**6. Disease control through sustainable approach**

The significant problems caused by *Fusarium* spp. in maize crop production worldwide include reduce in crop quality, decrease in yield, and higher production costs. Once soilborne *Fusarium* spread in the field, it is very difficult to control. The one of the major reason for not being able to control the *Fusarium* diseases of maize including ear and stalk rot diseases is, its nature of being survived in crop residue for longer period of time and being endophytic, many *Fusarium* spp. remain away from the contact of chemical control [16]. Currently, in order to control the soilborne pathogens the conventional methods of controlling like soil disinfection using fumigants, hot water, or solarization, or using resistant cultivars are very popular and even the chemical fertilizers are in great use [69]. However, the efficacy obtained with these treatments is not up to the mark and found less effective than expected. Many of these methods have been found to be very harmful not only to the plants only but man; animal and associated beneficial microbial communities are severely affected. Therefore, there is an urgent need of finding such an alternative option which could be eco-friendly and sustainable. There are many microorganisms dwelling in the rhizospheric zone of the host plant which significant not only in controlling the soilborne diseases causing pathogen but their role in plant growth promotion is quite commendable. These biocontrol agents have been proved to be eco-friendly, less expensive and more sustainable tools for disease management. Biocontrol agent use different mode of action in order to control the disease which include nutrient competition, antagonism, and production of toxic metabolites and induced systemic resistance (ISR) through the production of defense enzymes. Many microorganism like *Trichoderma* spp., *Penicillium*, *Bacillus* spp., *Rhizobium* and *Pseudomonas* spp. have been reported to function as plant growth promoting fungi (PGPF) and plant growth promoting rhizobacteria (PGPR) in addition to their potentiality to boost defense mechanism [70, 71]. Among the fungi, *Trichoderma* is often considered as universal biocontrol agent due to it extra ordinary function such as mycoparsitism, antibiosis, production of extracellular enzyme, competition for space and nutrients etc. [72].

#### **6.1 Antibiotic production**

Antibiosis, a kind of interaction takes place between two organisms when one produces antimicrobial metabolites called antibiotics that directly check the growth and metabolism of the other organism. Antibiotics are low molecular weight toxic organic compound produced by many organisms in order control the growth of pathogen. It is assumed to be one of most effective measures having antagonistic activity against wide range of phytopathogen. Bacteria can either produce single antibiotic and toxin or can produce them in multiple numbers. The antibiotic and toxin produce by bacteria include pioluteorin, pyrrolnitrin, hydrogen cyanide (HCN), oomycins, polymyxin, circulin, colistin and tensin etc. [73]. Bacteria and fungi of various genera, such as *Bacillus* spp. *Microsphaeropsis* sp. *Trichoderma* 

*harzianum* and nonpathogenic *Fusarium* spp. have been identified as microbial antagonists of *Fusarium* spp. through the mechanism of antibiosis elicited by a wide range of antifungal metabolites, including antibiotics [74]. The ability to produce multiple classes of antibiotics by various group of microorganism enhances the biocontrol activities against the phytopathogen. Microbiotacontaining fungi belonging to genus *Trichoderma* are found eliminate plant pathogens by producing specific and nonspecific antibiotics such as trichodemin, trichodermol, harzianun and harzianolide etc. [75].

#### **6.2 Production of extracellular enzyme**

Cell wall-degrading enzymes produced by biocontrol strains of bacteria and fungi have a definite role in restricting the growth of various pathogenic fungi including *Fusarium*. The exracellular enzyme such as chitinase and β-1,4-glucanase etc. interferes with fungal growth by lysing and degrading the cell and cell wall of the pathogenic fungi. *Trichoderma* spp. are very effective biocontrol agents because of their powerful extracellular lytic enzymes activity against fungi through lysis of cell walls [10]. Chitinases from bacteria and fungi are reported with fungicidal and insecticidal activities against *Fusarium* spp. showing extraordinary role in bio-control mechanism include *Streptomyces*, *Pseudomonas*, *Bacillus*, *Trichoderma* spp. and *Penicillium* spp. [71]. Various strains of *Bacillus* has been found to produce chitinase and glucanase for biocontrol mechanism against *Fusarium verticilloides* [76]. There is a report that *Trichoderma asperellum* along with other extracellular enzymes like chitinase and protease, produced β-glucanases against *F*. *graminearum* causing stalk rot of maize [77].

#### **6.3 Competition for root niche and nutrient**

Competition between pathogens and non-pathogens for nutrient resources is important for limiting disease incidence and severity. Rhizosphere is hotspot zone of microorganism and nutrient rich environment which provide a suitable platform for the interaction. Competition for these nutrients and niches is a fundamental mechanism by which beneficial microorganism both bacteria and fungi protect plants from phytopathogens. The interaction between them brings the beneficial microbes to control the disease causing pathogen. Soilborne pathogens, such as species of *Fusarium*, that infect through mycelial contact are more susceptible to competition from other soil and plant-associated microbes [78]. The nonpathogenic microorganisms extremely dependent on exogenous nutrient make them highly competitive with pathogenic microorganism. Competition for nutrient like carbohydrates in the nutrient rich environment in combination with competition for the limited amounts of nitrogen sources such as amino acids play the key roles in the antagonistic interaction [79]. Mycorrhizal fungi are also potential candidates for biocontrol through competition for space and nutrients by virtue of their ecologically obligate association with roots [80].

#### **6.4 Siderophore production**

Iron is one of the most common trace elements in nature required by almost all the living organism for their growth and metabolism. Siderophores are low molecular weight extracellular chelating compounds and have a great affinity for ferric iron that are produced by many microorganism like *Trichoderma* spp., *Penicillium* spp., *Bacillus* and *Pseudomonas* etc. in response to iron deficiency. They are secreted into the surrounding environment to dissolve iron minerals and hold it in a soluble form so that they can acquire them by diffusion and consequently enhance plant

#### *Fusarium Disease of Maize and Its Management through Sustainable Approach DOI: http://dx.doi.org/10.5772/intechopen.100575*

development by increased uptake of iron [81, 82]. Solubilization and the competitive acquisition of iron under limiting conditions restrict the availability of iron to soilborne pathogen, subsequently limiting their growth [83]. Many *Fusarium* spp. are found to be inhibited by both PGPF i.e. *Trichoderma* spp. and PGPR i.e. *Pseudomonas* through siderophore mediating competition. PGPR such as *Bacillus amyloliquefaciens* and *Microbacterium oleovorans* have been found to protect maize against *Fusarium verticillioides* [84]. Siderophores produced by *Pseudomonas* species (pyoverdine, pyochelin) has shown siderophore-mediated competition for iron and in the control of *Fusarium* [85].
