Biol/CSES 4684

TRICHODERMA

SOIL MICROBIOLOGY

This webpage was created by Bradley Norman Spain

1. IDENTIFYING CHARACTERISTICS

Facultative anaerobic.
Grows saprophytically or as a parasite on other fungi.
Grows toward hyphae of other fungi, coils around them, and attachs to host mycelium.
Conidiophores are erect and produce side branches bearing whorls of short phialides.
Branches are not swollen at the apex and bear terminal conidial heads.
Conidia are one-celled ovoid spores and are produced successively from tips of phailides which collect in small wet masses.
Individual cells range from 25-70 um in height and 2.5-3.5 um in diameter.
Colonies grow quickly producing white, yellow, or green cushions of sporulating filaments.

The picture to the right shows a nice contrast in size between Trichoderma conidial heads and filaments.

2. TAXONOMIC DESCRIPTION
Trichoderma are among the most common saprophytic fungi. They are within the subdivision Deuteromycotina which represents the fungi lacking or having an unknown sexual state (though many trichoderma are considered asexual). Further, it is part of the form class hyphomycetes. They are known as early invaders of roots and quickly occupy an ecological niche on the roots. Due to their ability to utilize comples substrates, they do not completely depend on the plant in their life cycle. They are also considered cellulolytic ascomycetes and among the organisms responsible for the destruction of cellulosic fabrics.

Trichoderma are found in nearly all agricultural soils and in other environments such as decaying wood. Most species grow rapidly, produce abundant conidia,and have a wide range of enzymes including cellulases. They have the hallmarks of ruderals. However, many species are still highly antagonistic to other species of fungi by many processes. These include production of soluble antibiotics (peptides), volatile and non-volatile antibiotics, or by direct parasitism. This is achieved when they coil around the hyphae of other fungi in a process called mycoparasitsm which limits the growth and activity of plant pathogenic fungi. The fungus has probably the most heavily studied cellulase system as it excretes large quantities of cellulases in growth media. Various strains have the ability to reduce plant root rot and increase root growth.

Above left: T. harzianum strangling Pythium. Above right: Electron micrograph of Pythium cell wall damage caused by T. harzianum.

There are many species of trichoderma with many differing characteristics. For example, T. harzianum is tolerant to stress imposed by nutrient scarcity. Often they are antagonistic towards one another. At high temperatures T. viride and T. polysporum are displaced by T. hamatum and T. koningii, while at low temperatures the opposite is true. Reasons like these are why some species are more prosperous during cooler months while others are more persistant during warmer months. Trichoderma is able to grow in soils having a pH range of 2.5 - 9.5, although most prefer a slight to moderately acidic environment. The species that prefer the more acidic soils are usually regarded as having a more stress-tolerant growth habit and are less aggressive. All species can produce colonies which have either white to yellow to green mature fruiting areas. Colonies can have either floccose and elliptical conidia, or tufted non-floccose globose conidia.

3. ISOLATION AND ECOLOGY
Different media for isolation purposes are used to grow Trichoderma. Some selective media are more efficient than others. Depending on the species, Trichoderma can show no growth to broadly spreading growth on Czapek's agar. Colonies are usually first white then develop yellowish tints until they become various deep shades of green. Conidiophores will arise as branches of aerial mycelia, septate, and grow up to 70 um in height. As mentioned, trichoderma are found in many environments but are abundant in decaying wood or in soil containing decaying wood.

Trichoderma are used in the commercial production of the enzyme cellulase. This capability makes the trichoderma very valuable in controling certain other pathogenic fungi such as rhizoctonia, botrytis, pythium, sclerotinia, and armillaria, which themselves are pathogens on fruits and vegetables. Currently, Trichoderma is being developed and marketed as a plant growth stimulator (by stimulation and protection). The two species used most for commercial applications are T. harzianum and T. koningii. Though they can inhibit pathogen growth, these species will kill other fungi with a toxin and then consume them using a combination of lytic enzymes. Despite many beneficial uses, they can be serious pests nonetheless and can cause problems in cultivated mushroom beds.

The picture at the right is a sketch of Trichoderma conidophores and the whorls of phialides they produce.

4. ADDITIONAL SOURCES OF INFORMATION
Merton, F. and H. Brotzman. 1979. Phytopathogenic Fungi: A Scanning Electron Stereoscopic Survey. Unversity of Missouri Columbia Extension Division. Columbia, Missouri. 204 pp.

Elsas, J., J. Trevors, and E. Wellington. 1997. Modern Soil Microbiology. Marcel Dekker, Inc. New York, New York. 250 pp.

Gilman, J. 1957. A Manual of Soil Fungi. The Iowa State University Press. Ames, Iowa. 212-214 pp.

Paul, E. and F. Clark. 1996. Soil Microbiolgy and Biochemistry. Academic Press. San Diego. 134 pp.

5. LINKS TO OTHER SITES ON TRICHODERMA
The Albright Seed Company explains their uses for Trichoderma.

This website from Cornell University discusses Trichoderma and biocontrol of plant pathogens.