Resistance of Raphanus raphanistrum to chlorsulfuron in the Republic of South Africa

Herbicide resistance poses a substantial threat to the agricultural industry throughout the world and during the past decade several reports regarding herbicide resistance have been published. Raphanus raphanistrum L., from two wheat farms located in the winter rainfall region of South Africa, showed indications of resistance to chlorsulfuron. Seeds from these suspected resistant biotypes as well as seeds from a susceptible biotype were collected and transported to the ARC-Small Grain Institute for herbicide resistance studies. Herbicides registered for R. raphanistrum control, i.e. chlorsulfuron, MCPA and bromoxynil, were used in this study. Significant differences in the degree of control were found between the susceptible and two resistant biotypes, when treated with chlorsulfuron. The LD₅₀ values for the resistant biotypes (WR 1 & WR 2) were 45 and 11.3 g a.i. ha^–1, respectively, whereas the LD₅₀ value for the susceptible biotype was 5.6 g a.i. ha^–1. The almost eightfold difference between the susceptible and resistant biotype (WR 1), indicated that resistance has developed to chlorsulfuron. Only twofold resistance was established between the other resistant biotype (WR 2) and the susceptible biotype. Significant differences between herbicide rates were also established with the MCPA and bromoxynil experiments. No significant difference could, however, be found between the susceptible and resistant biotypes when treated with MCPA and bromoxynil, indicating the importance of different modes of action of herbicide as a strategy to prevent herbicide resistance.