Characterization of the first cases of corn poppy ( Papaver rhoeas ) with multiple resistance to ALS-inhibiting and auxin-mimic herbicides in Ireland

Abstract Corn poppy ( Papaver rhoeas L.) is a problematic weed in cereal farms in Ireland, necessitating continuous herbicide use to protect crop yields. In 2022, poor control with acetolactate synthase (ALS) inhibitors and auxin-mimic herbicides was reported by growers/advisors in two resistance-suspect populations, namely PAPRH-R1 and PAPRH-R2. In this study, we quantified their resistance to both herbicide modes of action and investigated the underlying mechanisms. ALS inhibitor dose–response experiments revealed that both populations had ED 50 and GR 50 resistance indices (RIs) above 120 to thifensulfuron + tribenuron, mesosulfuron + iodosulfuron + amidosulfuron, and metsulfuron, and values of 4 to 13 to florasulam + pyroxsulam. Results of 2,4-D dose–response assays revealed higher resistance in PAPRH-R2 (ED 50 and GR 50 RI > 9.1) than PAPRH-R1 (ED 50 and GR 50 RI > 3.4). ALS gene sequencing detected Pro-197 mutations in all sequenced plants from both resistant populations. Pretreatment with piperonyl butoxide (PBO, a cytochrome P450 inhibitor) reversed 2,4-D resistance with GR 50 , the more sensitive response parameter, decreasing from 680.8 to 84.9 g ha −1 in PAPRH-R1 and from 2,508.9 to 456.8 g ha −1 in PAPRH-R2, both well below the recommended label rate of 1,000 g ha −1 . The 14 C-2,4-D assays ruled out reduced absorption or translocation as resistance mechanisms. Sensitivity screening further revealed incomplete control with other postemergence foliar herbicides tested at half and full recommended label rates, except for fluroxypyr + halauxifen, which provided complete control of PAPRH-R1 at the full rate, and glyphosate, which provided complete control of PAPRH-R1 and PAPRH-R2 at both rates. This is the first report of multiple resistance in P. rhoeas in Ireland involving ALS target-site mutations and likely P450-mediated 2,4-D metabolism. As cultural/nonchemical control of broadleaf species is difficult, the continued availability of preemergence or autumn residual herbicides, together with effective auxin mimics and stewardship practices, will remain essential for managing these species, including P. rhoeas , in cereal crops.