The term ‘mycorrhiza’ was coined by Frank in 1885. Mycorrhiza are highly specialized obligate symbiotic association between fungus and roots of land plants. Broadly, mycorrhiza is of two types; ectomycorrhiza and endomycorrhiza. Ectomycorrhiza are the symbiotic association mostly found with ornamental and forest plants belonging to families, Pinaceae, Gnetaceae, Salicaeae, Dipterocarpecae, Fagaceae, Myrtaceae. Ectomycorrhiza are identified by the presence of mantle and hartig-net. Endomycorrhiza or ‘arbuscular mycorrhiza’ are most common and found in association with most of the land plants.

Arbuscular mycorrhiza

Arbuscular mycorrhiza (AM), earlier know as Vesicular arbuscular mycorrhiza (VAM) are found in symbiotic association with more the 72 % of the land plants. AM fungi belong to phylum Glomeromycota. Glomus, Gigaspora, Acaulospora are some of the common AM fungi. AM fungi are obligate symbiont and can not be cultured in absence of host roots. AM association consists of intraradical (inside the root) and extraradical (outside the root in soil) structures. Intraradical structures consists of hyphae, arbuscules, spores, and vesicles. Hyphae ramify in the intercellular spaces of root cortex (arum type) and some times form coils within the cortical cells (paris type). Arbuscules are tree-like branched structures found inside the root cortex. Arbuscules are heart of AM symbiosis, as exchange of water and nutrient across the two partners occurs through the peri-arbuscular membranes. Vesicles are the bulbous storage organs. Vesicles are not found the in the family Gigasporaceae, instead auxillary cells are found outside the roots. Spores are bulbous structures borne terminal, in some cases, intercallary a and behave as propagules.

Arbuscular mycorrhiza
A dissected root showing AM association and inraradical structures

Extraradical structures consists of highly branched hyphae and spores. The hyphae of AM fungi reaches beyond the access of root. In-fact most of plants have mycorrhizal structures for nourishment in true sense.

Arbuscular mycorrhiza
Extraradical hyphae and spores of arbuscular mycorrhizal (AM) fungi

In AM association, fungus derive carbon (not all) from the host plants, whereas it provides water, minerals and defends host plants from numerous abiotic and biotic stresses. Plant pathogens are key biotic stress to the plants, which AM association mitigates. AM fungi are known to act as a potential biocontrol agent against several pathogens.

AM fungi as a biocontrol agent

AM fungi protect the host plants from many pathogenic diseases. Application of AM fungi as biocontrol agent reduces the dependency upon chemicals, which otherwise causes harm to both plants as well as ecosystem.There is no direct evidences regarding the interaction (competition, mycoparasitism, predation, etc.) of AM fungi with pathogens. However, following mechnanisms can be generalized.
  • AM fungi protect the plants by enhanced growth performance by increased uptake of nutrients.
  • Extraradical structures of AM fungi modify the soil habitat (rhizosphere and rhizoplanes), which changes the population and activities of pathogen antagonists.
  • In another way, AM fungi compete with pathogen in terms of spaces in the roots.
  • AM fungi also compete with the pathogen in terms of carbohydrates acquisition.
  • AM fungi changes the root morphology and anatomy, thereby suppress pathogen entry and establishment.
  • AM fungi systemic resistance in plants.

AM fungi suppress a number of plant diseases that are caused by fungi, bacteria, nematodes, etc. Some the examples are listed here.
  • AM fungi protect pepper plant against Verticillium dahliae.
  • AM fungi suppress the haustoria formation by angiospermic hemiparasite Pedicularis kansuensis in Elymus nutans.
  • AM fungi along with co-inoculation of Trichoderma sp. control the wilt disease of pigeon pea.
  • AM fungi reduces the infection of Alternaria solani in tomato.
  • AM suppress the infection of root by a number of nematodes.
  • AM fungi has biocontrol potential against Fusarium verticillioides causing root, stock and ear rot.
  • AM fungi suppress the proliferation of Phytophthora parasitica in tomato roots.
  • AM fungi effectively suppress the Fusarium oxysporum in the soil.
  • AM fungi suppress the nematode Nacobbus aberrans causing gall formation in tomato.

For effective control of plants pathogen in the soil, established symbiotic association of AM fungi with host root is prerequisite.

Mycorrhizal (AM fungal) inoculum

One of the major constrains in utilization of AM fungi in sustainable agriculture is the availability of AM fungal inoculum. As the AM fungi obligate, they can not be cultured or multiplied artificially, however, they can be multiplied in the living hosts.

Where to get AM fungal inoculum?

AM fungi inoculum can be obtained from a number of market supplier and laboratories. Here are the list of some sources of mycorrhizal inoculum.

How to multiply the inoculum?

Inoculums are produced in the sophisticated laboratories through the trained scientists, hence the cost of inoculum is high. Therefore, the multiplication of mycorrhizal inoculum before application is recommended.