Resistance to ACCase-inhibiting herbicides and isoproturon in UK populations of Lolium multiflorum: mechanisms of resistance and implications for control

Herbicide-resistant Lolium multiflorum (Italian rye-grass) was first report ed in the UK in 1993 and had been confirmed on 25 farms by 1999. In this st udy, resistance to five herbicides belonging to the aryloxyphenoxypropionat e, cyclohexanedlione and phenyl-urea classes was determined in six populati ons of L multiflorum from the UK under glasshouse and simulated field condi tions. Glasshouse conditions tended to exaggerate the degree of resistance, but experiments performed in both environments detected resistance in four populations oft multiflorum. Four populations (Essex Al, Lines Al, Wilts B 1, Yorks A2) were resistant to diclofop-methyl, fluazifop-P-butyl, tralkoxy dim and partially resistant to isoproturon, but only the population from Yo rkshire (Yorks A2) showed resistance to cycloxydim. Biochemical analyses of acetyl coenzyme A carboxylase (ACCase) activity, oxygen consumption by thy lakoids, diclofop metabolism and glutathione S-transferase activity showed that, in three of the resistant populations, an enhanced rate of herbicide metabolism conferred resistance. This is the first report world-wide of an enhanced metabolism mechanism of diclofop resistance in L multiflorum. In t he Yorks A2 population, an insensitive ACCase was detected (target-site res istance) which also conferred cross-resistance to all of the other ACCase i nhibitors investigated. (C) 2001 Society of Chemical Industry.