Confirming Glyphosate Resistance and Alternative Strategies for Controlling Red Brome (Bromus rubens L.) in Perennial Crops Grown in Southern Spain

The excessive use of the herbicide glyphosate on annual and perennial crops grown in Southern Spain has caused an increase in resistant weed populations. B. rubens L. also known as red brome, is an important winter-annual grass weed. Bromus rubens have begun to spread through olive and almond cultivars due to low glyphosate control over these species, whereas previously it had been well controlled with field dose (1080 g ae ha-1). Therefore, the aim the aims of this study are to identify different species of the Bromus genus using molecular markers, confirming B. rubens glyphosate-resistant (G-R) in Spain using rapid shikimic acid accumulation and dose-response bioassays. This study also identifies alternative herbicide control with different modes of action. Characterization using Simple Sequence Repeat (SSR) markers confirmed the presence of B. rubens collected in Andalusia. A rapid shikimic acid accumulation screening showed 17 resistant (R) populations with values between 300 and 700
g shikimate g-1 fresh weight and three susceptible (S) populations with values between 1200 and 1700 µg shikimate g-1 fresh weight. In dose-response experiments, the GR50 values agreed with previous results and the resistance factors (RFs: GR50 R/GR50 S (Br1)) were between 4.35 (Br9) and 7.61 (Br19). Foliar retention assays showed no differences in glyphosate retention in both R and S populations. The tests carried out in a resistant field (Br10) demonstrated the control efficacy of pre-emergence herbicides since flazasulfuron in the tank mix with glyphosate had up to 80% control 15 to 120 days after application (DAA) and grass weed postemergence herbicides, such as propaquizafop + glyphosate and quizalofop + glyphosate, had up to 90% control 15 to 90 DAA. Results confirm the first scientific report of glyphosate-resistant B. rubens worldwide;

however, the use of herbicides with another mode of action (MOA) is the best tool for integrated weed management. Future research should focus on advancing the characterization of resistance mechanisms including non-target site (NTSR) or target site (TSR) in these populations.