Bioassay-Directed Isolation and Identification of Phytotoxic and Fungitoxic Acetylenes from Conyza canadensis

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 mu M, while the (4Z,8Z)-matricaria lactone was less active (IC50 of 220 mu M). In a fungal direct bioautography assay, the two lactones at 10 and 100 mu g/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 mu M and about as active as the commercial fungicide captan against Col. gloeosporioides, while (4Z)-lachnophyllum lactone was less active.