Target site resistance to ALS inhibiting herbicides in Papaver rhoeas and Stellaria media biotypes from the UK

P>Acetolactate synthase (ALS) inhibiting herbicides play an important role in controlling broad-leaved weeds. Biotypes of Stellaria media and Papaver rhoeas showing resistance to ALS inhibiting sulfonylurea herbicides (e.g. metsulfuron) were first identified in 2000 and 2001, respectively, in the UK. Recent work has shown that resistance in both species is due to mutation of the ALS gene conferring an altered herbicide binding site. Three P. rhoeas test biotypes from three areas possessed ALS gene point mutations (Pro-197-Leu or Pro-197-His) conferring ALS target site resistance. All were highly resistant to the sulfonylurea herbicides metsulfuron-methyl and tribenuron-methyl. Several alternative herbicides were tested against resistant P. rhoeas, with MCPA and ioxynil + bromoxynil being most effective. Four resistant biotypes of S. media were identified from widely separated locations. Two different mutations conferring ALS target site resistance were confirmed in two of these biotypes. A mutation conferring Pro-197-Gln substitution in a biotype from Perthshire was associated with resistance to metsulfuron, but not to florasulam. A mutation conferring Trp-574-Leu substitution in the biotype from Aberdeenshire conferred resistance to both herbicides. This is the first characterisation of ALS gene point mutations in resistant S. media. Currently, there are good alternatives for control of resistant biotypes, including fluroxypyr and mecoprop-P. The potential loss of alternative modes of action, as a consequence of reforms to the regulatory system for pesticides in the EU, represents one of the main threats to continued control of ALS resistant S. media.