Triazine Resistance in a Biotype of Wild Radish (Raphanus raphanistrum) in Australia1

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Abstract
This study documents the first case of triazine resistance in wild radish and the resistance mechanism involved. The high survival (57 to 97%) of the resistant (R) biotype progeny plants treated at a rate four times higher than the commonly recommended rate of simazine or atrazine clearly established that the R biotype plants were resistant to triazines. All the plants of the susceptible (S) biotype plants were killed when treated at half the commonly recommended rate of atrazine (0.5 kg/ha) or simazine (0.25 kg/ha). The dry weight of the S biotype was reduced by 89 to 96% at the commonly recommended rate of atrazine or simazine, while the dry weight of the R biotype plants was reduced by only 36 to 54% even when treated at a rate four times higher than the commonly recommended rate of atrazine or simazine. The growth-reduction-ratio values indicated that the R biotype progeny plants were 105 and 159 times more resistant to atrazine and simazine, respectively, than the S biotype plants. Leaf chlorophyll fluorescence yield was reduced by 97% in the S biotype 24 h after application of triazine compared with only 9% reduction in the R biotype, indicating that the resistance mechanism involved is target-site based. The R biotype was effectively controlled by herbicides of different modes of action.