Cercosporella acroptili and Cercosporella centaureicola sp. nov.--potential biological control agents of Russian knapweed and yellow starthistle, respectively.

Russian knapweed (Acroptilon repens [L.] DC.) and yellow starthistle (Centaurea solstitialis L.) are invasive weeds in the western United States, and both weeds are targeted for biological control. Cercosporella acroptili (Bremer) U. Braun was identified as a possible biological control agent for A. repens, and a morphologically similar Cercosporella sp. recently was found damaging to C. solstitialis in the field. Because both fungi are potentially important for biological control of the respective weeds, studies were undertaken to ascertain whether the isolates were identical based on morphology, pathogenicity, growth and spore production, and genetics (molecular characterization of the internal transcribed spacer regions of the ribosomal RNA genes). Differences in these variables between the two isolates were sufficient to indicate that the isolate from C. solstitialis was distinct and justified a new description at the species level: Cercosporella centaureicola sp. nov.