Trichoderma harzianum is a filamentou fungu antagonit of plant pathogen, used in the biological control of dieae caued by phytopathogenic fungi. It is widely ued in agriculture due to its propertie.

Indeed, the scientific interest generated by this species is related to the control mechanisms against phytopathogenic fungi. Actions such as competition for nutrients and space, mycoparasitism and antibiosis, are biological control mechanisms.

The Trichoderma harzianum It is a cosmopolitan fungus, since it is distributed worldwide in various ecosystems and natural environments, and its development is common in places where organic plant material accumulates, such as crop residues or humic soils.

Plants with a high density of adventitious roots and an adequate rhizosphere favor their colonization. In fact, its great ability to adapt to various agrological conditions, make the Trichoderma a fungus with a wide spectrum of use.

At the nutritional level, Trichoderma it is capable of degrading complex substrates such as starches, pectins and celluloses. Later it uses these elements for its growth by virtue of the abundant enzymatic complex it possesses (amylases, pectinases, cellulases and chitinases).

characteristics

Aquatic and terrestrial ecosystems

These fungi are characterized by proliferating in terrestrial ecosystems (agricultural soils, grasslands, forests and deserts) and aquatic ecosystems. Some species are free-living in the soil, opportunistic, plant symbionts, and others are mycoparasites.

They also have the ability to colonize various environments, due to their great reproductive capacity. They can adapt and survive in extreme conditions of temperature, salinity and pH.

Reproduction and nutrition

In their vegetative state they have a simple, haploid mycelium or septa and their wall is composed of chitin and glucans. They are facultative anaerobes and reproduce asexually by conidia.

This species has low nutritional requirements, although its growth is favored by organic matter and humidity. The optimum temperature range for its growth and development is between 25º to 30º C.

Habitat

TO T. harzianum, it can be found in various organic materials and soils, they present a wide distribution due to their great adaptability. Some species prefer dry and temperate places, and other humid and cold places.

In particular, these fungi, as endophytic organisms, compete with the rhizosphere of the plant, managing to colonize the root surface. In fact, they penetrate through the intercellular spaces, up to the first or second layer of cells.

Importance

This fungal group is of great importance for plants, since they contribute to the control of phytopathogenic fungi. Indeed, they are widely known for their ability to produce toxins and antibiotics that control various pathogens.

The isolates of the genus Trichoderma They are among the most widely used biological control agents in agriculture. Research work has allowed to verify their effective control, since they act on a large number of soil pathogens.

Features

One of the main functions of the Trichoderma harzianum it is their ability to develop symbiotic relationships with plants.The fungus unfolds and grows in the rhizosphere of the crop, increasing its development to obtain more space to grow.

Furthermore, used as a biological control agent, it has the ability to produce enzymes that attack and inhibit phytopathogenic fungi. Indeed, incorporated into the substrate or the cultivation field before sowing it is very beneficial.

In this regard, its action as a competitive hyperparasite is based on the production of antifungal metabolites and hydrolytic enzymes. Originating structural changes at the cellular level, such as vacuolization, granulation, disintegration of the cytoplasm and cell lysis, on the controlled organisms.

Studies at the warehouse level have made it possible to determine the increase in the root system when using the Trichoderma harzianum in different doses. In this regard, it stimulates the germination of seeds and favors the growth of new seedlings.

It is recommended to include the T. harzianum in a disease control program in order to take advantage of its antagonistic capacity. It has been proven that applications of Trichoderma prevents and controls pathogens such as Fusarium, Pythium, Phytophthora, Rhizoctonia Y Sclerotium.

Taxonomy

The gender Trichoderma spp., was initially described by Persoon (1794), to classify four species not related to each other at present. Between them: Trichoderma viride, Xylohipha nigresce, Sporotrichum aureum, and Trichotecium roseum.

Subsequently, several classifications were made, based on microscopic characteristics, size and the presence of phialides. Then Rafai (1969) made a review of the genus and described 9 species of Trichoderma spp., where I include the Trichoderma harzianum.

The species T. harzianum (Rafai, 1969), belongs to the genus Trichoderma, Hypocreaceae family, Hypocreales order, Sordariomycetes class, Pezizomycotina subdivision, Ascomycota division, Fungi kingdom.

Taxonomic studies of Trichoderma harzianum, have been supported by variations in DNA polymorphism using PCR techniques. Within the genre T. harzianum (Rifai), four biological forms have been differentiated: Th1, Th2, Th3 and Th4.

Morphology

The gender Trichoderma it comprises a series of species with no obvious sexual phase. It is characterized by a septate mycelium, generally oval conidia, non-whorled hyaline conidiophore, singular or grouped phialides, and unicellular conidia.

At the macroscopic level, the colonies are easily recognizable by their white-green or yellow-green coloration. Furthermore, in areas with conidia, concentric rings are observed; and on the reverse of the colonies the coloration is yellow, amber or greenish-yellow.

At the microscopic level, erect, hyaline, branched and non-whorled conidiophores are observed, they appear in groups or solitary. The phialides are pear-shaped, single or in groups, swollen in the central region and thin at the apex.

The angle of insertion between the phialides and the conidiophores is right. Unicellular conidia are oblong or sub-globular, smooth or equinulate. Green or hyaline in color, and present in masses at the apices of the phialides.

Reproduction

The gender Trichoderma they do not have an advanced sexual period, they naturally reproduce by asexual spores. The life cycle of T. harzianum, begins when the organism grows and branches like a fungal hypha 5-10 microns in diameter.

Asexual sporulation begins when spores 3-5 microns in diameter are released in large numbers. Likewise, intercalated chlamydospores are formed individually, although sometimes two or more fused chlamydospores are observed.

Action mechanisms

The controlling effect of fungi Trichoderma It is verified by various mechanisms of action on the development of phytopathogenic fungi. Among the main mechanisms that exert a direct action are competition for space and nutrients, mycoparasitism and antibiosis.

The biocontroller action of Trichoderma harzianum it is increased by its ability to colonize the rhizosphere of plants. Furthermore, mechanisms such as the secretion of enzymes and the production of inhibitory compounds function as a biocontroller effect.

On the other hand, there are mechanisms whose indirect function contributes as a bioregulatory effect. Among them, the ability to activate compounds related to resistance, detoxification of toxins and deactivation of enzymes in the plant.

The ability of the fungus to facilitate the solubilization of nutritive elements, not available to plants in their natural form, constitutes a process that improves the nutritional conditions of the medium to supply nutrients to a crop.

Likewise, when it develops under favorable conditions, it is capable of abundantly colonizing the rhizosphere of plants, which allows it to create an environment favorable to radical development, improving the plant's tolerance to stress.

Competition

Competition is defined as an unequal behavior between two individuals to supply the same requirement, be it substrate or nutrients. The success of the competition results in the ability of one of the organisms to surpass the ability of the other.

The Trichoderma harzianum they have a great antagonistic ability due to the fact that they have a rapid rate of development. Its biocontroller effect is favored by its broad ecological adaptation and adaptability to adverse conditions.

In addition, it has a great ability to mobilize and take advantage of the nutrients in the soil, mainly nitrogen, carbohydrates and polysaccharides. In this way, it is able to rapidly colonize the environment, preventing the proliferation of other microorganisms in the same habitat.

Mycoparasitism

Mycoparasitism is defined as an antagonistic symbiotic interaction between the fungus and the pathogen. This mechanism involves the extracellular enzymes of the cell wall of parasitized fungi: chitinases and cellulases.

This action occurs in four stages: chemotrophic growth, recognition, adhesion and coiling, and lytic activity. During the last stage, the fungus generates extracellular lytic enzymes, degrades the cell wall of the pathogen and facilitates the penetration of hyphae.

The Trichoderma harzianum during mycoparasitism it grows chemotropically towards the pathogen, coils and penetrates the daughters of the host. Through the generation of special enzymes and the degradation of the pathogen's cell wall, it causes the weakening of the phytopathogen.

Mycoparasitism as an antagonistic mechanism of action in T. harzianum it depends on various factors. The development of each stage is due to the pathogens involved, the biotrophic or necrotrophic action of the antagonist, and environmental conditions.

• Chemotrophic growth: refers to the direct positive growth of an organism towards a chemical stimulus. Trichoderma detects the presence of the pathogen and its hyphae grow and reach the body in response to the chemical stimulus.

• Recognition: Research studies have determined that the Trichoderma it is an antagonist of specific phytopathogens. Molecules such as lectins-carbohydrates present in the host make it susceptible to being parasitized by the fungus Trichoderma.

• Adhesion and curl: the hyphae of Trichoderma they have the ability to adhere to the host, forming hook-like and appressor-like structures. This process involves enzymatic processes and the antagonistic association of a sugar on the wall of the fungus with a lecithin on the wall of the phytopathogen.

• Lithic activity: the degradation of the cell wall of the phytopathogen occurs, facilitating the penetration of the hyphae of the Trichoderma. The lytic enzymes involved in the process are basically chitinases, glucanases and proteases.

Antibiosis

It is the direct action of volatile or non-volatile organic compounds, produced by the Trichoderma on a susceptible host. Various strains of T. harzianum produce antibiotics or toxic metabolites that inhibit the growth of other microorganisms.

Applications

The Trichoderma harzianum it is widely used as a biological controller due to its rapid growth and development. In addition, it promotes a variety of enzymes capable of degrading other phytopathogenic fungi.

This fungus is a natural agent, not aggressive with plants or soil. Used as a biocontroller, it does not report toxicity on crops, it also reduces the environmental impact due to the absence of chemicals in the soil.

The biocontroller effect of T. harzianum It is carried out depending on the environments where the incidence of phytopathogens occurs. The control method and the application mode is carried out in the structure, area and space to be protected.

Generally, the control is carried out by means of controlled applications to the seeds, to the substrate in seedbeds or directly to the soil. The use of aspersions on leaves, flowers and fruits is common; and recently studies have been carried out to prevent postharvest pathogen attacks.

Biological control in seeds

Seed treatment with T. harzianum It is aimed at protecting the seed against intrinsic or soil pathogens. In addition, provide protection over time to the underground parts of the new plant once it has germinated.

Indeed, once the seed is inoculated with the fungus, it is capable of colonizing the rhizosphere of the plant, exerting its biocontrolling action. In addition to this, the amount of fungus applied to seeds is lower, it is compared with the amount to be applied to a field of cultivation.

For the application of Trichoderma Different methods are used on the seed: the use of a dry powder, application of biopreparation in the form of a paste, dissolution in dry clay or coating by pelletization.

Biological control in the soil

The soil is the conducive environment for the control of pathogens through Trichoderma harzianum. In fact, the rhizosphere of plants is the most favorable environment to exert its antagonistic action.

The application of the fungus on the seeds is carried out in order to establish the biocontroller locally in the rhizosphere. Hence, the biological control in the soil is directly related to the application of the fungus on the seed

Other methods include direct application to the furrow or broadcast, at planting time or during plant cleaning and hilling. In this case, it is applied in powder, granules or incorporated together with organic amendments.

Control over the leaf surface

Biological control through Trichoderma in foliar areas, such as flowers, fruits and foliage, it is subject to environmental conditions. Low availability of nutrients, temperature variations, solar radiation and wind are conditions that make it difficult for the fungus to establish.

In this regard, the formulations designed to apply the antagonist must contain adherents and nutrients that facilitate colonization of the Trichoderma. The moderate effectiveness of this method and its high cost have promoted the study of new control strategies at the foliar level.