IMIDAZOLINONE AND SULFONYLUREA RESISTANCE IN A BIOTYPE OF COMMON WATERHEMP (AMARANTHUS-RUDIS)

The incidence of weed resistance to acetolactate synthase (ALS) inhibi ting herbicides has increased in the United States. In 1993, a populat ion of ALS-resistant common waterhemp was discovered after two confirm ed applications of an imidazolinone herbicide. Following another imaze thapyr application in the glasshouse, the resistant biotype demonstrat ed 130-fold resistance to imazethapyr(3) at the whole plant level. The concentration of imazethapyr required to inhibit the ALS activity by 50% was 520 times greater for the resistant biotype than the susceptib le. Plants also demonstrated cross-resistance to the sulfonylureas, ch lorimuron and thifensulfuron, at the whole plant and enzyme levels. Th is particular discovery is of concern due to the low number of applica tions of the selection agent (imazaquin 1989, imazethapyr 1992, and im azethapyr in the greenhouse) and the high degree of cross-resistance e liminating several options for weed control.