Ectomycorrhizal association describes a structure that results from a mutualistic symbiosis between the roots of higher plants and root-inhabiting fungi. Within this symbiotic relationship, the role of the fungi is to help the host plants take up water and nutrients, receiving plant-derived carbohydrates from photosynthesis in return. About 6000 plant species in 145 genera and 26 families (approximately 5600 angiosperms and 285 gymnosperms) have been estimated to possess ectomycorrhizal symbiotic fungal partners (Brundrett 2009; Tedersoo et al. 2010). Ectomycorrhizal association helps both the fungi and their host plants to overcome environmental stresses caused by low nutrients, drought, disease, extreme temperatures and heavy metal contamination (Smith and Read 2008; Courty et al. 2010; Kipfer et al. 2012; Heilmann-Clausen et al. 2014). Moreover, ectomycorrhizae can improve soil structure and nutrients; protect the plants against root pathogens; promote plant growth by producing phytohormones; and increase the photosynthetic rate of the plants (Splivallo et al. 2009; Ramachela and Theron 2010; Makita et al. 2012). Ectomycorrhizae are dominated by members of the Basidiomycota, some Ascomycota, and, rarely, Mucoromycota (Taylor and Alexander 2005; Rinaldi et al. 2008; Tedersoo et al. 2010). Generally, ectomycorrhizae produce reproductive fruiting bodies appearing above- or below-ground that are essential to the food webs of forest ecosystems and their spore dispersal (Rinaldi et al. 2008; Wilson et al. 2011).
Plant seedling regeneration and restoration are of pivotal interest to forestry, but the survival of seedlings is often poor both in nurseries and natural plantation areas, especially in mine spoils, polluted areas, and other treeless areas. Therefore, the main purpose for the application of ectomycorrhizae is to improve the survival and growth of seedlings. The potential advantages of ectomycorrhizal association in nurseries are not only the positive growth responses of the seedlings, but also a reduction of fertilization costs in an environmentally friendly manner. The role of ectomycorrhizae in forest establishment and recovery has been well-established. Numerous studies on the ectomycorrhizae inoculation of seedlings have shown increases in plant growth and productivity, the viability of seedlings, and seedling establishment on a forest restoration programs (Teste et al. 2009; Dalong et al. 2011; Brearley et al. 2016; Velmala et al. 2018). Ectomycorrhizae are particularly important for the growth of economically
important trees, including species of beech (Fagus), dipterocarps (Dipterocarpus and Shorea), eucalyptus (Eucalyptus), oak (Quercus and Castanopsis), pine (Pinus) and spruce (Picea) (Tennakoon et al. 2005; Flykt et al. 2008; Dalong et al. 2011; Kayama and Yamanaka 2016). Cenocococum, Pisolithus, Laccaria, Rhizopogon, Russula, Scleroderma and Thelephora species have been shown to increase the rate of survival and growth of eucalyptus, pine and oak seedlings in both plantation and reforestation programs (Chen et al. 2006; Jha et al. 2008; Cram and Dumroese 2012; Kipfer et al. 2012; Zong et al. 2015).
Generally, three main types of ectomycorrhizal inoculants—soil, fruiting body/spore and vegetative mycelium—have been used in nurseries. Forest soil was used as a source of indigenous ectomycorrhizal fungi in an inoculation experiment mixed with planting substrates (Kaewgrajang et al. 2013; Dulmer et al. 2014; Restrep-Liano et al. 2014; Livne-Luzon et al. 2017). This method is still used in many parts of the world, particularly in developing countries. However, the use of forest soil inoculants has the major disadvantage that the ectomycorrhizal composition is unknown. Moreover, it requires large amounts of soil and hence risks introducing plant pathogens and weeds exits. Fruiting bodies/spores of various ectomycorrhizae are easily obtained from natural forests and can be easily applied to plant seedlings as inoculants. The variety of application methods include mixing with sand, clay, or vermiculite carrier before being added to planting substrate or soil, suspension in water and drenching or irrigating, spraying, and encapsulation or coating onto seeds. Ectomycorrhizae that are “gasteromycetes” (puffball fungi) with conspicuous basidomes are better sources than the gilled fungi if large numbers of spores are required, as they are easier to collect and use. For instance, species of the genera Pisolithus, Rhizopogon and Scleroderma produce a large quantity of spores, and the approximate spore concentration in a seedling inoculation may range from 105–107 spores/ml (Chen et al. 2006; Bruns et al. 2009; Rai and Varma 2011; Aggangan et al. 2013). Most previous studies resulted in acceptable levels of ectomycorrhizal association, improved seedling growth of pines in the nursery, and improved outplanting success following inoculation with Pisolithus and Rhizopogon spores (Bruns et al. 2009; Dalong et al. 2011).
There are of course limitations to fruiting body/spore inoculants: only those ectomycorrhizal species able to produce large numbers of fruiting bodies and spores can be used, and there may be a concern about the compatibility and efficiency of ectomycorrhizae to the plant species to be cultivated. As an alternative, vegetative mycelial inoculants obtained from pure cultures of ectomycorrhizae may be prepared from a pure culture using different methods, e.g. using mycelial suspensions and substrate carriers such as forest litter, cereal grains, peat moss, vermiculite, and alginate-beads (de Oliveira et al. 2006; Rossi et al. 2007; Lee et al. 2008a, b; Restrep-Liano et al. 2014; Kayama and Yamanaka 2016; Kumla et al. 2016). This inoculant type has proven to be the most suitable method because of their efficiency in promoting plant growth by selected fungal isolates. However, optimal conditions, including nutrition, temperature and substrate carrier, must always be established empirically for large-scale production.
Several commercial ectomycorrhizal products have been developed. For instance, the commercial mycelial inoculants of MycoRhiz®, Ectomycorrhiza Spawn®, Somycel PV and Mycobead® are available. BioGrow Blend®, MycoApply®-Ecto, Ectovit® and Mycor Tree® Ecto-Injectable are commercially available products with ectomycorrhizae spores. The commercial products produced by mixing endomycorhizae and ectomycorrhizae spores are MycoApply®-Endo/Ecto, BioOrganicTM Mycorrhizal Landscape Inoculant and Mycoke® Pro ARBOR·WP. In order to apply ectomycorrhizae in forestry, it is necessary to select ectomycorrhizal isolates of high compatibility and efficiency in the colonization of the target plant species. Inoculant types, as well as inoculation protocols and skills in nursery practices, lead to the success of an inoculation program under the proper environmental conditions in the plantation site.
The potential for arbuscular mycorrhizae to increase crop yields has been known for decades, but there are few published studies demonstrating the effectiveness of the large-scale inoculation of globally important crops, especially in the tropics where population growth is high (Rodriguez and Sanders 2015). Therefore, researchers need to study large-scale arbuscular mycorrhizae application to crops in the tropics where phosphate bioavailability is low and the application of arbuscular mycorrhizae has the strongest potential to increase food production and reduce the need to apply phosphate fertilizers (Ceballos et al. 2013). Manufacturers should ensure their arbuscular mycorrhizae products are free from other microorganisms and ensure product quality and sufficient weight for cheap transport. Farmers should have easy access to arbuscular mycorrhizae products, correctly apply them to the crops, and know how to produce on-farm arbuscular mycorrhizae inoculum for sustainable agriculture.