| Summary: | From Manitoba hardwoods and conifers, 183 collections were examined for Ceratocystis. Most previously described species, including many type specimens, were also studied. Among morphological characters used to separate species of Ceratocystis, those of the ascospore are the most reliable and stressed in this study. Three-dimensional examination of ascospores, particularly in species possessing spores with sheaths, is necessary to determine spore shape accurately. Four ascospore sheath types are recognized: uniform, single-brimmed, double-brimmed, and quadrangular or ossiform. Since the conidial state is distinctive in some species of Ceratocystis, and of diagnostic value in others, the conidial state is considered an integral part of the species diagnosis and the method of conidium formation is stressed. Based mainly on ascospore characters, the genus Ceratocystis is herein organized into four groups: 1. The Minuta Group; 2. The Ips Group; 3. The Fimbriata Group; and 4. The Pilifera Group. It is felt that species placed in a particular group are more closely related to species within that group than they are to species placed in any of the other groups. A key to the Groups of species and keys to the species of each of the four respective groups are provided. The following new species are described by the author: C. aequivaginata, C. arborea, C. cainii, C. columnaris, C. concentricia, c. conicicollis, C. cornata, c. crenulata, c. curvicollis, c. davidsonii, c. deltoideospora, c. fasciata, c. introcitrina, c. longispora, C. minima, C. ossiformis, C. pallidobrunnea, c. parva, C. populicola, c. pseudoeurophioides, C. pseudominor, C pseudonigra, C. spinifera, C. torticiliata, c. tubicollis. In addition, descriptions of the conidial states of the following species are given herein for the first time: c. angusticollis Wright and Griffin, C. dolominuta Griffin, C. spinulosa Griffin.
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