Resistance of Chenopodium albumto photosystem II-inhibitors.

Chenopodium album L. (fat-hen), a highly competitive and very prolific species, is a common weed in most spring- and summer-sown crops such as maize, sugar beet and vegetables. In the late seventies, C. album stepped into the limelight as a problem weed in maize. Frequent use of atrazine in maize monoculture did select for plants having a Ser-264-Gly mutation on the psbA gene resulting in atrazine-resistance and cross-resistance to other Photosystem (PS) II-inhibitors. The psbA gene encodes the D1 protein of PS II which is the target site of PS II-inhibitors. Introduction of new herbicides made it possible to control this atrazine-resistant biotype in maize, which allowed C. album to fade into the background again until it resurfaced some years ago as a problem weed in European sugar beet (Belgium, France, The Netherlands and Sweden). Greenhouse bioassays at Ghent University revealed that the unsatisfactory control of C. album in sugar beet is due to resistance to the triazinone metamitron, a key herbicide in sugar beet. The expected cross-resistance to atrazine and metribuzin was found in all populations except for a Swedish one, which is highly resistant to metamitron and metribuzin but not to atrazine. DNA sequence analysis confirmed the presence of a Ser-264-Gly mutation for all populations that are both metamitron- and atrazine-resistant. The Swedish population has an Ala-251-Val mutation on the psbA gene explaining its aberrant (cross)-resistance profile. The occurrence of C. album biotypes with resistance to metamitron but different genotypes and cross-resistance profiles could raise the question which herbicide(s) did select for the resistance. In Sweden, having no history of atrazine use, the triazinones metamitron, used in sugar beet, and metribuzin, used in rotational potato, could have selected for resistance. In Belgium, at least three different herbicides and/or crop rotations could have contributed to resistance development: (1) a record of continuous use of atrazine in maize resulting in triazine-resistant C. album in the seed bank, (2) metamitron use in sugar beet and (3) metribuzin use in potato.