments involved for the detoxification of mycotoxins are ammoniation, treatments with different acids, bases, oxidizing (e.g. ozone) or reducing (e.g. sodium bisulfite) agents and enzymatic degradation (Munkvold et al., 2019).
Awell-studied method of mycotoxin management through chemicals is the treatment of contaminated products with ammonia or ammonium hydroxide. In a study, the treatment of contaminated products with 2% ammonia caused reduction in fumonisinsup to 79% (Charmley and Prelusky, 1994). Treatments with calcium or sodium hydroxide have shown remarkable detoxification in feeds contaminated by aflatoxins, T-2, zearalenone and diacetoxyscirpenol from 45% to 99% depending upon the nature of the toxin as well as feed moisture level (Charmley and Prelusky, 1994;Karlovsky et al., 2016). Sodium bisulfite treatments were proved to be effective against deoxynivalenol in only animal feed corn as well as treatments with ozone and chlorine gas were only effective in detoxification of different mycotoxins in corn but not wheat (Young, 1986;Young et al., 1986). Formaldehyde and ammonium hydroxide were proved to be effective in decontaminating zearalenone affecting corn and corn grits,but the products treated with formaldehyde are unstable for human consumption (Charmley and Prelusky, 1994).
Biological detoxification of mycotoxins includes the use of different microorganisms or the use of different enzymes obtained from microorganisms. In some cases, there are reports of degraded products that are still toxic but, in some cases, there is the complete degradation of mycotoxins using mycotoxin-detoxifying microorganisms. Reduction in the level of deoxynivalenol up to 54%-56% has been seen in the feed when it is incubated with intestinal microflora of chickens (Charmley and Prelusky, 1994). For the detoxification of each of the common mycotoxins, at least one specific microorganism has been discovered (Zhu et al., 2016). Many microorganisms identified as mycotoxin degrading agents are bacteria especially Bacillus species.
The life cycle of Fusarium spp. Fusarium spp. follow both asexual and sexual life cycles. During both sexual and asexual stages, mycelial structures that are haploid are being established. Few species of Fusarium produce sexual (meiotic) spores viz ascospores and three types of asexual (mitotic) spores viz. microconidia, macrocondia and chlamydospores that are produced from conidiophores, from sporodochium and within or on hyphae, respectively. Both stages produce spores that are airborne in nature and hence may cause infection and mycotoxin contamination in plants (Dweba et al., 2017). The generalized life cycle of Fusarium spp. is depicted in Figure 1. Not all species of Fusarium produce all kinds of spores and the sexual cycle of only less than 20% of Fusarium spp. is known (Ma et al., 2013).

Sexual state of Fusarium spp.
Teleomorph, which is the sexual state, is known of few Fusarium species. All sexual states of known Fusarium species are part of Ascomycota but included in different genera viz. Genus Gibberella and Genus Nectria etc. The teleomorphic species of Fusarium can be both heterothallic and homothallic. During meiosis, the chromosomes of few of these species was seen under light microscope but due to the small chromosome size of Fusarium species, the accurate number of chromosomes or karyotyping is not determined therefore, for this purpose pulsed field gel electrophoresis (PFGE) has been used (Suga and Hyakumachi, 2004). All the known perfect states of Fusarium species are given in Table 2.
Morphological and microscopic characteristic of Fusarium spp. Fusarium spp. can grow on many media. When different Fusarium spp. are grown on potato dextrose agar, they may show white, lavender, pink, salmon, or gray-colored velvety to fuzzy cottony growth. Hyphae of Fusarium species is hyaline and septate, and it varies from 3 to 8μm in diameter. The species of Fusarium produce both macro as well as microconidia.
Macroconidia produced by different Fusarium species are hyaline, multicellular, septate, and sickleshaped which may appear in form of clusters while the microconidia are hyaline, unicellular, and ovoid to straight or slightly curved in shape. Sometimes chlamydoconidium are also produced by Fusarium species which may present as a single spore or in the shape of clusters or chains (Bullerman, 2003;Nucci and Anaissie, 2009).

Molecular identification of Fusarium spp.
Different molecular techniques which include Random Amplified Polymorphic DNA (RAPD) analysis, specific diagnostic PCR primers and DNA sequencing are being used to identify Fusarium species. The polymerase chain reaction (PCR) is considered   (Mulè et al., 2004) as the most reliable and rapid technique for the identification of different Fusarium species (Kachuei et al., 2015). Different primer sets have been designed for the identification of Fusarium species (Table 3).
Pathogenicity factors of Fusarium spp. Fusarium spp. uses different cellular signaling pathways and different toxins or enzymes which may include MAPKs, Ras proteins, G-proteins, Velvet complex, cAMP pathways and cell wall degrading enzymes to enter their hosts and cause infection. These pathogenicity factors maybe generally produced by different species of Fusarium or they may be hostspecific (Poppenberger et al., 2003).

Diseases caused by Fusarium spp.
Species of Fusarium cause many diseases like root rots, seedling blight (Bakker et al., 2016), vascular wilts (Michielse and Rep, 2009), infection in reproductive tissues as well as in developing seeds (Kazan et al., 2012) and diseases in storage (Gachango et al., 2012 (Table 4).
Disease symptoms Fusarium spp. produces different types of symptoms on hosts. Some of the common symptoms produced by this fungus are described below.

Vascular wilt diseases
Wilt diseases are mostly caused by different species of Fusarium. Mostly all wilts show common symptoms like the infected parts of the plant lose their turgidity, their color changes to light green or yellowish green then to brown and they wilt and finally die. Wilting is due to the blockage in xylem tissue of plants by the spores, mycelium, or polysaccharides of fungus which results in the less flow of water in tissues of plants. The fungus also produces different mycotoxins like fusaric acid and lycomarasmin in vessels that flow from vessels to the leaves. In leaves, they affect the process of photosynthesis by reducing the production of chlorophyll (Voigt, 2002).

Seedling blight diseases
The blight of seedlings mostly caused on corn and small grains as dark brown lesions that resemble brown rot and blight in both at pre-and post-emergence stage. The seedings may not develop properly, chlorosis will occur which finally results in the death of seedling (Voigt, 2002).

Head blight or Scab diseases
Scab appears on spikelet as water-soaked lesions which later decolorize. During warm and humid conditions, the head fully gets infected through mycelia and conidia and the kernels get dry and shriveled. Mycelium of fungus overgrows and appears as white, pink, or brown growth on infected kernels (Voigt, 2002).

Dry rot diseases
Dry rot may infect bulbs, corms, and tubers on both pre-and post-harvest stages. The most common hosts of dry rot diseases are onion, lily, gladiolus, and potatoes. The injuries caused during harvesting are the main way of pathogen invasion. Brown colored small lesions will form on tubers which increase in size and later wrinkles will form. Tubers become hard and mummified (Voigt, 2002).

Management of diseases caused by Fusarium species cultural control
Cultural control practices help to reduce the primary inoculum which is responsible for the development of secondary infection. Diseases in plants caused by Fusarium spp. can be controlled by achieving low plant density, using proper phosphate, potassium, and nitrogen fertilizers, and using resistant varieties of plants. Rotation of crops, tillage and proper seedbed preparation can also reduce the primary inoculum present in crop residues. The first-ever method used to control the plant diseases was crop rotation (Sumner, 1994). But it is not useful in the case of Fusarium spp. which are not specialized because they have a wide host range (Waller and Brayford, 1990). Losses caused by Fusarium spp. can be decreased by crop rotation with resistant or non-host crops, attaining suitable soil drainage and by using healthy and treated stock (Agrios, 1997). Disease development can also be controlled by changing the date of sowing because it is affiliated with the epidemic development. Crops like chickpea have been sown in southern Spain in early winters instead of early springs which helped in slowing the epidemics of Fusarium wilt which resulted in less disease development (Navas-Cortés et al., 1998). Sowing the crops in early winter instead of early spring resulted in more soil moisture and less temperature which are not favorable for Fusarium wilt thus it affects the disease development (Voigt, 2002). Primary inoculum of Fusarium can also be decreased by flooding fields for a large period or dry fallowing because it results in a low oxygen level which is not favorable for pathogen development (Manners, 1993). Infection of Fusarium in seeds can be controlled by decreasing the temperature and moisture in storage for several months because it decreases the activity of Fusarium graminearum in grains (Gilbert et al., 1997). By achieving the temperature below 10 o C, growth and mycotoxin production can be lowered in F.graminearum, F. moniliforme and F. proliferatum . Food preservatives like sorbic acid, acetic acid, formic acid and propionic acid, etc. can be used to decrease the mycelial growth and production of spores and mycotoxins by different Fusarium spp. like F. proliferatum (Marin et al., 1999) and F. oxysporum (Tzatzarakis et al., 2000) in food.

Chemical control
The use of chemical control strategies such as fungicide seed treatment and the application of fungicides on crops along with cultural control strategies can be effective to control diseases. Studies have been done to check the effect of treatments on seeds which helped in better understanding of different chemical treatment effects on viability, germination, emergence and vigor of seeds as well as the weight of roots (Gilbert and Tekauz, 1995;Gilbert et al., 1997). Rots caused by Fusarium spp. can be controlled by applying benomyl sprays on plants or by treating propagative plant materials with benomyl. Benzimidazoles and Benomyl works very well against the infections caused by Fusarium species. Benzimidazoles are very effective against the F. avenaceum, F. culmorum, F. equiseti and F. solani, but it is not effective against the F. sambucinum which causes tubers in potatoes because it is highly resistant to benzimidazole and its derivatives (Kawchuk et al., 1994). While prochloraz and tebuconazole are effective against F. culmorum and F. poae which are the causal agents of ear blight on wheat (Doohan et al., 1999). In storage conditions, diseases on crops can be managed by applying fungicides at the postharvest stage. For example, a pathogen causing dry rot of potato tubers only enters its host through physical injuries. After it enters its host, it develops inside the host and causes infection but its development inside the host can be controlled by treating the potatoes with thiabendazole at the post-harvest stage (Secor and Gudmestad, 1999). When plants are treated with benzimidazole then on the surface of plants they convert into methyl benzimidazole carbamate (MBC, carbendazim) which acts as a systemic fungicide and are fungistatic in nature (Manners, 1993). Organomercury fungicides act both as eradicants and protectants and they are used to control Fusarium diseases in cereal grains (Häni, 1981;Manners, 1993). In China, bakanae disease of rice has been successfully managed by treating seeds with formalin and organic mercury (Cook, 1967). But formalin and organic mercury-based fungicides have toxic effects on plants.
Another safe method to control Fusarium diseases is by treating the soil with fumigants like methyl bromide before the sowing of crops. This fumigant enters in the pores of soil, spread thoroughly and has no toxic effect (Ben-Yephet et al., 1994). Soil is treated with both volatile fumigant methyl bromide and insecticide chloropicrin together. Metalaxyl, diazoben, pentachloronitrobenzene (PCNB), ethazol, captan and chloroneb are the most common organic fungicides which are used to treat the soil. They are more effective, safe but are expensive than sulfurbased and copper-based fungicides. Captan which act as a protectant, reacts with sulfhydryl groups and stops the activity of enzyme which contains thiol (Manners, 1993). Many Fusarium spp. develop resistance against fungicides and fungicide may not remain effective against those isolates which will ultimately increase the diseases caused by Fusarium (Secor and Gudmestad, 1999). Therefore, along with chemical control we must use other control measures like use of proper cultivation techniques, proper treatment of seeds and usage of clean and healthy seed and propagating stock etc. (Voigt, 2002).

Biological control
Fusarium against Fusarium: Some species of pathogens contain virulent, avirulent or hypovirulent strains. These two avirulent or hypovirulent strains can be used against the virulent strain. These virulent or hypovirulent strains can protect the crop against its virulent strain (Sneh, 1998). The avirulent strains increase the resistance of host plants against its pathogen by competing with the virulent strains. Some strains of Fusarium also produce anti-fungal compounds like alpha-pyrones produced by F. semitectum which play an important role in the protection of plants (Evidente et al., 1999). For example, Wilts caused by Fusarium oxysporum in different crops was managed by using avirulent strains of the same fungus (Fravel and Engelkes, 1994).  (Schönbeck et al., 1994). The Trichoderma is also one of the successful bio-control agents of Fusarium. It affects the growth of Fusarium by causing parasitism (Ogawa et al., 2000). Trichoderma spp. effects and degrade the chitin which is the main constituent of the cell wall of fungus (Manocha and Govindsamy, 1998). Trichoderma viridae successfully controlled the diseases caused by F. moniliforme as well as reduced its mycotoxin production by 85% (Yates et al., 1999).

Fungi against
F. oxysporum f. sp. lycopersici which causes wilt was controlled in tomatoes by 30% using Penicillium purpurogenum as a bio-control agent (Larena and Melgarejo, 1996).

Bacteria against Fusarium:
Rots and damping-off diseases caused by Fusarium have been successfully managed by using Bacillus cereus which is a soil-borne bacteria whereas, Pseudomonads are being successfully used against F. oxysporum because they produced antibiotics such as N-butylbenzene sulfonamide which inhibits the activity of F. oxysporum (Kim et al., 2000). Pseudomonads especially P. fluorescens and P. putida are abundantly present in soil rhizosphere. They make soil suppressive against the wilts causing Fusarium by producing siderophores (Alabouvette et al., 1998). P. putida promotes the production of phenolic compounds in cucumber which are antifungal in nature. This helps in the increase of resistance against the pathogen (Ongena et al., 2000). The health of potato plants has been increased by the induction of Pseudomonas which promotes siderophores that are hydroxamate type which resulted in the production of hydrocyanic acid and indole acetic acid (Gupta et al., 1999). Salicylic acid is an important signaling molecule in plant defense systems which plays an important role in the induction of resistant mechanisms in plants (Mauch-Mani and Métraux, 1998). If the activation of salicylic acid is affected, then it increases the susceptibility level of the host against its pathogens (Delaney et al., 1994). Rhizobacteria increase the plant health by inducing ISR in plants through the production of compounds like indole-3-acetic acid and cytokinin along with the reduction of ethylene (Buchenauer, 1998).

Conclusions and Recommendations
Fusarium is an important genus among fungi including different plant as well as human pathogenic species. They not only cause infection but are also responsible for different mycotoxins that are toxic for both animals and humans. There is a dire need to control this menace so that losses can be minimized and to this end different control strategies can be employed to manage and control this fungus.

Novelty Statement
An up to date comprehensive review about Fusarium spp. For the first time in this review, mycotoxins and diseases caused by Fusarium spp. are discussed. All the possible control strategies for controlling this pathogen are discussed with special reference to biological control, an environmentally safe method. All the information is tabulated for the ease of students and researchers.

Author's Contribution
Saba Shabeer: Data collection, draft-ing the article, revision of the article. Riffat Tahira: Revision of the article. Atif Jamal: Conception of the work, revision of the article and final approval.