**2. Importance of mycorrhizal fungi as a biocontrol agent in suppressive soils**

The management of plant disease by chemical approach has been one of the classical methods in agriculture that has sustained productivity for ages. As much as this has been helpful, it has also resulted in nearly an uncontrollable levels of pesticide resistance among many plant pathogens. Also, their direct and indirect impacts have led to the destruction of non-target and beneficial soil organisms as well as raising various health concerns among human and animal populations [4, 11]. To manage this, an appreciable number of studies in recent times have focused on identifying and engineering micro-organisms (i.e., mycorrhiza, bacteria, fungi, and nematodes) that are naturally antagonistic to various plant disease causing pathogens. This approach comes as a more environmentally friendly approach to the application of synthetic pesticides [12, 13]. Among all the organisms, the mycorrhizal fungi are the commonest, largest in biomass and the most important beneficial fungi group. They also combine this with a target specific inhibitory or antagonistic reaction on various soil-borne phytopathogens [3]. By means of changing both the anatomical and morphological structures of plant roots, mycorrhizal fungi improves both the chemical and physical properties on the root-zone environment, hence activating various defensive and disease resistance systems in the plant [14]. Furthermore, they have the abilities to minimize the damage infringed by bacteria, fungi, nematode, as well as other phytopathogens of crops such as *Musa nana*, *Fragaria ananassa*, *Medicago truncatula*, *Cucumis sativus*, *Lycopersicon esculentum, Cucum ismelo*, *Olea europaea*, *Zea mays*, *Citrus reticulata*, *Solanum tuberosum*, among other plants [3, 4, 15].

Most mycorrhizal fungi, usually being present as a biotrophic symbiotic microorganisms in the soil rhizosphere usually have a common invasion and ecological niche as most soil-borne pathogens. This could mean that under fair conditions, there must be a spatial competition between pathogens and most mycorrhizal fungi. In this, mycorrhizal fungi, eg., the Arbuscular mycorrhizal fungi (AMF) have been reported to have the potential of reducing the initial and reinfection rates of most pathogens that infects the root epidemics. For example, various studies have reported some

competitive relationships between the AMF and an array of plant pathogens. i.e., bacteria, fungi, and nematodes [16–19].

Mycorrhizal fungi played some significant roles in regulating plant growth and development. For example, cucumber plants were found to have some higher levels of zeatin, GA, and IAA when it was inoculated with *G. terrestris*. This increment was further observed to have a bearing with an enhancement in the plant's resistance ability to *Rhizoctonia solanacearum*. In general, mycorrhizal fungi proves significant in the development of plants by inducing synthesis of various plant signaling substances, and improving and enhancing activities of enzymes.
