Distribution of metamitron-resistant Chenopodium album L. in Belgian sugar beet.

Chenopodium album L. (fat-hen) with a Ser264-Gly mutation is resistant to photosystem II-inhibiting herbicides like the triazinone metamitron, a key herbicide in sugar beet. In recent years, this resistant biotype may cause unsatisfactory weed control in Belgian sugar beet. However, the dimension of the problem was yet unknown. Therefore, a survey was conducted in 2008 covering the whole Belgian sugar beet area. In randomly selected fields, C. album plants surviving weed control were counted and sampled. First, the number of surviving plants was used to estimate the prevalence of fields with unsatisfactory control and to classify the surveyed fields. Then, the share of the resistant biotype in each field was determined with cleaved amplified polymorphic sequence-analysis (CAPS-analysis) on sampled leaves. Finally, all results were visualised on the map of Belgium. Twenty percent of the fields had more than 500 surviving plants per hectare and were thus classified as fields with unsatisfactory C. album control. The resistant biotype was present in 95% of these fields and even in 74% of the sampled fields with good weed control. No pattern was found during mapping. These results indicate that the metamitron-resistant biotype has spread over the whole sugar beet area but that it is not (yet) causing severe problems in every field. To get a more accurate estimation of the portion of resistant plants in the field and the effect of herbicide treatment on this biotype, an elaborate survey will be conducted in 2010 on fields that have both untreated and treated plots installed.