ATRAZINE RESISTANCE IN AMARANTHUS RETROFLEXUS (REDROOT PIGWEED) AND A. POWELLII (GREEN PIGWEED) FROM SOUTHERN ONTARIO

Abstract

Screening trials with the herbicide atrazine and a morphological examination of atrazine-resistant pigweed populations from southern Ontario and Washington state have established: (1) that the several resistant populations from the West Montrose area, Waterloo Co., Ontario and one from Washington state, previously reported as Amaranthus retroflexus, are, in fact, referable to A. powellii and (2) that the one resistant population near Ayr, Waterloo Co., Ontario, which had not been previously reported, is correctly identified as A. retroflexus. Features distinguishing the three pigweed taxa that are common in southern Ontario (A. powellii, A. retroflexus and A. hybridus) are reviewed.