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Sherriff, C., Whelan, M. J., Arnold, G. M., Lafay, J.-F., Brygoo, Y., and Bailey, J. A. 1994. Ribosomal DNA sequence analysis reveals new species groupings in the genus Colletotrichum. Experimental Mycology 18, 121-138. The relatedness of a range of isolates of Colletotrichum species, selected to represent the major morphological forms of the genus, was studied by comparing their morphology with an analysis of an 886-bp region of their 28S rDNA sequences and ITS-2 regions. rDNA was amplified by PCR. An initial dendrogram for the full sequence was drawn using cluster analysis (UPGMA), with trees then constructed using the neighbor-joining method. The similarity matrices and the resulting dendrogram and trees indicate that the genus can be divided into two groups. One group, consisting of Colletotrichum lindemuthianum, Colletotrichum malvarum, Colletotrichum orbiculare, and Colletotrichum trifolii, was distinct from all the other species. Their rDNA was highly homologous and they had consistent morphological features, including their failure to produce septa during conidial germination, which readily distinguished them from Colletotrichum gloeosporioides and from all other isolates. It is concluded that isolates within this group may represent a single species, which should be referred to as C. orbiculare (Berk. and Mont. (v Arx)), in which distinct host-specific forms exist. Examination of the other group indicates that isolates presently included within C. gloeosporioides represent more than one species. The data also confirmed present day inaccurate identifications; e.g., the causal agent of cowpea anthracnose is not a form of C. lindemuthianum. The distinction between species with falcate conidia, i.e., Colletotrichum capsici, Colletotrichum graminicola, Colletotrichum caudatum, and Colletotrichum truncatum, was verified. The relationships revealed by analysis of a 886-base region between the isolates examined was also evident from selective analysis of domain 2 (206 sites) and ITS-2 (158 sites) sequences, suggesting that further analysis of either of these regions should rapidly extend the understanding of the taxonomy of this important plant pathogenic genus. Copyright 1994, 1999 Academic Press