Of 78 biotypes of wild radish (Raphanus raphanistrum) collected from Western Australia (WA), 42% were resistant and 14% intermediate to acetolactate synthase (ALS)-inhibiting herbicides. Based on the LD50 and GR50 ratios, the resistant biotype K96071 was 81-fold more resistant to chlorsulfuron and 114- to 116-fold more resistant to metosulam than the susceptible biotype K96041. More resistant biotypes were found in northern zones than in southern zones of WA. Resistant biotypes evolved after five applications of chlorsulfuron in a predominantly cereal–lupin rotation. Where resistant biotypes were found, ALS-inhibiting herbicides were not rotated with herbicides with different modes of action as frequently as in fields with susceptible biotypes. Cross-resistance to chlorsulfuron and metosulam was found in the resistant biotypes even though only 15% of the 78 biotypes were exposed to two applications of metosulam over a 10-yr period. All 78 biotypes were effectively controlled by simazine and 2,4-D amine.

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.

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Tolerance to metribuzin herbicide is an essential agronomic trait for narrow-leafed lupin (L. angustifolius L.) grown in Western Australia (WA), however, metribuzin causes up to 30% yield loss in cv. Tanjil. Tanjil is widely used as a parent in the WA lupin breeding programme to provide anthracnose resistance. Hence, identification of genotypes tolerant to metribuzin and incorporation of this tolerance into the disease-resistant cultivar is necessary for maintaining lupin production. This study identified tolerance to metribuzin among lupin cultivars and advanced breeding lines under both controlled temperature and natural winter conditions. Differences in dose responses between cultivars revealed that cv. Gungurru was tolerant and cv. Tanjil susceptible to metribuzin. Gungurru seedlings survived metribuzin applications of up to 1600 g/ha, whereas Tanjil seedlings exhibited zero survival at 800 g/ha. The rate of herbicide application that caused a 50% growth reduction (GR50, excluding dead plants) for Gungurru was 2 times greater than that for Tanjil. The level of tolerance in Gungurru is adequate to protect plants against metribuzin damage in the field. Large and consistent differences in tolerance between genotypes were identified among cultivars and advanced breeding lines across controlled temperatures (20°C during the day and 12°C at night) and in natural winter conditions. One breeding line (95L208–13–13) showed marginally better tolerance than Gungurru. A number of advanced breeding lines were as susceptible to metribuzin as Tanjil, indicating that it is very important to select for metribuzin tolerance concurrently with disease resistance in the breeding programme. Of the 6 measures of tolerance used in this study, leaf score proved to be the simplest and most effective measure and could be used for the selection of individual tolerant plants in segregating populations. Tolerance was independent of early vigour, suggesting that it is possible to combine both early vigour and tolerance into a cultivar for better weed management. In conclusion, breeding for metribuzin tolerance in lupin is feasible, and the screening method tested here was simple and consistent, which would assist a breeding programme in making rapid progress towards herbicide-tolerant plants.

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