Resistance of Wild Radish (Raphanus raphanistrum) to Acetolactate Synthase-Inhibiting Herbicides in the Western Australia Wheat Belt1

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Abstract
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. Nomenclature: Chlorsulfuron; metosulam; simazine; 2,4-D; wild radish, Raphanus raphanistrum L. # RAPRA. Additional index words: Weed survey, cross-resistance, crop rotations, herbicide rotations. Abbreviations: ALS, acetolactate synthase; GR50, rate of herbicide required to inhibit growth by 50%; I, intermediate; R, resistant; S, susceptible; SU, sulfonylurea; WAP, weeks after planting.