Current Status of Fusarium and Their Management Strategies

Fusarium spp. is one of the most economically important plant pathogens causing a wide range of plant diseases with significant crop losses globally. Fusarium wilt is a major problem all over the world. Fusarium oxysporum , Fusarium solani , Fusarium fujikuroi are economic importance species in worldwide. Fusarium solani causing disease in many agriculturally crops and favored by high temperatures and warm moist soils. The fungus produces three types of asexual spores; microconidia, macroconidia and chlamydospores serve as propagules in infecting host plants and found endophytes and saprophytes. The color of the colony, length and shape of the macroconidia, the number shape of microconidia and the presence or absence of chlamydospores are key features for the differentiation of Fusarium species. Pathogens, forms over 100 formae speciales cause disease in dicot and monocot plant species and infecting a variety of hosts. Vegetative compatibility Groups (VCG) is used to differentiate their races. Resistant cultivars and bio-control agents ( Trichoderma spp., and Psedomonas spp.) have been used to manage the disease.


Introduction
Soil-borne pathogens caused infection in soil via the roots. Fusarium is a complex genus and worldwide distribution, causing diseases in plants, animals, and humans as well as the presence of non-pathogenic Fusarium in the natural ecosystem [1]. Fusarium wilt pathogen is one of the most destructive soil-borne pathogens around the world occurring in both saprophytic and pathogenic [2,3]. Non-pathogenic and pathogenic F. oxysporum strains are in the soil, but the pathogenic strain causes severe vascular wilt disease in more than 150 agricultural crop species are banana, tomato, melon, watermelon, and cotton to be infected by vascular wilt [4]. Cereals and other food grains can be contaminated by Fusarium toxins and causes many diseases syndromes in mammals, moldy sweet potato toxicity, and poisoning in bean hulls [5]. Fusarium is one of the most important fungal genera that can produce mycotoxins. Fusarium mycotoxins are fumonisins, zearalenone, deoxynivalenol, and additional trichothecenes, cosmopolitan genus and numerous species are plant pathogens [6]. Cob rots in maize, caused mainly by F. graminearum and F. verticillioides, and both species produce mycotoxins which contaminate the grain and some strains of Fusarium solani cause collar rot of legume seedlings such as peas and bean. Fusarium species have their ability to grow on a wide range of substrates and their efficient mechanism for dispersal [7]. Many species are saprophytes which occur commonly in soil, colonize diseased roots, stems and grow quickly Current Status of Fusarium and Their Management Strategies DOI: http://dx.doi.org /10.5772/intechopen.100608 Fusarium species were recently included in the list of the top ten plant pathogenic fungi with both economic and scientific importance [11]. This genus interacst with plants as endophytic root colonizers [40]. they may be responsible for a wide range of human infections [41]. Fusarium genus has more than 1500 species and several strains occur on plants/animals producing mycotoxins. Fusarium belongs to Phylum-Ascomycota, Order-Hypocreales and Family-Nectriaceae. Fusarium species are complex includes plant pathogens, human pathogens, and non-pathogens. Pathogenic strains are morphologically indistinguishable from nonpathogenic strains. Fusarium pathogens are persisting in the soil as chlamydospores, cause infect through the feeder rootlets and then colonize the vascular system, leading to severe wilting and death of plants. Important species are Fusarium oxysporum, F. solani, F. fujikuroi and F. graminearum well known plant-pathogens. It's characterized by fast-growing colonies with floccose aerial mycelium, colony pigmentation from pale, rose, burgundy to bluish violet depending on species and growth conditions. Fusarium usually produces pale violet to the dark magenta pigment in agar media (some do not produce). Conidia are often produced in sporodochia which are slimy dots in the culture, macroconidia are fusiform, multi-celled by transverse septa and characteristic foot-shaped basal cell pointed apical cell. Some species also produce microconidia are mostly single-celled, in some cases three to five celled and vary from globose, oval and fusiform. A few species produce microconidia in chains and others in slimy. Fusarium characteristics by morphological conidia in size and shape of macroconidia, the presence or absence of microconidia and chlamydospores, colony color and conidiophore structure [42]. Macroscopic and microscopic features, such as the color of the colony, length and shape of the macroconidia, the number, shape and arrangement of microconidia, and presence or absence of chlamydospores are key features for the differentiation of Fusarium species [43]. The Fusarium oxysporum is a soil borne fungi found in cultivated and uncultivated soils worldwide [7]. F. oxysporum have high functional and genetic diversity [6]. F. oxysporum can affect perennial and annual plants, including aquatic plants (lotus), cause wilts and crown rot on field crops, garden, ornamental crops and weeds (broomrape and witchweed). Strains with the same host range are grouped into forma specialis. In some formae speciales are subdivided into races by cultivar specialization [44]. Based on size and shape of macroconidia, presence or absence of microconidia and chlamydospores, colony color, and conidiophore Fusaria classified [42]. Morphological pictures of plant pathogenic, saprophytic and bio-control strains of F. oxysporum are indistinguishable. Based on taxonomic fusaria recognizing more than 100 species [23][24][25]. Pathogenic strain is very host specific, attacking only one or a few species and certain cultivars and designated as formae speciales and race of the pathogen. Proposed a system of classification of F. oxysporum strains, on basis of vegetative compatibility group (VCG), but not a universal tool of identify formae speciales or non-pathogenic isolates [45]. Nitrate reductase and phosphate permease have been used successfully to distinguish Fusarium species [46]. The presence or absence of microconidia is a primary character in Fusarium taxonomy. Fusarium teleomorphs have been described, classified into several different genera (Gibberella, Nectria) (Figure 1) [47].

Identification
There are three basic concepts for identification of Fusarium sp., by morphology can differentiate species, biological as sexual viable and phylogenetic as the common origin of the same species. Colonies character on potato dextrose agar of Fusarium species. F. oxysporum and F. solani can establish in suppressiveness soil than other species. Fusarium microconidia are oval to kidney-shaped, generally one-celled produce on short conidiophores on aerial mycelia and enter into the sap stream transported upward, macroconidia are fusiform having three to five cells and produced large numbers on sporodochia and chlamydospores are usually two types one within the macroconidium and other within the mycelium, formed singly/in pairs or chains with thick-walled, survive in the soil for a long time. A system of classification of strains of F. oxysporum, based on their vegetative compatibility as a described method based on pairing nitrate non-utilizing mutants to determine the vegetative compatibility group (VCG) of each strain and use of various molecular tools that group together genetically similarity in strains [45]. VCG cannot be used as a universal tool to identify formae speciales or nonpathogenic isolates only molecular tools can provide information for a taxonomic framework for species identification to relationships among species. Sequences of the β-tubulin region have been useful to distinguish some Fusaria [48]. Use nuclear restriction fragnment length polymorphism (RFLP) and VCG to determine F. oxysporum f. sp. radicis-lycopersici [49]. Use random amplified fragment length polymorphisms (RAPD) to differentiate races of Fusarium oxysporum f. sp. vasinfectum on cotton [50]. DNA sequences of the ITS regions are very useful in distinguishing species in many eukaryotic organisms, but not is very informative for Fusarium [51]. Random amplified polymorphic DNA identify sequence-characterized amplified region (SCAR) markers. Many formae speciales are known to be polyphyletic, making it difficult to identify specific molecular markers [52,53]. Molecular methods, such as 28S rRNA gene sequencing, may be used for rapid identification of Fusarium strains to species and subspecies levels [54]. Polymerase chain reaction (PCR) based rDNA detection method [55] and detection of protein banding patterns by SDS-PAGE and esterase isozyme electrophoresis [56]. Cultures of Fusarium species grown on Sabouraud Dextrose Agar at 25°C produce wooly, cottony, flat or spreading colonies [57]. F. oxysporum are responsible for severe damage on many economically important plant species and show a high level of host specificity, based on infection of the plant species and plant cultivars they are classified into more than 120 formae speciales and races [58]. Molecular tools are providing species identification as well as evolutionary relationships among species.

Fusarium oxysporum
F. oxysporum is soil-borne pathogen that survive in the soil for a long time in the form of chlamydospores, penetrates the roots and colonizes in xylem vessels, systemic appear as yellowing, wilting, and death in plants. F. oxysporum are saprophytic and able to grow and survive for long periods on organic matter in soil and in the rhizosphere of plant species [59]. Some strains of F. oxysporum are pathogenic on plant species causing wilt and responsible for severe damage on many economically important crops and show host specificity based on the plant species and plant cultivars. They are classified more than 120 formae speciales and races [58]. Some strains can penetrate roots, but do not invade the vascular system [60]. F. oxysporum strains are responsible for two types of symptoms, such as vascular wilting and rotting. Vascular wilt resulting in yellowing and wilting of the plant [61]. Rotting of root without reaching the vascular system is called basal rot, stem rot, crown, root rot and also affect storage organs such as bulbs, corms, tubers and rhizomes. The first rot reported on lupine was caused by F. oxysporum f. sp. radicis-lupini. The term "radicis" can differentiate rot-producing strains from wilt-producing strains. The "radicis" name of the forma specialis to allow for identification of the type of symptoms. Some formae speciales such as cepae, lilii, and opuntarium, cause rotting but are not referred as formae speciales "radicis-host plant name. F. oxysporum causes disease on vanilla, described to as forma specialis radicis-vanillae. Two different formae speciales are causing two types of symptoms in tomato as the forma specialis lycopersici causing wilt and radicis-lycopersici causing rot, Table 1 [62-66].

Fusarium oxysporum 'formae speciales ( f.sp.)' and race
Host range of plant species are grouped into a forma specialis and subdivided into races by cultivar specialization [44]. More than 100 formae speciales of F. oxysporum causing diseases in different plant species. Forma specialis is determined by testing the fungus for pathogenicity on various plants species and race is determined by pathogenicity on cultivars of a single plant species. Molecular tools can identify pathogenic strains and in some cases races of the pathogen. A forma specialis of fusarium fungus normally affects only one primary host species, but colonize endophytically in the roots of secondary hosts. Many formae speciales were named according to the host plant either the genus name/species name. A reported 106 formae speciales and 58 additional host plants which have no forma specialis as characterized and races based on cultivar identified 25 of the 106 formae speciales ( Table 2) [67].

Diseases
The genus Fusarium species cause vascular wilts, root, stalk and cob rots, collar rot of seedlings, and rots of tubers, bulbs and corms, some species also produce mycotoxins in contaminating grain, diseases as ear and kernel rot of corn, scab of rice and wheat and stalk rot and grain mold infection of sorghum. Fusarium species are causing diseases such as crown rot, head blight and scab on cereal grains; vascular wilts on a wide range of horticultural crops; root rots; cankers; and other diseases such as pokkah-boeng on sugarcane and bakanae disease of rice. Wilts are important in many cultivated crops. Fusarium pathogens survive as chlamydospores in soil for long periods. Wilt pathogens colonize in the root cortex of some non-host plants. Chlamydospores form in the cortex when the plant dies. Fusarium produces harmful secondary metabolites known as mycotoxins [18]; toxicity to animals, humans, plant pathogens, and also in food and feeds [68]. Mycotoxins are secondary metabolites produced by Fusarium species and threat to animal and human health. Earlier infections during the harvesting some of them produce mycotoxins in agricultural products [69,70]. Mycotoxins, are the trichothecenes, fumonisins, and zearalenone [71]. Vascular wilt fungus produces the characteristic xylem vessel clogging and wilting of plants. Colonization and clogging of vessels in addition to the secretion of several toxins by the fungus including fusaric acid, lycomarasmin, dehydrofusaric acid, play a major role in the development and progression of wilt symptoms [18]. First detected fusaric acid (in-vivo) in wilted cotton plants and suggested that responsible for the production of wilt symptoms [72]. Fusaric acid is a toxin in tomatoes and cotton [73]. Fusaric acid is well-known for its phytotoxicity and role in the pathogenesis of Fusarium wilts [74]. Fusarium species as plant pathogens, causing diseases such as crown rot, head blight, and scab on cereal grains; vascular wilts on a wide range of horticultural crops root rots; cankers; and other diseases such as pokkah-boeng on sugarcane and bakanae disease of rice [23].
Fusarium -An Overview on Current Status of the Genus 8

Symptoms
The pathogen colonizes in the xylem, growing up the vascular system in the stem, disease development and symptom expression of host plants depend on the colonization of vessels by the pathogen [75]. Hyphae and chlamydospores of diseased plant debris in the soil infect young rootlets and enter the xylem vessels. Colonization in the plant causes a reaction, producing brown phenolic compounds and tyloses. Browning of vascular tissue is a key symptom of pathogens that cause vascular wilt disease. Blocking of the xylem decreases water movement, causing the infected plant to wilt and die. Yellowing, wilting and stunting are general symptoms of many diseases of the root and stems. Early symptoms appear as leaf yellowing, slight wilting during the day and stunting. Wilt starts vein clearing on the younger leaves and drooping of the older lower leaves, followed by stunting, yellowing of the lower leaves, defoliation, marginal necrosis and plant death. This seed and soilborne plant pathogen showing symptoms like chlorosis, necrosis, immature leaf fall, vascular system browning, and finally wilting. Fusarium vascular wilt diseases are more severe in warm, wet conditions. The Fusarium infects through wound sites as made by the nematode as associated with roots. Pathogenic strains of F. oxysporum are produce two types of symptoms, vascular wilting and rotting, penetrates the host roots and reach the xylem vessels, colonizes caused vascular wilt, and progressive yellowing and wilting [61].

Disease cycle
Fusarium is a monocyclic, soil-borne, diversified fungus including pathogenic and saprophytic [9,10]. Dispersing by soil, plant debris, farm machinery [76] and seeds [77] and survive more than 15 years without host plants [78]. Pathogens spread through water and farm equipment over short distances but extensive areas through contaminated soil, seeds, or seedlings. A report indicated that spread by seeds [79]. Fusarium wilt does not spread from plant to plant within a season. The fungus infects the plants by germinating spores, growing through the wounds and openings near the root hair [80]. Fungal hyphae penetrate the vascular tissue produce microconidia [80]. Microconidia are released into the xylem, which travels upward though the water and colonizes the vascular tissue [81]. In stressful environmental fungi produce chlamydospores into the soil. Fusarium wilt accomplishes by spreading chlamydospores as the primary survival of the pathogen [15].

Management
Several Bacillus spp. strains suppressive effect against plant diseases caused by soil-borne diseases. B. subtilis, produce volatile compounds and activate plant defense mechanisms by triggering induced systemic resistance [82,83]. Bacillusmediated plant growth promotion due to promote phytohormone, biosynthesis, thereby enhancing nutrient uptake ability in the host and stimulating plant defense responses against biotic and abiotic stresses [84,85]. Bacillus species can produce lytic enzymes like chitinase and β-1,3-glucanase, involved in the degradation of the fungal cell wall [86]. Trichoderma spp. show a wide genetic diversity, and are producers of several extracellular proteins, enzymes. Arbuscular mycorrhizal fungi (AMF) protect plants against phytopathogens and abiotic stresses [87][88][89]. Chemicals can prevent infection, but do not cure a plant once infected and these compounds affect beneficial soil microbiota and accumulate in the food [90]. Resistant plant varieties carrying resistance genes is currently the most effective Current Status of Fusarium and Their Management Strategies DOI: http://dx.doi.org /10.5772/intechopen.100608 in terms of economy, ecology, and disease control. However, genetically encoded resistance is seldom durable and sooner or later new races emerge that overcome resistance [91,92]. Fusarium oxysporum resistance genes are not available in the germplasm of all crops [93].
F. oxysporum are genetically varied in phytopathogens, saprophytes and bio-control agents. Management of Fusarium wilt use broad-spectrum chemical fumigates in the soil before planting that are environmentally unsafe and also living thinks, only cost-effective, environmentally safe method is resistant cultivars when these are available. Resistant crop varieties are available against some Fusarium wilt pathogens. However, resistant variety is not resistant to all races of the particular forma specialis. In case develop new races of the pathogen overcome host resistance. Managing Fusarium wilt is very difficult to manage because of chlamydospores persistent in nature for about 10 to 15 years and the development of new physiological races [15,94]. Fusarium wilts diseases are difficult to control because the chlamydospores persist for a long time in soil. These fungi can survive infecting the root cortex of some symptomless, non-host crops. Only biocontrol agents are useful in the management of diseases. Non-pathogenic strains generally developed as bio-control agents and show several modes of action in their bio-control capacity, easy to massproduce and formulate. The use of nonpathogenic strains of F. oxysporum to control Fusarium wilt has been reported for many crops [94][95][96][97][98][99][100][101][102][103][104]. On the infection sites on the roots trigger plant defense reactions; plants protect themselves from microbes by activating defense reactions such as systemic acquired resistance. During growth, plants are continuously challenged by a wide spectrum of environmental stimuli, by abiotic and biotic. Plants usually protect themselves from microbes by activating defense reactions such as systemic acquired resistance (SAR) after recognizing microbial stimuli. When plants are exposed to abiotic stimuli, the plants can acquire an improved defense by chance. Chemical stimuli, such as probenazole (PBZ), acibenzolar-S-methyl (ASM), tiadinil (TDL), and isotianil, have been used as plant activators and can induce disease resistance in plants. Foliar spray with validamycin A effectively controls soil-borne Fusarium diseases tomato wilt, and banana panama disease by inducing SAR. Once soil-borne fusaria pathogens spread in the field, their removal is very difficult. Soil treatments often are less sufficient and need to reduce their usage because of adverse effects on the environment. Biological control and resistance cutovers are alternatives to control fusarium diseases. Trichoderma lignorum was registered as a fungicide on the Agricultural Chemicals Regulation Law in Japan in 1954 to control Rhizoctonia disease in tobacco. This was the first registered bio-fungicide in the world. A non-pathogenic strain of F. oxysporum was registered in 2002 as a bio-fungicide to control soil-borne wilt of sweet potato plants caused by F. oxysporum f. sp. batatas. Trichoderma atroviride was registered as a bio-fungicide to control rice 'Bakanae' by seed or nursery-box treatment.

Conclusion
Fusarium is a large genus of imperfect fungi and numerous species are important plant pathogens. Fusarium oxysporum all strains are saprophytic, based on phenotypic and genetic characterize the strains and showed the diversity. Interactions between pathogenic and non-pathogenic strains result in the control of the disease. Complex fusarium species are the economic importance of their pathogenic/nonpathogenic activity. The development of molecular-based genomic tools to study in relation and its characterization. As 106 formae speciales have been clearly described within F. oxysporum. The pathogenic activity of F. oxysporum on plants of economic interest, many wild plants also infect by new formae speciales. Greater diversity in

Author details
Amar Bahadur College of Agriculture, Tripura, Lembucherra, Agartala, India *Address all correspondence to: amarpatel44@rediffmail.com; agcollege07@gmail.com F. oxysporum and within formae speciales may be revealed over time by using new plant genotypes resulting from breeding. Fusarium species has a significant role in socio-economic and international trade for food security as ability to destroy crop yields and contaminate plant products. New populations of Fusarium pathogens will continue to emerge through micro-evolution and the invention of exotic pathogens. Need the research on the biology of the fungus to determine their role of non-host crops and length of survival of chlamydospores in soil.
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