**3. 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].

## **4. 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].

#### **4.1 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].

## **5. 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 [87–89]. 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

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 [94–104]. 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.
