Cochliobolus Drechsler; anamorphs Bipolaris Shoemaker, Curvularia Boedijn.
Ascomata perithecial, unilocular, with little or no stromatic development, dark brown, thick-walled. Asci cylindrical to saccate, thick-walled, apparently with two discernible layers but not exhibiting the “jack-in-the-box” discharge of many “bitunicate” ascomycetes, usually 8-spored. Ascospores helically coiled within the ascus, filiform, multiseptate, sometimes surrounded by mucus.
Conidiomata absent. Conidiophores dark brown, more or less erect, thick-walled, the conidia being produced sequentially from a series of conidiogenous loci in the usually knobbly apical cells, the loci appearing as pale dots, conidial dispersal frequently being delayed so that clusters of conidia are visible at the tips of conidiophores. Conidia dark brown, slightly (in Bipolaris) or strongly (in Curvularia) curved, with many thick septa (Bipolaris) or relatively few thin septa (Curvularia).
These fungi are most prominent as plant pathogens, but may occasionally be encountered in industrial situations, from soil and air isolations, and acting as agents of biodeterioration of textiles etc. They are most frequently encountered in the tropics. In addition, there is interest in enzyme, metabolite and toxin production by some species of Cochliobolus and Pyrenophora (q.v.), and at least one isolate is subject to patent restrictions for this reason. Some species are also implicated as disease-causing organisms (McGinnis et al., 1986b)
The fungi are most frequently seen in their anamorphic (conidium-producing) condition, and at one time were merged with Drechslera (the anamorphic state of Pyrenophora) and several other form-genera as Helminthosporium Link. This assemblage was shown to be unnacceptably heterogeneous more than thirty years ago (Hughes, 1953), and the use of that last name since than has been restricted to a small group of saprobic fungi of no industrial interest. The fungi are in most cases more difficult to identify using teleomorphic rather than anamorphic characteristics.
Bipolaris and Curvularia are very similar, and share the same teleomorph genus. Curvularia species have conidia which are strongly curved and have relatively few, thin, septa, while those of Bipolaris in general are less strongly curved, and have larger numbers of thick (disto-) septa (Alcorn, I983a) . The two groups intergrade, and may be merged in the future (Sivanesan, 1987).
Most species produce only the anamorph in culture, and in many cases it is difficult to maintain even conidium production after successive subculturing.
Cochliobolus geniculatus Nelson; anamorph Curvularia geniculata (Tracy & Earle) Boedijn.
Conidia are 18-37 x 8-14µm in size in culture (often rather longer on natural substrata), strongly curved, and almost always 4-septate, with the end cells paler. It is heterothallic. The anamorphic state was formerly known as Helminthosporium geniculatum Tracy & Earle.
Cochliobolus lunatus Nelson & Haasis; anamorph Curvularia lunata (Wakker) Boedijn.
Conidia are 20-32 x 9-15µm in size, strongly curved, 3-septate, with the central cells dark brown and the apical ones much paler. It also is heterothallic.