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Trichoderma is a genus of
fungi
that is present in all soils, where they are the most prevalent
culturable fungi. Many species in this genus can be characterized as
opportunistic avirulent plant symbionts.
Cultures are typically
fast growing at 25-30°C, but will not grow at 35° C. Colonies are
transparent at first on media such as cornmeal dextrose agar (CMD) or
white on richer media such as potato dextrose agar (PDA). Mycelium are
not typically obvious on CMD, conidia typically form within one week in
compact or loose tufts in shades of green or yellow or less frequently
white. A yellow pigment may be secreted into the agar, especially on
PDA. Some species produce a characteristic sweet or 'coconut' odor.
Conidiophores are
highly branched and thus difficult to define or measure, loosely or
compactly tufted, often formed in distinct concentric rings or borne
along the scant aerial hyphae. Main branches of the conidiophores
produce lateral side branches that may be paired or not, the longest
branches distant from the tip and often phialides arising directly from
the main axis near the tip. The branches may rebranch, with the
secondary branches often paired and longest secondary branches being
closest to the main axis. All primary and secondary branches arise at or
near 90° with respect to the main axis. The typical Trichoderma
conidiophore, with paired branches assumes a pyramidal aspect. Typically
the conidiophore terminates in one or a few phialides. In some species
(e.g.
T. polysporum) the main branches are terminated by long, simple or
branched, hooked, straight or sinuous, septate, thin-walled, sterile or
terminally fertile elongations. The main axis may be the same width as
the base of the phialide or it may be much wider.
Phialides are typically enlarged in the middle but may be
cylindrical or nearly
subglobose. Phialides may be held in whorls, at an angle of 90° with
respect to other members of the whorl, or they may be variously
penicillate (gliocladium-like).
Phialides may be densely clustered on wide main axis (e.g.
T. polysporum,
T. hamatum) or they may be solitary (e.g.
T. longibrachiatum).
Conidia
typically appear dry but in some species they may be held in drops of
clear green or yellow liquid (e.g.
T. virens,
T. flavofuscum). Conidia of most species are ellipsoidal, 3-5 x 2-4
µm (L/W = > 1.3); globose conidia (L/W < 1.3) are rare. Conidia are
typically smooth but tuberculate to finely warted conidia are known in a
few species.
Synanamorphs are formed by some species that also have typical
Trichoderma pustules. Synanamorphs are recognized by their solitary
conidiophores that are verticillately branched and that bear conidia
in a drop of clear green liquid at the tip of each phialide.
Chlamydospores may be
produced by all species, but not all species produce chlamydospores on
CMD at 20° C within 10 days. Chlamydospores are typically unicellular
subglobose and terminate short hyphae; they may also be formed within
hyphal cells. Chlamydospores of some species are multicellular (e.g. T.
stromaticum).
Teleomorph
Teleomorphs of Trichoderma are species of the ascomycete genus
Hypocrea. These are characterized by the formation of fleshy,
stromata in shades of light or dark brown, yellow or orange. Typically
the stroma is discoidal to pulvinate and limited in extent but stromata
of some species are effused, sometimes covering extensive areas.
Stromata of some species (Podostroma) are clavate or turbinate.
Perithecia are completely immersed. Ascospores are bicellular but
disarticulate at the septum early in development into 16 part-ascospores
so that the ascus appears to contain 16 ascospores. Ascospores are
hyaline or green and typically spinulose. More than 200 species of
Hypocrea have been described but few have been grown in pure culture and
even fewer have been described in modern terms.
Habitat
Trichoderma colony in nature
Trichoderma species are frequently isolated
from forest or agricultural soils at all
latitudes.
Hypocrea species are most frequently found on bark or on
decorticated wood but many species grow on bracket fungi (e.g.
H. pulvinata),
Exidia (H.
sulphurea) or bird's nest fungi (H.
latizonata) or agarics (H.
avellanea).
Biocontrol
agent
Several
strains of Trichoderma have been developed as biocontrol agents
against fungal diseases of plants.[2]
The various mechanisms include antibiosis, parasitism, inducing
host-plant resistance, and competition. Most biocontrol agents are from
the species
T. harzianum,
T. viride and T. hamatum. The biocontrol agent generally
grows in its natural habitat on the root surface, and so affects root
disease in particular, but can also be effective against foliar
diseases.
Causal
agent of disease
Trichoderma
aggressivum (formerly T. harzianum
biotype 4) is the causal agent of green mold, a disease of cultivated
button mushrooms..
Trichoderma
viride is the causal agent of green mold rot of
onion.
Industrial
use
Trichoderma,
being a saprophyte adapted to thrive in diverse situations, produces a
wide array of enzymes. By selecting strains that produce a particular
kind of enzyme, and culturing these in suspension, industrial quantities
of enzyme can be produced.
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