Biological control of plant diseases has emerged as a promising area in phytopathology. These methods not only minimize reliance on synthetic pesticides but are also comparatively economical, feasible, robust, and sustainable [1,2]. Among the commercial biofungicides/fungal antagonists, Trichoderma has been broadly acknowledged as a source of potential biocontrol agent, particularly for lowering soil-borne phytopathogens such as Fusarium oxysproum, Rhizoctonia solani [3], Macrophomina phaseolina [4], Sclerotinia rolfsii, and others [5,6,7]. Trichoderma spp. are saprophytic, avirulent, and soil-inhabiting fungi, and they control pathogens with various biocontrol methods such as mycoparasitism [8], retard the pathogen growth by secreting cell-wall-destroying enzymes [9,10], nutrient uptake competition, and rhizospheric competence [2]. Because of its effectiveness against phytopathogens, its market demand is increasing yearly [11].

According to Harman (1991), [12] key points that are required for the production of any biological control system are (1) a potential biocontrol agent, (2) viable propagule with amplified shelf-life, (3) bioprotective delivery mechanisms that may offer the biocontrol agent an advantage to compete against existing microflora, and (4) steady field performance [13]. As mentioned above, the first and most principal step before mass production of a biocontrol agent is the identification of a robust propagule (hyphae, chlamydospores, and/or conidia) [14]. Once a reliable and efficient biocontrol agent is identified, large-scale production, design, and application ideas need to be performed cautiously for the product’s stability during storage and later use. Both liquid and solid formulations are utilized in developing sufficient amounts of viable and active Trichoderma inoculums. Conidia and chlamydospores are the preferred propagules for formulations since they can withstand rigorous treatment procedures, whereas hyphae cannot be used because they are not dehydration-resistant [12,15]. Several reports (Table 1) related to the effectiveness of Trichoderma strains and their formulations have been published periodically [13]; however, reports concerning advancements in mass production, persistent viability, and related field performance of Trichoderma species are fragmentary. Furthermore, the challenges related to the isolation and development methods for elevating the effectiveness and sustainable use of Trichoderma formulations in the field of health security and food also need to be addressed. In this review, we intend to fill this gap and unravel the recent developments regarding isolation, identification, preservation, substrates, consortium, quality control, mass production, delivery approaches, field performance, and registration and commercialization of Trichoderma formulations.

Trichoderma was initially reported as a biocontrol agent in the 1930s [18,19]. Since the discovery of mycoparasitic activity of Trichoderma by Weindling against Rhizoctonia solani and other phytopathogens, several other researchers have also obtained positive results with Trichoderma isolates as biocontrol agents of plant pathogens [46]. Trichoderma continues to hold a significant position among commercial biological control agents (BCAs) in a wide range of crop and disease management, either as a single ingredient or in combination with other ingredients [6,45,47,48]. Up till now, more than 80 species of Trichoderma have been reported [13], and among these, T. harzianum, T. virens, and T. viride are frequently deployed biocontrol agents. In India, commercialization of Trichoderma is limited to only two species, namely T. viride and T. harzianum [13]. However, there are ample reports on the effectiveness of T. virens and T. asperellum, but these are still unregistered under the Central Insecticide Board and Registration Committee (CIBRC) in India. This may be due to several hurdles, such as toxicity assessment, environmental effects of microbes and their formulation, and optimization of technology for mass-scale production [49]. Apart from these, some other constraints include multi-location trials for the purpose of proving its safety, followed by registration. In addition, some other challenges are inconsistent field presentation and low shelf-life of formulation, lack of patent protection, preliminary testing, high registration cost, alertness about the beneficial effect of Trichoderma formulations, and training and education shortfalls. Registration also requires good and effective documentation and other confirmations. Moreover, the evaluation process itself (compilation and analysis of data) can be prolonged and costly. In the past, high registration fees were clearly seen as a delay or barrier to BCA market growth, especially for medium and small enterprises that are the chief manufacturers of biocontrol agents. Due to these reasons, there are now diverse products in the market that claim to be plant growth promoters or biofertilizers for managing plant diseases but are not yet registered. The biocontrol activity of Trichoderma spp. is represented in Table 2.

However, their safe use cannot be guaranteed without toxicological and efficacy data [50]. Combined with the competitive pesticide industry, BCA companies are finding it hard to generate enough revenue from product sales to justify the registration cost. Unluckily, this has led to some products being removed from the stores. For instance, T. harzianum T-39 (Trichodex) was launched in Europe and Israel in 1993 for the control of Botrytis fruit rot through biological methods but was detached from the market. It went bankrupt in 2005 due to low sales and the rising cost of registration. Table 3 contains information on gene identification in Trichoderma species.

Trichoderma spp. have the ability to produce metabolites, modulate the plant defense responses, and act as a hyperparasite [93,94]. Moreover, Trichoderma strains are effective BCAs because of their high reproductive potential, resilience to harsh environments, efficiency in utilizing nutrients, ability to alter the rhizosphere, aggressiveness against phytopathogenic fungi, and effectiveness in fostering plant growth and defense mechanisms. Due to these characteristics, Trichoderma may be found in all habitats and at high population densities [95]. Trichoderma BCAs can even have a beneficial impact on plants by promoting biofertilization, which increases plant development and enhances plant defense systems [96]. Trichoderma uses indirect and direct methods to control plant pathogens. The power of these mechanisms in the biocontrol method relies upon the type of Trichoderma strain, the antagonized pathogen, including its host, and the ecological situation [96]. The direct mechanism includes mycoparasitism and coiling, whereas the indirect mechanism includes challenges for nutrients and space, systemic acquired resistance, and antibiosis. Among them, mycoparasitism, competition, and antibiosis play a major role in Trichoderma-mediated biological control.

Parasitism is one of the important mechanisms of fungal antagonist, where one fungus parasitizes (mycoparasite) another fungus (host), and this process is known as mycoparasitism. It has been observed that mycoparasitism involves four sequential steps: chemotaxis, recognition, attachment, and wrapping, as well as penetration of the pathogen cell wall and host digestion [97,98,99]. Trichoderma is widely used as a biofungicide against phytopathogens such as B. cinerea as well as the soil-borne pathogens Rhizoctonia, Sclerotinia, Pythium, and Fusarium spp. [14,100].

Trichoderma species employ several mechanisms to antagonize and mycoparasitize other pathogenic fungi, which includes competing for nutrients [101], releasing antibiotic metabolites, and activities like encircling the host and enlargement of the appressorium-like structure [51,102,103]. The degradation of host tissues containing pathogenic organisms occurs due to the enzymatic breakdown of cell walls facilitated by hydrolytic enzymes (such as chitinase, β-1,3-glucanase, and cellulase) that are synthesized by Trichoderma spp [104]. In T. atroviride, the nag1 gene coding for N-acetylglucosaminidase has a significant effect on chitinase induction, followed by biocontrol [105]. In the parasite interaction between Trichoderma and R. solani, host-released dispersal factors are responsible for inducing ech42 (encoding endochitinase 42) gene transcription prior to physical touch [30,106]. Lectins present in the host cell wall cause the parasites to cover the hyphae of the host after direct contact [27,100,106].

Antibiosis is a phenomenon where one organism is prevented/inhibited by another microorganism through secondary metabolites (SMs). Trichoderma spp. synthesizes SMs (pyrone, heterocyclic compounds, terpenoids, polyketides, etc.) [107] and also produces specific low molecular weight compounds/antibiotics for combatting plant pathogens [108]. Antibiosis has been reported to occur during contact among pathogen, plant, and Trichoderma spp., which triggers Trichoderma to produce antibiotics and SMs to reduce the growth of phytopathogens. More than 180 secondary metabolites have been extracted from Trichoderma spp., showing different classes of chemical compounds [109,110]. On the basis of their biosynthetic origin, these compounds can be classified as peptaiboles, polyketides, and terpenes [111]. Many species of Trichoderma genus are known to synthesize peptidols that are non-proteinogenic amino acids (α-aminoisobutyric acid, a polypeptide antibiotic with a 500 to 2200 Da molecular weight). These compounds are acetylated at the N-terminus and contain an aminoalcohol at the C-terminus [112].

Different types of Trichoderma produce different antibiotics; for example, T. viride produces mucortoxins A and B, mucorin, trichophyton, and mucorin. Similarly, mucorin A and B were isolated from T. mucorin. T. harzianum produces tricholongins BI and BII, while longibrachins and trichokonins were extracted from T. koningii. Atroviridines A-C and neoatroviridines A-D have been obtained from T. atroviride culture. In addition, other antibiotics and fungicidal compounds have been isolated from T. harzianum, T. koningii, T. aureoviride, T. virens, T. hamatum, and T. lignorum [110].

Growth of soil pathogens such as Phytophthora solani, P. middletonii, P. cinnamomi, Bipolaris sorokiniana, and Fusarium oxysporum was adversely affected in the vicinity of Koninginin D [113]. Similarly, viridins obtained from Trichoderma spp. like T. viride, T. koningii, and T. virens inhibit the spore germination of Colletotrichum lini, Botrytis allii, Penicillium expansum, Fusarium caeruleum, Stachybotry satra, and Aspergillus niger [114]. Harzianic acids derived from T. harzianum show antimicrobial activity against Sclerotinia sclerotiorum, R. solani, and Pythium irregulare [115]. It has been found that Trichoderma spp. And Gliocladium suppressed the growth of various soil-borne plant pathogens (Fusarium spp., Macrophomina, Sclerotium rolfsii, and Sclerotinia spp.) [116,117]. Silva et al. (1998) studied the antibiosis mechanism of Trichoderma against Colletotrichum spp. [118].

Limitation and competition for nutrient sources can lead to the natural control of fungal pathogens [119]. Trichoderma is a cosmopolitan fungus and is found in all kinds of soils because of its outstanding competitive potential. It can fight with phytopathogens for nutritive sources, such as C, N, and Fe, and also acts as a biological antagonist towards soil-borne pathogens. It is also an aggressive competitor that grows rapidly and quickly colonizes its substrate and controls slow-growing pathogens [120]. Trichoderma is more competitive with other microorganisms due to certain abilities, such as a higher growth rate and enhanced aptitude to mobilize and utilize nutrients from soil/substrate [121,122]. Thus, competition for macro and micronutrients plays a key role in the interaction between Trichoderma-plant pathogen [123] because Trichoderma species compete with bacteria in the rhizosphere of crops for nutrients and sites of infection [124]. Compared to other rhizosphere bacteria, Trichoderma shows a better ability to produce and take up nourishment from soil; therefore, the management of certain disease-causing entities, such as Botrytis cinerea, using Trichoderma through food competition is possible [125]. It is observed that there are four major features of any organism that contribute to its saprophytic ability and inoculum potential: (i) fast germination of fungal propagule, rapid hyphal growth towards nutrients, (ii) production of suitable enzymes for carbon constituents of the host plant, (iii) secretion of growth inhibitor compounds (fungistatic and bacteriostatic), and (iv) tolerance to fungistatic substances produced by competing microorganisms. Antagonistic fungi can compete with the pathogens for food and space by colonizing the normal environment, i.e., plant tissue, rhizosphere, or phyllosphere [126]. It depends on the colonization level of the host plant and acclimatization to the environmental situations in which they are living [127]. In order to successfully compete with other fungal phytopathogens for food and space, Trichoderma should exhibit efficient strategies for colonization of the plant and should be plentiful in an area where competition with other microorganisms occurs [126].

Trichoderma spp. produces iron chelating agents and siderophores, which make iron unobtainable for rhizospheric bacteria, which eventually leads to the extinction of the disease. Thus, Trichoderma acts as a competitor that helps control plant diseases [128]. Apart from this, due to its ability to colonize the rhizosphere and outcompete for nutrients, T. harzianum (T35 Strain) reduces the availability of nutrients and the amount of rhizospheric space available for the fungal wilt agent of watermelon (Fusarium oxysproum f.sp. meloni) to colonize [129]. Srinivasan et al. (1995) demonstrated the importance of competition between siderophore-producing Trichoderma strains and wood decay Basidiomycetes fungi [130].

Mokhtar et al. 2013 studied the interaction between T. harzianum and a few fungal species, such as Alternaria alternata, Fusarium acuminatum, and A. infectoria. The results revealed that lack of nutrients caused death of the pathogenic fungi [131]. It has also been found that Trichoderma can compete with plant pathogens, including Colletotrichum sp., Botrytis sp., and Phytophthora sp., for complex and simple substrates of carbon [132]. In order to successfully compete with other fungal phytopathogens for food and space, Trichoderma should exhibit efficient strategies for colonization of the plant and should be plentiful in an area where competition with other microorganisms occurs [126].

Trichoderma colonization of roots commonly improves nutrient absorption and utilization, crop yield, tolerance to abiotic stressors, and root growth and development [133]. Trichoderma hamatum or Trichoderma koningii can boost crop production up to 300% after addition in the field. In greenhouse experiments, a substantial increase in yield was reported after treating the seedlings with Trichoderma spores [95]. The ability of Trichoderma BCAs to produce metabolites that either prevent spore germination (fungistatic), kill cells (antibiosis), or alter the rhizosphere, for example, making the soil acidic, leads to biocontrol that is unsuitable for pathogen proliferation [96]. Trichoderma strains quickly proliferate when introduced to the soil because they are inherently resistant to a wide range of hazardous substances, including insecticides, fungicides, and herbicides like DDT [95].

It has been shown that the Trichoderma species produce a large number of secondary metabolites, about 370 of which are members of several chemical compound classes with potent antagonistic activities [126,134]. Peptaibols and polyketides are the most significant non-volatile and volatile organic compounds (VOCs) produced by the majority of Trichoderma strains [2]. The volatile antibiotic 6-phenyl-pyrone (6PAP), responsible for the distinctive coconut scent and the biological control of F. oxysporum, is produced by the T. viride, T. harzianum, and T. koningii species [135]. In addition, T. harzianum also produces harzianic acid, a tetramic acid that has strong antifungal action as well as the capacity to stimulate plant development and function as a chelator [136].

Trichoderma can trigger a host plant’s defensive mechanism while preventing harmful pathogens from proliferating and growing, and it can also encourage crops to build self-defense mechanisms to gain local or systemic disease resistance [137]. Two methods are used to achieve Trichoderma-induced plant disease resistance: first, control the elicitors or elicitors that trigger the plant disease resistance response; and second, release oligosaccharides from the cell-wall-degrading enzymes produced by Trichoderma to cause plant resistance [138]. Saravanakumar et al. (2016) found that Trichoderma coated corn seeds dramatically increased the peroxidase (POD) and phenylalanine ammonia lyase (PAL) activity, and the plants were resistant to Curvularia leaf spot of corn [139].