Recognized by its cellular and more or less round fruiting structures (pycnidia) containing masses of 1-celled colourless to yellow or pink spores (conidia). The conidia are borne from inconspicuous peg-like phialides lining the inner wall of the pycnidium. Species of Phoma having spines or setae on their pycnidia are sometimes confused with those of Pyrenochaeta. However, the two genera can be distinguished on the basis of their conidium-bearing structures: conidia of Phoma species are produced from simple phialides while those of Prenochaeta arise from phialides ocurring along the sides of elongated conidiophores.
Phoma is a taxonomically difficult genus and is still not fully understood. Species identification is often difficult. Much of our present knowledge has come from the work of Dr. G.H. Boerema and his colleagues in The Netherlands. Boerema (in Aa, et al., 1990) divided Phoma into five sections, separable by the following key:
1. Multicellular chlamydospores produced along the vegetative hyphae, often resembling conidia of Alternaria or Ulocladium.
1. Chlamydospores one-celled or absent - 2
2. Pycnidia with hard, thick walls
2. Pycnidia with thin walls - 3
3. Pycnidia with hairs, spines or setae on the upper part, especially around the ostiole (opening)
3. Pycnidia lacking hairs or setae - 4
4. Conidia always one-celled
4. Conidia (5-95%) septate
Species of Phoma are common in soils, dung, and both living and dead plants. Holomorphs: many genera of ascomycetes, for example, Didymella and Leptosphaeria. Ref: Boerema, 1993; Boerema et al., 1994; Boerema and Dorenbosch 1973; Dorenbosch 1970; Gruyter and Noordeloos, 1992. James Scott's eclectic and interesting Sporometrics site (see our LINKS page) presents a key to pycnidial fungi commonly occurring indoors, including several Phoma species.