Cross-resistance profile of metamitron-resistant Chenopodium album L. biotypes from sugar beet

in recent years, in several of the Belgian sugar beet growing regions, fanners have been confronted with unsatisfactory control of fat hen (Chenopodium album L.) by metamitron, a key herbicide in the modem low rate weed control programs in sugar beet. Greenhouse bioassays conducted on reference C. album populations and on "suspected" populations from sugar beet fields where poor fat hen control had been observed, revealed that all "suspected" populations were resistant to metamitron. These metamitron-resistant biotypes were all cross-resistant to atrazine. Since cross-resistance, particularly negative cross-resistance or reversed resistance, is known to play a major role in resistance management, other herbicides used in sugar beet and/or in rotational crops were tested to determine the cross-resistance profile of metamitron-resistant biotypes. Greenhouse bioassays were conducted using herbicides from different chemical families representing different modes of action. Cross-resistance was found for metribuzin, lenacil and chloridazon, all HRAC Group C, herbicides that inhibit photosynthesis at PS II. The metamitron-resistant C. album populations examined showed negative cross-resistance to S-metolachlor (HRAC Group K-3: inhibition of cell division), prosulfocarb (Group N: lipid synthesis, not ACCase, inhibition), aclonifen and clomazone (both Group F-3: inhibition of carotenoid biosynthesis).