**5. Use of mycorrhizal fungi in plant growth and disease suppression**

Plants are major source of energy for both human and animals providing about 80% of food consumed by humans and primary source of nutrition for livestock. Production of adequate food to feed the ever growing global is threatened by the high prevalence of diseases caused by biotic agents such as bacteria, fungi, nematodes, viruses and oomycetes. Plant diseases reduces quantity and quality of yield, thereby affecting food security and safety of produce for consumption. It is estimated that diseases account for yield loss ranging from 13 to 22% with billions in economic losses due to inputs purchase for their management [80]. Yield and storage losses attributed to diseases have significantly been linked to global starvation and malnutrition millions of people [80, 81]. Diseases reduce yield of plants by altering several physiological process such as the absorption and transportation of water and nutrients needed for plant use, photosynthesis, flower and fruit development [82].

Plant diseases results from positive interactions of host, pathogen and environment. To overcome the negative impact of diseases on plant growth, multiple strategies have been developed and successfully used to manipulate host-pathogen interactions to favor growth of the host whilst suppressing reproduction, establishment and transmission of the pathogen. Some of the approaches to controldiseases include the use of chemicals, physical, genetic and cultural means. Host resistant approach is economical, effective and environmentally friendly, however, rapid breakdown due to continuous pathogen evolution limits its use in commercial and modern crop production tilted towards intensification and mono-cropping which provides ideal environment for pathogen evolution. In situations where reliance on resistant varieties to suppress diseases has not been achieved, utilization of chemicals have become inevitable. Chemicals are highly effective but its harsh effect on human and animal health, non-target organisms and the environment resulting from excessive use has unfortunately defeated its mass promotion and utilization. An alternative to chemical pesticide is the use of biological control. According to [83, 84], biological control limits diseases causing pathogen, improves plant immunity, modifies environment through efficient cropping systems. Biological control agents offer advantages over chemical control agents by being antagonistic to specific pathogens with less risk compared to chemical pesticides. Contrary to its benefit, application of BCAs is heavily challenged by several biotic and abiotic factors as well as frequent pathogen evolution which makes field application frequently inconsistent [85]. Notwithstanding this, recent reports have shown that application of mycorrhizal fungi strains as biological control agents is an important option to reduce threats posed by diseases.

Mycorrhizal fungi exists closely with over 80% of plants species on land offering plethora of benefits to its host. These fungi may reside within the cortex of plant roots. Mycorrhiza fungi-host association contributes significantly to carbon, nitrogen and phosphorus cycling in the ecosystem. In addition to these, mycorrhiza fungal activities improve water uptake by increasing quantity of available soil water [86] thereby improving plant productivity, diversity and contributing significantly to plant growth and fitness. According to [87] these fungi alters root morphology by increasing root branching and growth to favor root vigor due to the high nutrient uptake [88] hence influencing plant growth and yield. Mycorrhizal fungi have successfully been used and reported to increase growth and yield of several crops such as carrot [89], yam [90], maize [91].

Biocontrol by mycorrhizal strains against multiple diseases is achieved by triggering defense mechanisms in the host to improve plant tolerance to pathogens. Earlier studies [92–94] have shown improved tolerance and suppressive ability of plants to vascular diseases caused by Fusarium, Verticillium and Bacteria. Other studies have demonstrated improved tolerance of cucumber, olive, date palm, and tomato seedlings to fusarium and bacteria wilt following application of mycorrhizal strains [95, 96]. Research findings by [21, 97] showed that multiple root branching resulting from root alteration due to interaction with host reduced infection of *Phytophthora fragariae* in strawberry. Root necrosis in cowpea caused by *Rhizoctonia solani* and *Pythium aphanidermatum* in pepper were reported to decrease in the presence of both *Glomus clarum* and *Gnypeta deserticola*. Although most success has been achieved in the use of Mycorrhizal fingi (MF) to manage soil borne fungal pathogens, other works have reported on their biocontrol potential against aerial pathogens like *Alternaria solani* in tomato [37] and other necrotrophic and biotrophic pathogens [98]. Apart from fungal pathogens, suppression of Plant Parasitic Nematodes (PPN) by MF has been reported in plants such as banana, coffee and tomato [99, 100]. Similarly, [101, 102] concluded that AMF can attack soybean cyst nematodes and reduce severity of nematode infection in crops such as soybeans, cotton, cucumbers, tomatoes and citrus. MF antagonizes activities of PPN by reducing infection, reproduction and enhances tolerance. Although many research outputs have concluded on the biocontrol potential of MF, mass application in the field is sporadic due to variability in performance, on host, pathogen isolate and environmental condition [103]. There is the need therefore to improve communication on the efficacy and safety associated with MF application biocontrol agents.

### **6. Mycorrhizal fungi for sustainable agricultural systems**

Sustainable agricultural systems use available natural resources to achieve acceptable level of productivity, food quality, and quantity without compromising the environmental impacts [27]. As defined, sustainable agriculture is the use of ecological sound, economic viable and socially responsible practices to obtain higher productivity, numerous plant health practices contribute to sustainable agriculture through the control of soil-borne diseases by increasing soil microbial activity thereby enhancing symbiosis, competition, and parasitism within the rhizosphere [104, 105]. The current research focus has been the search for suitable alternatives to the use commercial synthetic pesticides. Many have been achieved though, but the efficient exploration of microorganisms to improve soil fertility and at the same time enhancing plant growth and protection is being pursued.

To improve crop protection, synthetic pesticides have been used extensively to mitigate effects of pest and diseases, over reliance has been a problem and therefore biological processes that will enhance plant health such as mycorrhizae, earthworm and other symbionts should be encouraged [106, 107]. Mycorrhizae association with plants are beneficial for sustainable agriculture as they reduce pests and diseases infestation [7].

### **7. Conclusion**

This chapter has demonstrated that Mycorrhizae fungi especially AM fungi play critical role in plant protection. Apart from making nutrient available for the plant

*Role of Mycorrhizae in Crop Protection DOI: http://dx.doi.org/10.5772/intechopen.109020*

uptake, AM fungi provide protection to the plant thereby enhancing plant growth and yield. The AM fungi live symbiotically with the plant by benefiting from nutrient rich environment provided by the plant in the rhizosphere and the plant also benefit from the AM fungi through deprivation of other pathogen from getting direct contact with the plant thereby enhancing its ability to resist pathogens attack.
