Colletotrichum incanum sp. nov., a curved-conidial species causing soybean anthracnose in USA

Soybean anthracnose is caused by a number of species of Colletotrichum that as a group represent an important disease that results in significant economic losses. In the current study, Colletotrichum species were isolated from soybean petioles and stems with anthracnose symptoms from soybean fields in Illinois. Multigene sequence phylogenic analysis, combining rDNA internal transcribed spacer, actin, β-tubulin, glyceraldehyde-3-phosphate dehydrogenase and histone H3 gene regions, revealed a group of isolates collected in this study to be distinct from other established Colletotrichum species. This new group was phylogenetically closer to C. liriopes, C. tofieldiae and C. verruculosum than to C. truncatum, another species with curved conidia commonly found on soybean. A representative isolate from this new group was used to examine its morphology, cultural characteristics and pathogenicity to soybean; it was found to differ from C. truncatum in colony culture characteristics and sizes of conidia and appressoria. As a result of the molecular phylogenetic, morphological and pathogenicity analyses, we named this species Colletotrichum incanum. Of the 84 Colletotrichum isolates collected from soybean petioles, 40 were C. incanum, indicating that this species may commonly occur.