Stagonospora cirsii is a pathogen of Cirsium arvense, causing necrotic lesions on leaves of this noxious weed. The fungus produced toxic metabolites when grown in liquid culture. A new phytotoxin, named stagonolide, was isolated and characterized as (8R,9R)-8-hydroxy-7-oxo-9-propyl-5-nonen-9-olide by spectroscopic methods. Stagonolide was shown to be a nonhost-specific but selective phytotoxin. Leaves of C. arvense were most sensitive and leaves of tomato and pepper (both Solanaceae) were less sensitive to stagonolide, which was assayed at 5 x 10(-3) M, than other plants. Stagonolide assayed at 5 x 10(-6) M was demonstrated to be a strong inhibitor of root growth in seedlings of C. arvense and some other Asteraceae species. Seedlings growth in wheat and radish was much less affected by the toxin, and seedlings of cucumber were insensitive to it.