A biotype of Sonchus oleraceus L. and two biotypes of Sisymbrium orientale Torn., SSO 3 and NSO 1, are the first dicot weeds in Australia to develop resistance to ALS-inhibiting herbicides. The resistant biotypes had been exposed to varying periods of selection with sulfonylurea herbicides. All three biotypes are resistant to a range of sulfonylurea and imidazolinone herbicides. The S. orientale biotypes are also resistant to the triazolopyrimidine herbicide, flumetsulam. LD(50) ratios of resistant Sonchus oleraceus for sulfonylurea and imidazolinone herbicides are greater than 64-fold and 4.5-fold, respectively. GR(50) ratios are greater than 9 for sulfonylureas and 7.4 for imazapyr. The LD(50) ratios for both S. orientale biotypes for chlorsulfuron, sulfometuron-methyl, metsulfuron-methyl, flumetsulam and imazethapyr are greater than 110-, 15-, 7-, 24- and 29-fold, respectively. All resistant biotypes are susceptible to MCPA, diuron and diflufenican, herbicides which do not inhibit ALS.
