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Cirsium arvense is a noxious perennial weed which has become an increasing problem in north European countries. Biological control by natural antagonists is of increasing interest to supplement mechanical and chemical control. Several attempts to use fungi such as Alternaria cirsinoxia, Puccinia punctiformis and Sclerotinia sclerotiorum as biocontrol agents have been initiated. No mycoherbicides against C. arvense have been developed and the search for aggressive pathogens continues. In a Danish survey, several fungi were isolated from C. arvense. Four are new records in Denmark: the white rust fungus Pustula andropogonis, the leaf spot fungi Ramularia cirsii, Septoria cirsii and Phomopsis cirsii. Our study shows that P. cirsii is pathogenic to C. arvense, causing stem canker and die back, and that it may have potential as a mycoherbicide against its host. Growth characteristics of P. cirsii on artificial media are described, as well as a scale measuring severity of visible symptoms of P. cirsii on C. arvense. The taxonomic characteristics of Phomopsis spp. are compared and discussed in relation to other records of Phomopsis spp. found on Cirsium spp. and C. arvense.