Trichoderma viride is a common, naturally occurring soil fungus found globally. This filamentous fungus belongs to the family Hypocreaceae and is characterized by its greenish-yellow conidia and branched conidiophores, giving it the common name “green mold.” It reproduces asexually through spores, known as conidia. This widespread microorganism plays a significant role in various ecosystems.
The Fungus in its Natural Environment
Trichoderma viride thrives as a saprophyte, meaning it obtains nutrients by decomposing dead organic matter. This ecological role is significant in environments like forests, grasslands, and agricultural fields, where it contributes to nutrient recycling by breaking down complex organic materials. Its mycelium can grow directly on wood, which is largely composed of cellulose, and on other fungi, whose cell walls primarily consist of chitin.
The fungus is widely distributed across temperate and tropical regions, demonstrating broad adaptability to diverse habitats. It is frequently found in soil, on decaying wood, and within the rhizosphere—the narrow zone of soil directly influenced by plant roots. Trichoderma viride tolerates temperatures between 15 °C and 40 °C, with optimal growth between 25 °C and 35 °C, and prefers acidic soils with a pH around 5. Its rapid growth rate allows it to quickly colonize various substrates.
Mechanisms of Plant Protection
Trichoderma viride protects plants through several distinct biological mechanisms, primarily mycoparasitism, competition, and antibiosis. These actions collectively inhibit the growth of harmful plant pathogens. The effectiveness of Trichoderma often depends on multiple mechanisms working together, with different strains exhibiting varied biocontrol capabilities.
Mycoparasitism
Mycoparasitism involves the fungus directly attacking other fungi, particularly plant-pathogenic species. Trichoderma viride physically interacts with host hyphae by coiling around them and penetrating their cell walls. This process is facilitated by the secretion of enzymes like chitinases and β-1,3-glucanases, which degrade components of the pathogen’s cell walls, leading to the host’s eventual death.
Competition
Competition is another mode of action, where Trichoderma viride rapidly colonizes the root zone and surrounding soil, effectively outcompeting harmful microbes for essential resources. Its fast mycelial growth allows it to quickly occupy space, consume available nutrients, water, and oxygen, thereby making the environment less hospitable for slower-growing pathogens. This competitive exclusion prevents the establishment and proliferation of disease-causing organisms.
Antibiosis
Antibiosis describes Trichoderma viride’s ability to produce and secrete various compounds that inhibit or kill pathogens. It releases secondary metabolites with antimicrobial properties, including specific enzymes and antibiotic compounds. Examples of such compounds include trichoviridin and trichodermin, which suppress plant-pathogenic fungi and nematodes. These secreted substances actively interfere with pathogen growth, spore germination, and mycelial development.
Agricultural and Horticultural Applications
The protective mechanisms of Trichoderma viride have led to its widespread use in agriculture and horticulture as a biological control agent. It is formulated into various commercial products, including wettable powders, granules, and liquid suspensions. These formulations are designed for different application methods to suit diverse cropping systems.
Seed Treatment
Seed treatment involves coating seeds with Trichoderma viride formulations before planting. This method typically uses about 8-10 grams of formulation per kilogram of seed. This provides early protection to seedlings against seed-borne and soil-borne pathogens during germination and initial growth.
Soil Drenching or Incorporation
Soil drenching or incorporation involves applying the fungus directly to the soil. For drenching, a common practice is mixing 1-2 kilograms of formulation in 200 liters of water and applying it to one acre of soil. Alternatively, it can be mixed with well-decomposed farmyard manure or compost and broadcast into the field or incorporated into the soil during seeding or transplanting. This method helps establish a protective colony around the root zone, safeguarding plants from root rot, damping-off, and wilt diseases.
Foliar Sprays
Foliar sprays are used to apply Trichoderma viride to plant leaves to combat certain surface pathogens. While primarily known for soil-borne disease control, some formulations can be applied as foliar sprays to colonize the phyllosphere, competing with and inhibiting foliar disease development. This versatility allows for protection against a range of diseases affecting both roots and aerial parts of plants.
Human and Environmental Safety Profile
Trichoderma viride is considered safe for use in agricultural settings and is widely approved for organic farming. It is non-pathogenic to humans, pets, and wildlife. Regulatory bodies like the U.S. Environmental Protection Agency (EPA) have found no adverse dietary, aggregate, or cumulative risks associated with its use as a pesticide active ingredient. The fungus occurs ubiquitously in nature and exhibits a low toxicity profile.
Like any mold, Trichoderma viride produces spores that can act as respiratory irritants or allergens for sensitive individuals. When handling powdered forms of Trichoderma viride products, it is advisable to wear a mask to minimize inhalation of spores. This precaution helps prevent respiratory discomfort.
The use of Trichoderma viride as a biocontrol agent contributes to environmental health. By offering a biological alternative to synthetic chemical fungicides, it helps reduce the reliance on and potential environmental impact of these chemicals. Its role in promoting plant health and decomposing organic matter also supports sustainable agricultural practices and overall soil ecosystem well-being.