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Conyza canadensis (L.) Cronquist syn. (horseweed) is a problematic and invasive weed with reported allelopathic properties. To identify the phytotoxic constituents of the aerial parts, a systematic bioactivity-guided fractionation of the dichloromethane extract was performed. Three active enyne derivatives, (2Z,8Z)-matricaria acid methyl ester, (4Z,8Z)-matricaria lactone, and (4Z)-lachnophyllum lactone, were identified. The lactones inhibited growth of the monocot Agrostis stolonifera (bentgrass) and the dicot Lactuca sativa (lettuce) at 1 mg mL(-1), while the (2Z,8Z)-matricaria acid methyl ester was less active. In a dose-response screening of the lactones for growth inhibitory activity against Lemna paucicostata , (4Z)-lachnophyllum lactone was the most active with an IC50 of 104 μM, while the (4Z,8Z)-matricaria lactone was less active (IC50 of 220 μM). In a fungal direct bioautography assay, the two lactones at 10 and 100 μg/spot inhibited growth of the plant pathogenic fungi Colletotrichum acutatum , Colletotrichum fragariae , and Colletotrichum gloeosporioides . In a dose-response screening of the lactones against six different plant pathogenic fungi, (4Z,8Z)-matricaria lactone was more active than the commercial fungicide azoxystrobin on Col. acutatum , Col. fragariae , and Col. gloeosporioides at 30 μM and about as active as the commercial fungicide captan against Col. gloeosporioides , while (4Z)-lachnophyllum lactone was less active.