Torula herbarum (Pers.)Link    

Kingdom: Fungi
Phylum: Ascomycota  (anamorphic fungi - hyphomycetes)

Systematic position within the Ascomycota uncertain.

British distribution: Common on dead, herbaceous stems.
World distribution: Cosmopolitan.

A very common hyphomycete on all kinds of dead, decaying herbaceous stems, especially in humid conditions. It forms dense, olive green to black areas of "mould", the colour that of the abundant conidia.

The photograph shows part of an old, dead nettle (Urtica dioica) stem (against a millimetre scale).
Torula herbarum can be seen as the fuzzy black fungus ('Th').

Nettle stems can be hosts to a large number of microfungi. Also visible here are the conical pseudothecia of Leptosphaeria acuta ('La') and (slightly out of focus) the apothecia of the discomycete Pyrenopeziza urticicola ('Pu'), both common, host-specific species on nettle. Note the dark colour of each of these fungi — they are exposed on the stem surface and the pigment presumedly protects the asexual or sexual spores from excessive light (see below).

The conidia are brown under the microscope, multiseptate and anything from 20 to 70 µm in length. The walls are conspicuously though minutely roughened to spiny (verruculose to echinulate).

T. herbarum has no known sexual state and the conidia can be regarded as its only means of dispersal and survival. Consequently (?) the conidia are thick walled and complex in structure (whereas conidia in species that also produce sexual spores are more often, though not always, thin walled, simple and adapted for rapid germination and only limited survival). The pigmented walls (making this a 'dematiaceous' hyphomycete) also relate to the longevity of the conidia, in relation to exposure to light on the substrate surface.

Photographed material from Pencelli Forest, Pembrokeshire, 1987.

In T. herbarum, the conidial chains break up into conidia that are typically 4-5 septate (up to 10-septate) and 5-9 µm in width. There are a few other species of torula, mostly in warmer parts of the world, and the conidia of these differ in width and numbers of septa (see Ellis, 1971). Rutola graminis, formerly regarded as a Torula, is a similar species on grasses, differing in having conidial chains that generally break up into single-celled conidia, 4-6 µm in diameter. T. herbarum itself is common on grasses.

•   Ellis, M.B., (1971). Dematiaceous hyphomycetes, CAB International (as CMI), Kew.

© A.J. Silverside
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