British distribution: Probably ubiquitous on dead and dying plant material, but under-recorded.
A very common hyphomycete on leaf surfaces, as a secondary pathogen or saprotroph. It does not usually form noticeable colonies but its characteristic conidia are often found during examination of leaf lesions caused by rusts and other primary pathogens. It appears to be a truly plurivorous species, i.e. found on a large number of different host plants.
The conidia are club-shaped, up to 95 µm in length and conspicuously septate, with both transverse and short, longitudinal septa. They are produced singly or in short chains from short, inconspicuous conidiophores. Fine illustrations of conidia from a range of host plants are given by Joly (1964).
A. tenuissima has no known sexual state and the conidia can be regarded as its only means of dispersal and survival. As in a number of other asexual leaf-surface fungi (e.g., see Torula herbarum), the conidia are thick walled, pigmented and complex in structure, adapted for longer-term survival, in contrast to those species that produce larger numbers of smaller, thin-walled, colourless conidia adapted for rapid germination.
There are numerous other Alternaria species, that need careful discrimination by the shape and size of the conidia, length of the narrowed "beak", details of the septation, etc. Descriptions of many are given by Joly (1964) and Ellis (1971, 1976).