*2.7.1 Microbial antagonisms*

Microorganisms that have the ability to grow in plant rhizophere are considered to be ideal for use as biological control agents. The rhizophere provides a leading edge defense for plants roots against disease causing microorganisms by suppressing pathogens growth and infestation. Pathogen-antagonizing metabolites produced by beneficial microbes that colonize the plant root, help to suppress phytopathogens' growth and thus preventing them from penetrating the root system [87]. Furthermore, this antagonistic relationship displayed between the beneficial microbes and pathogens often results to significant disease control, in which the established metabolites produced by active beneficial microbes protects plants either by directly antagonizing pathogen activity directly, by outcompeting

pathogens or by stimulation of host plant defenses (priming) [88], also displays its antagonism against pathogens by antibiosis which is the secretion of diffusible antibiotics, volatile organic compounds, and toxins, as well as the development of extracellular cell wall degrading enzymes such as chitinase, β-1,3-glucanase, betaxylosidase, pectin methylesterase and many more [87, 89]**.**

#### *2.7.2 Plant-microbe mutualistic interaction*

Microbes that inhabit plant rhizophere are nourished with nutrients obtained from plant roots in the form of root exudate and lysates. The plant-microbe interaction is not only beneficial to the microbe but it also improves plant nutrition, growth and proliferation and do enhances plant's ability to prevail over biotic and abiotic stress. This associoation gives the plant a good competitive advantage due to the presence of rhizophere [90]. Various endophytic bacteria and free-living rhizobacteria that inhabit the root surface and rhizosphere secrete metabolite substances that suppress deleterious pathogen growth and activity which invariably leads to the control plant diseases caused by fungi or bacteria [91–94].

Furthermore, microorganisms can be directly involved in plant growth promotion, by acting as agents for stimulation of plant growth and management of soil fitness, for example through the production of auxin [95].

#### *2.7.3 Production of allelochemicals/antimicrobial compounds*

Allelochemicals/antimicrobial compounds produced biological control bacteria helps improve the plant-microbe rhizophere niche. Example of such compounds include iron-chelating siderophores, antibiotics, biocidal volatiles, lytic enzymes (chitinases and glucanases), and detoxification enzymes. These chemical may have detrimental effect on target pathogens, some help the plant to induce resistance against pathogen infestation and attack while some assist in nutrient absorption which promotes plant growth [96–98]. For example, rhizobacteria include antibiotic-producing strains such as *Bacillus* sp. producing iturin A and surfactin, *Agrobacterium* spp. producing agrocin 84, *Pseudomonas* spp. producing phenazine derivatives, pyoleutorin and pyrrolnitrin, and *Erwinia* sp. producing herbicolin A [99, 100], that are tenacious in the rhizosphere [101, 102]. The mycoparasitism of phytopathogenic fungi of the Trichoderma and Streptomyces genera have important roles in secretion of chitinases and glucanases [103]. A common feature of successful biocontrol strains and a crucial factor for plant root pathogen suppression is the production of antibiotic compounds and fluorescent siderophores that enable effective competition for iron [104].

*Trichoderma* spp., are universally known as BCAs and used to prevent plant pathogens and increase plant immunity in field and greenhouse conditions [105]. This is due to its ability to interact with plants (maize, cotton, cucumber) through production of auxin like compounds and secondary metabolites [106–108]. BCAs of *Trichoderma* spp. have ultimate functions in promoting the plant beneficial microbial community and decreasing the pathogen attack through the specific interactions with host-pathogen. In maize, growth-promoting and antifungal compounds-producing bacteria have been shown to have inhibitory effects on southern leaf blight disease caused by the fungus *Cochliobolus heterostrophus* [109, 110].

#### *2.7.4 Induced systemic resistance (ISR)*

Van Peer et al. [111] first discovered rhizobacteria-induced systemic resistance or ISR, also referred to in its early stage as priming. It is as an enhanced defensive

#### *Microbiological Control: A New Age of Maize Production DOI: http://dx.doi.org/10.5772/intechopen.97464*

capacity of the whole plant to multiple pathogens induced by beneficial microbes in the rhizosphere [112] or elicited by specific environmental stimuli which lead to potentiation of the plant's innate defense against biotic challenges [113]. Nonpathogenic rhizobacteria are capable of activating defense mechanisms in plants in a similar way to pathogenic microorganisms, including reinforcement of plant cell walls, production of phytoalexins, synthesis of PR proteins and priming/ISR [112]. Plants that possess ISR displays stronger and/or faster activation of defense mechanisms after a subsequent pathogen or insect attack or as a response to abiotic stress, when inoculated with rhizobacteria [114].

### **2.8 Entomopathogens**

Entomopathogens are microorganisms that are pathogenic to arthropods such as insects, mites, and ticks. Various species of naturally occurring bacteria, fungi, nematodes, and viruses infect a several arthropod pests and play an important role pest management. Some entomopathogens are produced in large scale as in vitro (bacteria, fungi, and nematodes) or in vivo (nematodes and viruses) and sold commercially. In some scenario, they are also produced on small scale for noncommercial local use. The use of entomopathogens as biopesticides is an alternative method to chemical control and a novel approach pest management, which can be a profound part of integrated pest management (IPM) against several pests [24].

#### *2.8.1 Entomopathogenic fungi*

They typically cause infection when spores come in contact with the arthropod host. Fungal spores germinate and breach the insect cuticle through enzymatic degradation and mechanical pressure to gain entry into the insect body provided the environmental conditions such as moderate temperatures and high relative humidity are in place. Once inside the body of the insect, the fungi multiply, invade the insect tissues, emerge from the dead insect, and produce more spores [24]. Fungal pathogens have an eclectic host range and are especially suitable for controlling pests that have piercing and sucking mouthparts reason being that spores do not have to be ingested. However, entomopathogenic fungi are also effective against a variety of pests such as wireworms and borers that have chewing mouthparts [24].

The potential use entomopathogenic fungus has been reported by some researchers. For example, *Beauveria bassiana* (Bal.) Vuillemin (Deuteromycotina: Hyphomycetes) can be used against the following stored-grain insects: rice weevil (*Sitophilus oryzae*), corn weevil (*S. zeamais*), granary weevil (*S. granarius*), lesser grain borer (*Rhyzopertha dominica*), red and confused flour beetles (*Tribolium castaneum* and *T. confusum*), *Oryzaephilus surinamensis*, and *Prostephanus truncatus* [115–121], and for another entomopathogenic fungus *Metarhizium anisopliae* (Metch.) Sorokin (Deuteromycotina: Hyphomycetes) against the following storedgrain insects: rice weevil (*S. oryzae*), lesser grain borer (*Rhyzopertha dominica*), and red flour beetle (*Tribolium castaneum*) [120–125].

#### *2.8.2 Entomopathogenic bacteria*

Entomopathogenic bacteria are well known for their ability to produce a plethora of protein insecticidal toxins [126]. Bacterial toxins acting as virulence factors have been shown to range from very specific to broad insecticidal spectrum ever since it was first discovered in the 19th century. When compared with chemical insecticides, bacterial toxins displayed high diversity of simultaneous action, contributing

to the sustainability of bacteria-based bio-pesticides by limiting insect resistances. *Bacillus thuringiensis* (Bt) has been profoundly used in biocontrol of insects and it represents approximately 95% of microorganisms used in biocontrol [127].

*B. thuringiensis* produces protein-based δ-endotoxins known as "Cry", which are lethal for several species of various insect orders [128]. Presently, about 170 different "Cry" toxins have been identified, which are effective against several coleoptera, lepidoptera, and diptera species [129]. These proteins are produced upon sporulation, and are contained in crystal inclusions. Once ingested, crystals inclusions are solubilized by the insect proteases in the midgut, inadvertently activating the "Cry" proteins [130]. A vast number of research work has produced various of Bt-based insecticides, ranging from wettable powder or liquid formulation to transgenic crops, thereby facilitating their use in organic farming and integrated pest management (IPM) programs.

#### *2.8.3 Entomopathogenic viruses*

As compared to entomopathogenic bacteria, entomopathogenic viruses are also required to be ingested by the insect host and are therefore ultimate in controlling pests that have chewing mouthparts. Diverse lepidopteran pests are important hosts of baculoviruses including nucleopolyhedroviruses (NPV) and granuloviruses (GV). These related viruses have various types of inclusion bodies in which the virus particles (virions) are implanted. Virus particles attack the nucleus of the midgut, fat body or other tissue cells, compromising the integrity of the tissues and liquefying the cadavers. Before the insect pathogen dies, infected larvae climb higher in the plant canopy, which helps in dispersing virus particles from the cadavers to the lower parts of the canopy. This conduct assists in the proliferation of the virus to cause infection in healthy larvae. Viruses are host specific and can cause remarkable reduction of host populations. Examples of some commercially available viruses include *Helicoverpa zea* single-enveloped nucleopolyhedrovirus (HzSNVP), *Spodoptera exigua* -enveloped nucleopolyhedrovirus (SeMNPV), and *Cydia pomonella* granulovirus (CpGV) [24, 131].

#### *2.8.4 Entomopathogenic nematodes*

They are microscopic, soil-inhabiting worms that are detrimental to insects. Diverse species of *Heterorhabditis* and *Steinernema* are obtainable in multiple commercial formulations, majorly for managing soil insect pests. Infective juveniles of entomopathogenic nematodes actively explore their hosts and penetrate through natural openings such as the mouth, spiracles, and anus or the intersegmental membrane. Immeddiately the get into the host body, the nematodes extricate symbiotic bacteria that kill the host through bacterial septicemia. *Heterorhabditis* spp. carry *Photorhabdus* spp. bacteria and *Steinernema* spp. carry *Xenorhabdus* spp. bacteria. *Phasmarhabditis hermaphrodita* is also available for controlling slugs in Europe, but not in the USA [24].

#### **2.9 Application of biocontrol**

#### *2.9.1 Seed dressing*

A suitable method for suppressing plant pathogens in the spermosphere and rhizosphere is dressing seeds with biocontrol agents [132]. Recently, bacterial inoculants have been used to antagonize soil-borne plant pathogens such as *Fusarium verticillioides (Fv)* and to promote plant growth. *Bacillus subtilis*

*Microbiological Control: A New Age of Maize Production DOI: http://dx.doi.org/10.5772/intechopen.97464*

and *Pseudomonas cepacia* have been used to control root rot caused by *Fv* in Argentina [133]. *Bacillus amyloliquefaciens* or *Microbacterium oleovorans* can reduce the fumonisin content in harvest grains during three evaluated seasons [134]. *Burkholderia* spp. stimulate plant growth and suppress disease caused by *Fv* in maize [45], and species like *Bacillus amyloliquefaciens* and *Enterobacter hormaechei* reduce the *Fv* infection and fumonisin accumulation in maize kernels [135]. Another example, is the application of *Gliocladium virens* and *Trichoderma viride* isolates on corn seeds for the reduction of Pythium and Fusarium-induced damping-off [136].

#### *2.9.2 Rhizophere inoculation*

Inoculation of rhizophere with biocontrol agents by alters the rhizosphere microbiota, thereby antagonizing soil-borne plant pathogens and promote plant growth. *Bacillus subtilis* and *Pseudomonas cepacia* have been used to control root rot caused by Fv in Argentina [133].

#### *2.9.3 Conventional spraying*

Entomopathogens viz., fungi, bacteria, virus and nematodes have an important place in the biological control because they have a wide host range, are harmless to the environment and human, and could be applied with conventional sprayers. They can be used more against stored product pests with the development of new biotechnical methods such as collecting pests in some stations to meet them with entomopathogens [137].

#### **2.10 Advantages of microbiological control**

### *2.10.1 Reduced use of Insecticides*

Many farmers have adopted the use of microbiological control agents (MCAs). Bt maize is an example of MCA, it has provided maize farmers testimonies coupled with both economic and environmental advantages. Many farmers quote unique opportunities to protect yield and reduce handling (and use) of insecticides to explain their rapid adoption of Bt maize [138].

#### *2.10.2 Protected yields*

Over the years, maize farmers had challenges in controlling corn borers because insecticides are not successful after larvae have tunneled into the stalk. In 1990, entomologists experimented the use of Bt maize and found out the "bullet proof" effect it gave to corn borer. Until then, plant breeders were able to increase host plant resistance, but none of these plants were "bullet proof". That has been the reason why farmers chose to use Bt maize which resulted in higher yields due to this reduced insect injury [139].

#### *2.10.3 Improved grain quality*

The use of Bt maize also helps to reduce the occurrence of ear mold on the field. This is as a result of the reduction of insect attacks that provides a site for infection by molds, Bt-protected maize can have lower levels of toxins produced by molds (i.e., mycotoxins), especially fumonisin and deoxynivalenol [140, 141]*.* Consequences of contamination with mold may be serious, as fumonisins can cause fatal leukoencephalomalacia in horses, pulmonary edema in swine, and cancer in laboratory rats. Economic analysis suggests that USA farmers save \$23 million annually through reduced mycotoxins [142] and mycotoxin reduction also could be a significant health benefit in other parts of the world where maize is a diet staple [143].
