Trichoderma Persoon ex Gray, 1801

Trichoderma is a genus of fungi in the family Hypocreaceae 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. This refers to the ability of several Trichoderma species to form mutualistic endophytic relationships with several plant species. Trichoderma species are also responsible for green mold disease in mushroom cultivation. The genomes of several Trichoderma species have been sequenced and are publicly available from the JGI.

Taxonomy

The genus was described by Christiaan Hendrik Persoon in 1794, but the taxonomy has remained difficult to resolve. For a long time, it was considered to consist of only one species, Trichoderma viride, named for producing green mold.

Subdivision

Pachybasium (20 species)

Longibrachiatum (21 species)

Trichoderma

Saturnisporum (2 species)

Hypocreanum

With the advent of molecular markers from 1995 onwards, Bissett's scheme was largely confirmed but Saturnisporum was merged with Longibrachiatum. While Longibrachiatum and Hypocreanum appeared monophyletic, Pachybasium was determined to be paraphyletic, many of its species clustering with Trichoderma. Druzhina and Kubicek (2005) confirmed the genus as circumscribed was holomorphic. They identified 88 species which they demonstrated could be assigned to two major clades. Consequently, the formal description of sections has been largely replaced by informal descriptions of clades, such as the Aureoviride clade or the Gelatinosum clade.

Characteristics

Cultures are typically fast-growing at 25–30 °C (77–86 °F), but some species of Trichoderma will grow at 45 °C (113 °F). 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 a wide main axis (e.g., T. polysporum, T. hamatum), or they may be solitary (e.g., T. longibrachiatum).

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