Crude extracts of Xanthium strumarium inhibited mycelial growth and zoospore germination of Phytophthora drechsleri, the causal agent of Atractylis rot, in vitro. Fresh sap from X. strumarium at 50-fold dilution was highly effective in controlling the disease incidence in pot and field trials. Purified extracts from cocklebur inhibited mycelial growth and zoospore germination in vitro at a concentration of 12.5 mg/ml and 15.6 mg/ml, respectively. Hyphal tips affected by the compound showed malformation. The antifungal compound purified from X. strumarium was identified as 4-oxo-1 (5), 2,11, (13)-xanthatriene-12,8-olide, known as "deacetyl xanthumin".