The Nature and Application of Biocontrol Microbes II: Trichoderma spp.
Overview of Mechanisms and Uses of Trichoderma spp.
Gary E. Harman
Cornell University, Geneva, NY 14456.
Accepted for publication 2 September 2005.
Harman, G. E. 2006. Overview of mechanisms and uses of Trichoderma
spp. Phytopathology 96:190-194.
Fungi in the genus Trichoderma have been known since at least the
1920s for their ability to act as biocontrol agents against plant pathogens.
Until recently, the principal mechanisms for control have been assumed to
be those primarily acting upon the pathogens and included mycopara-
sitism, antibiosis, and competition for resources and space. Recent
advances demonstrate that the effects of Trichoderma on plants, including
induced systemic or localized resistance, are also very important. These
fungi colonize the root epidermis and outer cortical layers and release
bioactive molecules that cause walling off of the Trichoderma thallus. At
the same time, the transcriptome and the proteome of plants are sub-
stantially altered. As a consequence, in addition to induction of pathways
for resistance in plants, increased plant growth and nutrient uptake occur.
However, at least in maize, the increased growth response is genotype
specific, and some maize inbreds respond negatively to some strains.
Trichoderma spp. are beginning to be used in reasonably large quantities
in plant agriculture, both for disease control and yield increases. The
studies of mycoparasitism also have demonstrated that these fungi pro-
duce a rich mixture of antifungal enzymes, including chitinases and β-1,3
glucanases. These enzymes are synergistic with each other, with other
antifungal enzymes, and with other materials. The genes encoding the en-
zymes appear useful for producing transgenic plants resistant to diseases
and the enzymes themselves are beneficial for biological control and
For about 70 years, Trichoderma spp. have been known to be
able to attack other fungi, to produce antibiotics that affect other
microbes, and to act as biocontrol microbes (44,45). During this
time, we have learned much about their mechanisms of action and
how they might be used commercially for various purposes.
Some landmarks along the way include the discoveries that
these fungi frequently increase plant growth and productivity (30)
either in the presence (9) or absence (30) of other microorganisms
and that they can induce disease suppression in soils (11). Com-
posted materials may be suppressive and that suppression may
occur in part as a consequence of activities of Trichoderma spp.
by several different mechanisms (25), as will be discussed in this
session (Hoitink). Further, strains differ remarkably in their abili-
ties to colonize roots (i.e., to be rhizosphere competent) (1) and
the most effective strains will colonize roots and provide benefits
for at least the life of annual crops (20). In addition, the complex
mechanisms of mycoparasitism, which include directed growth of
Trichoderma toward target fungi, attachment and coiling of Tricho-
derma on target fungi, and the production of a range of antifungal
extracellular enzymes, were elucidated, initially in large part by
I. Chet and his students and colleagues (summarized in Chet 
and Chet et al. ).
Indeed, Trichoderma spp. were demonstrated to be very effi-
cient producers of extracellular enzymes, with cellulases as the
first example (36,37). Later, these fungi were found to produce a
wide range of other extracellular enzymes and some of these were
implicated in the biological control of plant diseases (17). In the
early 1990s, a series of papers by a range of labs demonstrated
that the mixtures of biocontrol enzymes were very complex, and
the genes encoding many of these were isolated and sequenced
(summarized in Benitez et al.  and Lorito ). The enzymes
themselves were found to be fungitoxic and mixtures of enzymes
were synergistic in their antifungal properties (34). Different
classes of chitinolytic or glucanolytic enzymes from Trichoderma
are synergistic as are enzymes from different organisms (34).
Genes encoding these proteins have been inserted into plants,
where they have been shown to induce resistance to a range of
plant-pathogenic fungi (6,7,35). The remainder of this paper will
summarize some recent advances.
Trichoderma spp. as opportunistic plant symbionts. A
number of plant-associated microbes are free-living and strongly
beneficial to plants. Fungi in the genus Trichoderma (21) and
rhizobacteria in the genera Pseudomonas, Bacillus, Streptomyces,
Enterobacter, and others (2,38,39,43) have evolved multiple
mechanisms that result in improvements in plant resistance to
disease and plant growth and productivity.
A new set of models of the mechanisms of action of Tricho-
derma spp. has recently been proposed (21). These new models
do not replace others, which may include inhibition of enzymes
necessary for pathogens to penetrate plant surfaces (49) and com-
petition for nutrients (16) including those necessary for pathogen
propagules to germinate near planted seeds (26,27). There are a
number of mechanisms whereby these fungi act as biocontrol
Our understanding of the genetic control of mycoparasitism has
changed significantly. It has been known for many years that
Trichoderma can sense the presence of target fungi and appeared
to grow tropically toward them (13). More recently, the gene en-
coding green fluorescent protein was inserted downstream of the
regulatory regions of genes encoding an endo- and an exo-
chitinase that have biocontrol abilities. When paired with a target
fungus, the endochitinase gene is activated before the fungi come
into contact, while the activation of the exochitinase occurs only
after contact is made (48). Different strains may follow different
patterns of induction but the fungi apparently always produce low
Corresponding author: G. E. Harman; E-mail address: firstname.lastname@example.org
© 2006 The American Phytopathological Society
Vol. 96, No. 2, 2006 191
levels of an extracellular exochitinase. Diffusion of this enzyme
catalyzes release of cell wall fragments from target fungi and this,
in turn, induces expression of fungitoxic cell wall degrading en-
zymes (8) that also diffuse and begin the attack on the target fungi
before contact is actually made (42,48). These cell wall fragments
are highly potent inducers of enzymes and induce a cascade of
physiological changes within the fungus, including an enhance-
ment in Trichoderma growth. This system will be described more
fully in this session by Lorito. Thus, there are numerous ways by
which Trichoderma spp. attack or otherwise directly inhibit other
However, the direct effects of Trichoderma spp. on plants are
remarkable and at least as significant as their direct effects on
other fungi and have only recently been described. First, the fungi
are highly efficient inducers of systemic and localized resistance
in plants, a fact perhaps first conclusively demonstrated by
Bigirimana et al. (5) although it was suggested by numerous other
workers earlier. A recent review lists 11 separate reports demon-
strating control by Trichoderma spp. of a wide range of plant
pathogens, including fungi, oomycetes, bacteria, and one virus, by
elicitation of induced systemic or localized resistance (21). The
fungi, especially rhizosphere competent ones, colonize root sur-
faces and penetrate the epidermis and into the cortex (46). Along
the way, the fungi may coil about root hairs in a manner reminis-
cent of mycoparasitism (46). Once Trichoderma hyphae penetrate
the roots, a series of bioactive metabolites from the fungus is
produced that induces walling off and biochemical mechanisms
that limit growth of the Trichoderma to a small area. This reaction
may not always occur; for example, there now are known endo-
phytic Trichoderma strains that colonize vascular systems of cer-
tain plants, as will be discussed elsewhere in this session
(Samuels). This may be similar to responses of other root coloniz-
ing biocontrol fungi including binucleate Rhizoctonia species (29)
and nonpathogenic Fusaria (3). The bioactive molecules may in-
clude several different proteins (19), avr-like proteins and cell
wall fragments released by action of extracellular enzymes, as has
been demonstrated in the mycoparasitic reaction (21).
This interaction results in both localized and systemic resis-
tance (21). Among other surprising findings are the demonstration
that, contrary to long-held opinion, the ability of T. virens to con-
trol seedling disease in cotton caused by Rhizoctonia solani is not
due to antibiotics or mycoparasitism but is mediated by the abili-
ties of the biocontrol strains to induce terpenoid phytoalexins
(28), as will be discussed in this session. T. virens on cotton re-
sults in localized resistance, but with most other plant–Trichoderma
systems, the resistance induced is systemic (21).
The systems for induced resistance appear to be in at least some
ways similar to those induced by rhizobacteria. Yedidia et al. (47)
demonstrated that mRNA for pathogenesis-related (PR) proteins
was only expressed transitorily in the absence of pathogens. How-
ever, if leaves of beans whose roots were colonized by Tricho-
derma were inoculated with the bacterial pathogen Pseudomonas
syringae pv. lachrymans, there was strong expression of mRNA
for several different PR proteins. If either the pathogen alone was
inoculated onto foliage or T. asperellum was not present on roots,
then the induction of the PR transcriptomes did not occur, or
occurred at a lower rate (21,47). Thus, in common with the effects
of plant growth promoting rhizobacteria, there appears to be a
priming effect of the root symbiont that is expressed when the
plant is challenged by a pathogen.
The resistance induced may be temporally and spatially distant
from the site of application or existence of the Trichoderma.
For example, T. harzianum strain T22 (41) was added at trans-
planting to tomato roots. After 90 to 120 days, symptoms of
late blight appeared on the leaves, but in trials over 2 years, there
was up to 80% reduction in disease in the presence compared
with the absence of T22 even though T22 was present only on
Mention has already been made of the ability of root symbiotic
Trichoderma strains to increase plant growth. This effect has been
studied for some time in this lab with maize as the system. The
general concepts have been published several times and are sum-
marized below. Among the positive effects on maize that have
been noted over the past 5 to 10 years in work conducted by Ad-
vanced Biological Marketing, Cornell University and others include
the following (documented in Harman  and Harman et al. ):
• Control of root and foliar pathogens
? Induced resistance
? Biological control of diseases by direct attack of plant-
• Changes in the microfloral composition on roots
• Enhanced nutrient uptake, including but not limited to nitrogen
• Enhanced solubilization of soil nutrients
• Enhanced root development
• Increased root hair formation
• Deeper rooting
For some time, the possibility that different plants respond differ-
ently to biocontrol agents or plant symbiotic fungi has been dis-
cussed. However, since Trichoderma spp. are broadly effective
across a range of plant species, we did not view strong genetic
interactions with plants as a major factor. However, in maize this
is not true, at least for the enhanced growth response.
There have been more than 500 documented field trials that
compared field corn grown from seed, treated or not treated with
T. harzianum strain T22, and the average yield increase is about
5 bu/acre. However, trial results have shown tremendous variabil-
ity, with ranges between +50% to actual yield decreases. This
clearly indicates that there are uncontrolled or poorly understood
variables that affect results. The greatest yield increases appeared
when the variety tested had some genetic weaknesses (e.g., Nutri-
dense varieties) or where there were biotic (e.g., anthracnose or
rust) or abiotic (e.g., soil compaction, drought, or nutrient insuf-
ficiency) stresses present.
The reasons for the occasionally observed yield decreases were
at first attributed to unusual field variations, but a large trial (160
hybrids +/– T22 in three different sites in the U.S. corn belt) con-
ducted by a commercial company, Advanced Biological Market-
ing, suggested that there was a maize genetic component to the
interaction of T22 as well.
A first priority for research at Cornell University was to
identify a genetically homozygous inbred that responded strongly
to T22. An initial screen resulted in the discovery that seedling
growth (measured 2 weeks after planting) of maize inbred Mo17
is strongly enhanced by T22 and that this increased growth re-
sponse continues for the life of the plant (23). The same treatment
also induces systemic resistance. Further, T22 on roots increased
levels of total proteins and activity of the putative PR proteins
chitinase and β-1,3 glucanase in both shoots and roots (23).
Field trials and lab experiments have confirmed the observation
that there is a maize genetic component to the T22 response. In
field trials in 2002, a hybrid, Sgi860 × Sgi861, was used that had
not been evaluated before. The experiment was a replicated block
design that compared +/– T22 over a range of organic (composted
chicken manure) or inorganic (ammonium nitrate) fertilizer con-
centrations to give different levels of added nitrogen ranging from
0 to 100 kg/ha. From the earliest growth of the maize, reduced
growth in the presence of T22 was observed regardless of treat-
ment, and the end of the season yields in the presence of T22
were reduced about 7% over a total of 14 fertilizer treatments.
This hybrid was tested in the 2-week seedling assay developed
previously for Mo17. There was a statistically significant reduc-
tion in growth in this assay as well. This finding was significant
for two reasons, as follows:
• T22 reduced growth of Sgi860 × Sgi861 both in the field
across a range of fertilizer levels and in our lab assay with
• The 2-week seedling assay retrospectively appeared to have
predictive ability for the yield performance of maize in the
field. This result is consistent with repeated observations that
changes in growth of maize by T22 can be observed within a
short time of plant emergence from soil and can be tracked
throughout the growing season.
The two parental inbreds were evaluated in the 2-week lab
growth test and only Sgi861 exhibited growth reduction, while
growth of the other parent was only slightly affected by seed
treatment with T22. We also made an F1 hybrid between Mo17
and Sgi861. The hybrid gave increased growth responses similar
to the responses of Mo17.
Thus, a hybrid between a line with little growth response to
T22 and a negative responding line gave an F1 hybrid (Sgi860 ×
Sgi861) that also gave a negative growth response. Another cross
between a negative responding line and a strongly positive re-
sponding line gave a hybrid (Mo17 × Sgi861) with a strong posi-
tive response. These data may suggest that the T22 responses in
maize are largely conditioned by dominant genes. Sgi lines are
proprietary and we could not proceed further with them, so we
examined a series of publicly available lines using the 2-week
assay. The responses of various inbred lines are as follows:
• T22 induces strong positive growth responses
? Mo17, Mo46, Va26, C103, C123, NYD410, WF9, B14
• T22 has little effect upon plant growth
? RD402, RD 6503, Oh43, Pa875, Va17, Va35, RD3013, B73
• T22 negatively affects plant growth
? A661 and Pa33
Thus, there clearly are strong genetic components to the response
of at least maize to T22.
Strain T22 of T. harzianum generally increases plant growth
and development and controls diseases in both commercial use
and controlled experimental settings (20). However, because of
variations in results, it is currently used on only a small percent-
age of the total maize acreage in the United States. The under-
standing of its genetic interactions with maize is expected to
substantially reduce the variability that has limited its use thus far.
Clearly, if T22 was only applied to seeds of varieties that respond
positively, then the overall yield benefit should increase and the
variability should decrease. Maize also is an ideal crop for appli-
cation of an inexpensive, long-term method for reducing disease
via systemic resistance. Since T22 is a root colonist, only small
amounts need to be applied to the seeds for long-term effects.
Further, total control is not necessary; even a relatively modest
decrease in disease would be a valuable addition to maize
culture since foliar fungicides are generally too expensive for
For nearly all commercial uses of Trichoderma for biological
control and for enhancement of plant growth and yield, an under-
standing of the mechanisms and Trichoderma–pathogen, Tricho-
derma–plant, and Trichoderma–plant–pathogen interactions is
essential. For example, if the maize–Trichoderma interactions are
fully understood, and the promise of substantially increased yields
are realized, then Trichoderma will be used on a sizable percent-
age of the total 36 million acres of the crop in the United States.
Further, with a clear understanding of mechanisms we can develop
strategies to produce improved strains. For example, there was
great concern regarding screening techniques for biocontrol strains
of Trichoderma; the first and quickest ones were screens for
antibiotic production and/or mycoparasitism in petri dish assays.
Unfortunately, although the results were frequently clear-cut, they
had almost no predictive value for biocontrol efficacy. Therefore,
we, and most other labs, used plant–pathogen interaction assays
for testing, and this was effective and predictive (24). Petri dish
assays as systems for good biocontrol strains had several funda-
mental drawbacks, based on our current knowledge. One was that
delivery and production systems were rudimentary, but the other
was the fact that the plant and soil were left out of the assay. If
induced resistance is the mechanism by which biological control
was achieved, then clearly assays containing only Trichoderma
and the pathogen were doomed to fail. Modern screening will
assess effects of the candidate strain on the plant, both on roots
and on foliage even if the biocontrol agent is present only on
roots. Understanding mechanisms and development of rapid
screening protocols will involve modern molecular tools such as
measuring specific gene expression. This advance alone is ex-
pected to pay dividends in practical biological control.
Uses of biological control and related enzymes and the
genes that encode them. We know that there is a wide range of
chitinolytic and glucanolytic enzymes with possible roles in bio-
logical control and no doubt many more will be discovered as the
Trichoderma genome and proteome are analyzed. There probably
are at least 30 chitinases alone, each with a different gene struc-
ture and protein composition. Some have exo-enzyme activity and
others act as endochitinases; no doubt there are also chitin de-
acetylases and chitosanases. There also is a rich mixture of gluca-
nases. Thus, the number of genes and proteins just in these two
groups of biocontrol genes is very diverse. Most of the gene
products are antifungal.
These enzymes, if they act differently (for example, exo- and
endochitinases), are synergistic in their activity. Thus, an effective
dose at fifty percent (ED50) for a typical chitinase may be around
40 µg/ml, but the ED50 for a mixture of an exo and an endo would
be about 10 µg/ml of total protein. For a mixture of two chitinases
plus a glucanase, the comparable value would be 1 to 3 µg/ml of
total protein (32). Furthermore, they are strongly synergistic in
their antifungal activities when combined with essentially any
chemical fungicide that directly or indirectly has an effect upon
the cell membranes of target fungi (33). Thus, it is surprising that
these enzymes are not already used in the control of unwanted
fungi; however, a major limiting factor has been the lack of avail-
ability of large-scale quantities of enzymes for product develop-
Other uses for these versatile and stable enzymes exist. For
example, very large quantities of crustacean chitin are discarded
each year. There is a large market for glucosamine, which is a
chemically degraded form of the monomer of chitin, N-acetyl
glucosamine (NAG). However, NAG has the potential to be more
valuable than glucosamine; for example, it has proven useful in
the treatment of inflammatory bowel disease and related bowel
disorders (40) while glucosamine lacks this ability. Crustacean
chitin is difficult to process chemically to NAG and the material is
highly resistant to most chitinolytic enzymes due to its crystalline
structure, the presence of calcium and other factors. However, a
synergistic mixture of bacterial chitinases and Trichoderma chiti-
nases has proven to give essentially complete release of NAG (14).
In addition, the genes encoding these antifungal proteins have
been transferred to plants where they confer resistance to fungal
plant pathogens (6,7). Recently, highly resistant rice plants ex-
pressing Trichoderma genes have been produced by T. Xu, PRC.
The primary impediment to commercial use of Trichoderma genes
has been the public resistance to transgenic plants. Because of
this, the first commercial use of transgenic plants that contain
Trichoderma biocontrol genes may be in China.
Uses of Trichoderma in pollution remediation. Trichoderma
spp. probably have significant wide-scale uses in the remediation
of pollutants in soils and waters (22). First, as has been noted
earlier, highly rhizosphere competent strains of Trichoderma,
such as T. harzianum strain T22, enhance root growth of a range
of plants. This enhanced root growth, when combined with a plant
that hyper-accumulates toxicants, will increase the volume of soil
colonized by roots, including enhancement of deep root penetra-
tion (20). Further, Trichoderma spp. on roots increase uptake of
nitrates and other ions (20) and may also increase uptake of
various toxic metals and metalloids. Thus, it should assist in
Vol. 96, No. 2, 2006 193
Further, Trichoderma spp. are highly resistant to a range of
toxicants, perhaps at least in part because of highly active ABC
transport systems (21). Included on the list of toxic materials they
resist is cyanide. Recently, fungi in the genus Trichoderma were
shown to be capable of catabolizing cyanide (18). These fungi
constitutively produce cyanide hydratase (EC 184.108.40.206) and
rhodanese (thiosulfate sulfurtransferase, EC 220.127.116.11). Experiments
were performed in which the fungi were added to soils spiked
with cyanide solutions, and then wheat or pea seeds were sowed
on this soil mix. In the absence of cyanide, the seedlings grew
normally from seed, but the addition of 10 mM cyanide severely
limited seedling growth. However, in the presence of any of
several different Trichoderma strains, the germinating seeds pro-
vided a nutrient source for the fungus, permitting its growth. The
fungi produced enzymes that degraded the cyanide and permitted
normal growth of seedlings even at 50 or 100 ppm.
Recently published research from Cornell University and the
University of Southern Illinois, Carbondale, suggests that the shrub
willow, Salix eriocephala, is an excellent candidate plant species
for phytoremediation of cyanide- and ferrocyanide-contaminated
soils and groundwater (15). S. eriocephala plants demonstrated
the ability to take up and degrade ferrocyanide in hydroponic cul-
tures with 15N-labeled cyanide or ferrocyanide, yet no cyanide re-
mained in the aerial plant tissues. The combination of Tricho-
derma with shrub willows is expected to provide an effective
method to degrade and remove cyanide and metallocyanides from
a variety of polluted sites. These examples are a portion of the
potential for use of Trichoderma spp. in remediation of polluted
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