Hershenhorn, J., Department of Plant Pathology, Montana State University, Bozeman, MT 59717, United States
Vurro, M., Istituto tossine d micotossine da parassiti vegetali, Consiglio Nazionale delle Ricerche, viale Einaudi 51, 70126 Bari, Italy
Zonno, M.C., Istituto tossine d micotossine da parassiti vegetali, Consiglio Nazionale delle Ricerche, viale Einaudi 51, 70126 Bari, Italy
Stierle, A., Department of Plant Pathology, Montana State University, Bozeman, MT 59717, United States
Strobel, G., Department of Plant Pathology, Montana State University, Bozeman, MT 59717, United States

Septoria cirsii causes leaf spot of Canada thistle (Cirsium arvense L.) and no other plat tested was susceptible to this pathogen. Because this pathogen possesses host specificity, and because it is so devastating to Canada thistle plants in the field, it is proposed for consideration as a biological control agenet. In culture, S. cirsii produced copious amounts of a known phytotoxin which was identified as ß-nitropropionic acid. The toxin is inhibitory to seed germination, root elongation and causes the typical symptoms of chlorosis and necrosis on the leaves of Canada thistle which is one of the most toxin-sensitive plants of all of those tested. © 1993.