Resistance to ACCase and ALS-inhibiting herbicides detected in targeted sampling of Lolium rigidum (rigid ryegrass) populations from cereal crops in Morocco, Algeria, and Tunisia

Lolium rigidum Gaud. (rigid ryegrass) is one of the most widespread weeds in cereal crops in Morocco, Algeria and Tunisia. This weed has evolved resistance to various herbicide modes of action in this region. ACCase and ALS inhibiting herbicides are mainly used in the major-cereal growing regions to control rigid ryegrass. Through a questionnaire, regions where farmers reported less control of herbicide treatments were registered in the three Maghreb countries. Registered fields were visited for collection and 75 field populations were screened with two ACCase and two ALS herbicides. Target site resistance (TSR) was diagnosed using Illumina Next Generation Sequencing (NGS) technology. The sensitivity bioassay results revealed over 60% of sampled populations to be resistant to pinoxaden and/or clodinafop and about 40% to be resistant to iodosulfuron + mesosulfuron and/or pyroxsulam. In addition, 53% of populations displayed resistance (R) to the two herbicide modes of action tested among the regions. In total, 16 ACCase and 11 ALS mutant alleles were identified, carrying out an amino-acid substitution and conferring herbicide resistance in 3700 of the analyzed plants. Most ACCase and ALS mutations were detected at codons Ile1781 and Pro197, respectively. Not only does this study demonstrate the presence of both cross and multiple resistance, it also highlights the non-ACCase and non-ALS -based resistance mechanisms that could confer resistance to herbicides with different modes of action which complicates the resistance management strategies. In the three Maghreb countries, this challenge is even more prominent due to few modes of action being available for rigid ryegrass control due to low-cost market and the prevalence of generic herbicides.