Trichoderma spp., including T. harzianum, T. viride, T. koningii, T. hamatum and other spp.
(Ascomycetes, Hypocreales, usually Hypocrea spp., are sexual anamorphs, this life stage is lacking or unknown for biocontrol strains)
by G. E. Harman, Cornell University, Geneva, NY 14456
What are Trichoderma?
Trichoderma spp. are fungi that are present in nearly all soils and other diverse habitats. In soil, they frequently are the most prevalent culturable fungi.
They are favored by the presence of high levels of plant roots, which they colonize readily. Some strains are highly rhizosphere competent, i.e., able to colonize and grow on roots as they develop. The most strongly rhizosphere competent strains can be added to soil or seeds by any method. Once they come into contact with roots, they colonize the root surface or cortex, depending on the strain. Thus, if added as a seed treatment, the best strains will colonize root surfaces even when roots a meter or more below the soil surface and they can persist at useful numbers up to 18 months after application. However, most strains lack this ability.
In addition to colonizing roots, Trichoderma spp. attack, parasitize and otherwise gain nutrition from other fungi. Since Trichoderma spp. grow and proliferate best when there are abundant healthy roots, they have evolved numerous mechanisms for both attack of other fungi and for enhancing plant and root growth. Several new general methods for both biocontrol and for causing enhancement of plant growth have recently been demonstrated and it is now clear that there must be hundreds of separate genes and gene products involved in these processes. A recent list of mechanisms follows.
Most Trichoderma strains have no sexual stage but instead produce only asexual spores. However, for a few strains the sexual stage is known, but not among strains that have usually been considered for biocontrol purposes. The sexual stage, when found, is within the Ascomycetes in the genus Hypocrea. Traditional taxonomy was based upon differences in morphology, primarily of the asexual sporulation apparatus, but more molecular approaches are now being used. Consequently, the taxa recently have gone from nine to at least thirty-three species.
Most strains are highly adapted to an asexual life cycle. In the absence of meiosis, chromosome plasticity is the norm, and different strains have different numbers and sizes of chromosomes. Most cells have numerous nuclei, with some vegetative cells possessing more than 100. Various asexual genetic factors, such as parasexual recombination, mutation and other processes contribute to variation between nuclei in a single organism (thallus). Thus, the fungi are highly adaptable and evolve rapidly. There is great diversity in the genotype and phenotype of wild strains.
While wild strains are highly adaptable and may be heterokaryotic (contain nuclei of dissimilar genotype within a single organism) (and hence highly variable), strains used for biocontrol in commercial agriculture are, or should be, homokaryotic (nuclei are all genetically similar or identical). This, coupled with tight control of variation through genetic drift, allows these commercial strains to be genetically distinct and nonvariable. This is an extremely important quality control item for any company wishing to commercialize these organisms.
So far as the author is aware, different strains of Trichoderma control every pathogenic fungus for which control has been sought. However, most Trichoderma strains are more efficient for control of some pathogens than others, and may be largely ineffective against some fungi. The recent discovery in several labs that some strains induce plants to "turn on" their native defense mechanisms offers the likelihood that these strains also will control pathogens other than fungi.
Fungal thalli are shown in the figure at the beginning of this web page. The organism grows and ramifies as typical fungal hyphae, 5 to 10 µm in diameter. Asexual sporulation occurs as single-celled, usually green, conidia (typically 3 to 5 µm in diameter) that are released in large numbers. Intercalary resting chlamydospores are also formed, these also are single celled, although two or more chlamydospores may be fused together.
Trichoderma spp. possess innate resistance to most agricultural chemicals, including fungicides, although individual strains differ in their resistance. Some lines have been selected or modified to be resistant to specific agricultural chemicals. Most manufacturers of Trichoderma strains for biological control have extensive lists of susceptibilities or resistance to a range of pesticides.
Uses of Trichoderma
These versatile fungi are used commercially in a variety of ways, including the following:
Foods and textiles:
As a source of transgenes.
There are a number of references that provide a wealth of information about these fungi. Some of these are listed below.
Kubicek, C. P. and Harman, G. E. 1998. Trichoderma and Gliocladium. Vol. 1. Basic Biology, Taxonomy and Genetics, Taylor & Francis, London. 278 pg.
Harman, G. E. and Kubicek, C. P. 1998. Trichoderma and Gliocladium, Vol. 2, Enzymes, Biological Control and Commercial Applications. Taylor & Francis, London. 393 pg.
The preceding two multi-author books contain most pertinent information on these fungi known through 1997. They can be purchased on the Amazon.com web site.
The following books contain substantial amounts of information regarding biocontrol with Trichoderma, and also include information on other topics.
Chet. I. 1987. Innovative Approaches to Plant Disease Control. Wiley-Interscience, New York, 372 pg.
Lumsden, R. D. and Vaughn, J. L. 1993. Pest Management: Biologically Based Technologies. Am. Chem. Soc., 435 pg.
Chet, I. 1993. Biotechnology in Plant Disease Control. Wiley-Liss, New York, 373 pg
Recently, there have been several papers published that have substantially improved our understanding of, and use of, Trichoderma spp. for biocontrol and related uses. Selected papers follow:
New mechanisms of biocontrol
Altomare, C., Norvell, W. A., Björkman, T., and Harman, G. E. 1999. Solubilization of phosphates and micronutrients by the plant-growth promoting and biocontrol fungus Trichoderma harzianum Rifai 1295-22. Appl. Env. Microbiol. 65:2926-2933.
Elad, Y., and Kapat, A. 1999. The role of Trichoderma harzianum protease in the biocontrol of Botrytis cinerea. Eur. J. Plant Pathol. 105:177-189.
Howell, C. R., Hanson, L. E., Stipanovic, R. D., and Puckhaber, L. S. 2000. Induction of terpenoid synthesis in cotton roots and control of Rhizoctonia solani by seed treatment with Trichoderma virens. Phytopathology 90:248-252.
Yedidia, I., Benhamou, N., and Chet, I. 1999. Induction of defense responses in cucumber plants (Cucumis sativus L.) by the biocontrol agent Trichoderma harzianum. Appl. Environ. Microbiol. 65:1061-1070.
New uses and summaries of mechanisms of biocontrol
Harman, G. E. 2000. The myths and dogmas of biocontrol: changes in perceptions derived from research on Trichoderma harzianum strain T-22. Plant Disease 84 (in press, will be published in the April issue).
Trichoderma genes for use in producing disease-resistant transgenic plants
Bolar, J., Norelli, J. L., Wong, K.-W., Hayes, C. K., Harman, G. E., and Aldwinckle, H. S. 2000. Increased resistance to scab of endochitinase transgenic McIntosh apple lines. Phytopathology 90: 72-77.
Lorito, M., Woo, S. L., Garcia Fernandez, I., Colucci, G., Harman, G. E., Pintor-Toro, J. A., Filippone, E., Mucciflora, S., Lawrence, C. B., Zoina, A., Tuzun, S., and Scala, F. 1998. Genes from mycoparasitic fungi as a source for improving plant resistance to fungal pathogens. Proc. Natl. Acad. Sci, USA 95:7860-7865.
Links to commercial and related web sites
Companies with registered biocontrol products, or who are developing biocontrol products or uses, are encouraged to contact the web master in order to become listed. However, this site will not knowingly list an address for any company that sells any Trichoderma-based product for biocontrol or plant growth promotion that has not undergone governmental review for safety, efficacy or other required evaluations and that therefore sells a non-registered product(s).http://www.bioworksbiocontrol.com/
Links to individual labs and organizations
Academic or private laboratories or other research organizations with web sites are encouraged to contact the web master in order to become listed at this site.