MECHANISMS OF CROSS AND MULTIPLE HERBICIDE RESISTANCE IN ALOPECURUS-MYOSUROIDES AND LOLIUM-RIGIDUM

Alopecurus myosuroides and Lolium rigidum have developed resistance to herbicides with several modes of action in many herbicide classes. A. myosuroides biotype Peldon A1 from England exhibits non-target site c ross resistance to substituted urea and aryloxyphenoxypropionate herbi cides (APP) due to enhanced metabolism. L. rigidum biotype SLR 31 from Australia has multiple resistance mechanisms, including both non-targ et site cross resistance and target site cross resistance. The majorit y of the SLR 31 population has enhanced metabolism of chlorsulfuron an d diclofop-methyl and a mechanism correlated with altered plasma membr ane response, which correlates with resistance to some APP and cyclohe xanedione (CHD) herbicides. A small proportion of the population also has target site cross resistance to APP a nd CHD herbicides. While A. myosuroides and L. rigidum share common biological elements, they are not unique. Non-target site cross resistance and multiple herbicide re sistance is predicted to develop in other weed species. The repercussi ons of cross and multiple resistance warrant proactive measures to pre vent or delay onset.