First case of three-way multiple herbicide resistance in Amaranthus palmeri outside the Americas: Resistance mechanisms and alternative management strategies

The rapid spread of herbicide-resistant Amaranthus palmeri S. Watson across the Mediterranean region poses a severe threat to crop production. This study characterizes the resistance profile of an A. palmeri population from Gonen, Israel, which survived multiple post-emergence herbicide applications in maize. Greenhouse dose-response and screening trials were conducted across four modes of action (MOAs). The Gonen population exhibited three-way multiple resistance to acetolactate synthase (ALS), photosystem II (PSII) and 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitors, marking the first confirmed case outside the Americas. High-level resistance was demonstrated for the ALS inhibitor foramsulfuron and the PSII inhibitor metribuzin, maintaining 40-70% and 80-100% survival, respectively, at double the labelled field rate. Sequencing of the ALS gene revealed a Trp574 to Leu substitution in resistant plants. The population showed differential PSII sensitivity, as atrazine provided complete control despite high metribuzin survival. Sequencing revealed no known psbA mutations, and malathion pre-treatment caused only a slight reduction in metribuzin survival. Partial resistance was observed for the HPPD inhibitor tembotrione (30-50% survival), where malathion pre-treatment reduced survival by 20-30%. In contrast, pre-mergence applications of S-metolachlor (very long chain fatty acid inhibitor), isoxaflutole (HPPD inhibitor), and a fomesafen + terbutryn mixture [protoporphyrinogen oxidase (PPO) + PSII inhibitors] significantly reduced emergence. These findings confirm multiple-resistant A. palmeri in the region and highlight the urgent need for integrated weed management incorporating effective pre-emergence herbicides rather than relying solely on post-emergence chemical control in summer cereal rotations.