Species of Colletotrichum on Xanthium (asteraceae) with comments on some taxonomic and nomenclatural problems in Colletotrichum

Abstract

Five species of Colletotrichum are recognized on Xanthium. Examination of the type and other collections, and cultural studies, show that the common anthracnose fungus on X. spinosum in eastern Australia is C. orbiculare, the cucurbit anthracnose organism. Other hosts of C. orbiculare include some cvs of safflower (Carthamus tinctorius) and other genera of Asteraceae and Apiaceae. On X. occidentale in Queensland and New South Wales, C. acutatum occurs commonly on stem lesions in close association with the rust Puccinia xanthii. Study of the type collection of C. xanthii, described from the United States of America in 1892, has shown it to be identical to C. acutatum. Telia of Puccinia xanthii were also present in the American collection. Colletotrichum dematium has been found on stem and cotyledon lesions of X. occidentale and C. coccodes has been isolated once from cotyledon spots on the same host. In the United States, isolates of C. truncatum pathogenic to soybean (Glycine max) have been reported from field-infected Xanthium spp. These findings are discussed in the light of various taxonomic and nomenclatural problems in Colletotrichum, including (i) species concepts, particularly in C. gloeosporioides sensu lato, (ii) the value of host range as a taxonomic character, (iii) the nomenclature of C. acutatum, and (iv) concealment of information in synonym lists.