Biofertilizers are produced from organic matter or agroindustrial wastes, which act as substrate for propagation of inoculum of selected microorganisms (Kaewchai et al. 2009). There are two approaches to developing potential biofertilziers: either the application of a single superior species with multifunctions, or groups of microorganisms (consortia) beneficial to plants (Vassilev et al. 2015). Biofertilizers have been used in agriculture, horticulture, landscape restoration, and soil remediation since the late 1980s (Hart and Trevors 2005). The long-term use of biofertilizers is economical and also eco-friendly to plant, animal and human health, and biofertilizers are renewable and low-cost resources which are accessible to marginal and small farmers (Dubey and Maheshwari 2008; Pal et al. 2015). Thus, the use of biofertilizers is recommended over chemical fertilizers. Details regarding biofertilizers such as term, role, types and advantages have been described by Kaewchai et al. (2009), Pal et al. (2015), Vassilev et al. (2015) and Itelima et al. (2018).

Several studies have applied fungal inocula as biofertilizers in greenhouse and/or field trials (Grigera et al. 2007; Rahi et al. 2009; Goetten et al. 2016; Zhang et al. 2016a; Wang et al. 2018c). Mycorrhizal fungi are widely used in agriculture, as they form root symbiotic relationships and provide many benefits to plants, such as improved plant growth and development, increased nutrient uptake and enhanced plant tolerance to disease (Whipps 2004; Liu and Chen 2007; El-Shaikh and Mohammed 2009; Smith et al. 2010; Herna´ndez-Montiel et al. 2013; Goetten et al. 2016; Janousˇkova´ et al. 2017). Strains of the genera, such as Alternaria, Aspergillus, Chaetomium, Fusarium, Penicillium, Serendipita (Piriformospora), Phoma, and Trichoderma have been reported as plant growth promoting fungi (Soytong et al. 2001; Muhammad et al. 2009; Salas-Marina et al. 2011; Varma et al. 2012; Bitas et al. 2015; Murali and Amruthesh 2015; Zhang et al. 2016a; Zhou et al. 2018). These potential plant growth-promoting fungi can be further researched and developed as potent fungal biofertilizers.

Numerous commercial fungal biofertilizer products have been manufactured globally and are available on the market today. There are various formulation types, such as granules, wettable powder, pellets and liquids, which comprise one or multiple fungal inocula. Aspergillus, Chaetomium, Penicillium and Trichoderma species have been used in biofertilizer products. For example, Ketomium® has been developed and improved from strains of Chaetomium spp. in pellet and powder form. The product was used in greenhouse and field trials of tomato, corn, rice, pepper, citrus, durian, bird of paradise and carnation plants in Thailand (Soytong et al. 2001). Plants treated with Ketomium® showed better plant growth and higher yield than non-treated control plants. In addition, Ketomium® had the ability to control Phytophthora sp., causing citrus root rot in the field.

Biofertilizers increase the uptake of nutrients from the soil or atmosphere, and produce bioactive compounds, enzymes and hormones which stimulate plant growth and enhance root growth (Chi et al. 2010; Abdel-Fattah et al. 2013; Pal et al. 2015). Fungal biofertilizers are able to solubilize and mobilize unavailable organic and inorganic forms of phosphorus into soluble forms, making them available to plants. For example, Aspergillus niger was mixed with Bacillus megaterium to form phosphate solubilizing microorganisms. These microorganisms were applied as biofertilizers in India (Pal et al. 2015). Arbuscular mycorrhizae have been used as phosphate mobilizing biofertilizers (Zhang et al. 2018). Biofertilizers play an important role in the recycling of plant nutrients and in enhancing the rate of compost degradation (Pal et al. 2015). Some biofertilizers act as antagonists and suppress the incidence of soil borne plant pathogens while helping in the biocontrol of plant diseases (Thamer et al. 2011; Pal et al. 2015).

Fungal derived stimulants, or elicitors, are fungi or fungal compounds that enhance the production of secondary metabolites, or elicit growth or immune response in a target plant species upon application. Plant responses include the upregulation of genes involved in plant defense, as well as the increased production of antimicrobial compounds, lignin, secondary metabolites, and certain proteins (Vassilev et al. 2015). Potential uses for such elicitors include the enhanced production of commercially valuable compounds/metabolites, or the artificial enhancement of plant defenses when pathogens are detected (Radman et al. 2003). A novel approach for the use of elicitors is to incorporate them with immobilized stimulants, such as with arbuscular mycorrhizal inoculum. Additionally, plant derived elicitors, which enhance the growth and development of beneficial fungi such as arbuscular mycorrhizae, also show promise in advancing this field of study (Akiyama et al. 2005; Besserer et al. 2006). Elicitors have a high potential for enhancing plant productivity and improving plant defenses against pathogens, and given that elicitors can be used in combination with other types of biofertilizers, they hold much potential for wide scale application in the future.

Fungal biofertilizers are applied on a very small scale in agriculture as compared to chemical fertilizers due to their limited shelf life and slower rate of effect. Olivian et al. (2004) reported using sterilized peat as solid support for Fusarium oxysporum inoculation, storing this admixture at room temperature without loss of activity. Growth and formulations based on recycling agro-industrial wastes can be expected to employ nitrogen-fixing and other microorganisms with different characteristics, such as biocontrol, P-solubilization, lignocellulolytic activity. For example, combinations between Trichoderma spp. and P-solubilizing fungi can be cultured based on agroindustrial-wastes, leading to mineralization of the matrix/substrate by the combined enzyme actions. We could apply immobilization of fungal cells together with enhanced biotechnology and in combination with elicitors. Immobilized cell technologies permit the use of two and more microorganisms, which result in highly effective synergies benefiting all the organisms involved, including the plants (Vassilev et al. 2015). In order to effectively implement the use and gain the full benefits of biofertilizers, an integrated approach engaging a variety of mechanisms should be considered. Such an approach could be tailored to suit specific industry needs and target defined outcomes, such as improved growth, upregulation of key metabolites, or enhanced plant defenses.