Resistance of Canada Thistle (Cirsium arvense) Ecotypes to a Rust Pathogen (Puccinia obtegens)

Puccinia obtegens (Link) Tul., an autoecious rust pathogen, is a potential biological control agent of Canada thistle [ Cirsium arvense (L.) Scop.]. Ten ecotypes of Canada thistle were inoculated with uredospores of P. obtegens and sporulation was observed on all ecotypes. Infection types varied among and within ecotypes, indicating that host-resistance is one factor limiting rust infection. No correlation was found between Canada thistle susceptibility to the rust and host plant ecotype classification, stomatal density, amount of leaf pubescence, or spore germinability on leaf surfaces.