Abstract
Understanding the resistance spectrum and underlying genetic mechanisms is critical for managing herbicide-resistant populations. In this study, resistance to acetyl CoA carboxylase (ACCase) and acetolactate synthase (ALS) inhibitors was investigated in four suspected resistant populations of Alopecurus myosuroides (ALOMY-001 to ALOMY-004) and Lolium multiflorum (LOLMU-001 to LOLMU-004), collected from cereal production fields in Ireland. Glasshouse assays with three ALOMY-active herbicides [propaquizafop, cycloxydim (ACCase) and mesosulfuron + iodosulfuron (ALS)] or five LOLMU-active herbicides [pinoxaden, propaquizafop, cycloxydim (ACCase) and mesosulfuron + iodosulfuron, pyroxsulam (ALS)], and target-site resistance mechanism studies, based on pyrosequencing, were carried out in each of those populations. For A. myosuroides, Ile-1781-Leu ACCase mutation contributed to propaquizafop and cycloxydim resistance (shoot dry weight GR50 resistance factor (RF) = 7.5–35.5) in all ALOMY populations, and the independent Pro-197-Thr or Pro-197-Ser ALS mutation contributed to mesosulfuron + iodosulfuron resistance (RF = 3.6–6.6), in ALOMY-002 to ALOMY-004. Most of the analyzed plants for these mutations were homo/heterozygous combinations or only heterozygous. For L. multiflorum, phenotypic resistance to mesosulfuron + iodosulfuron (RF = 11.9–14.6) and pyroxsulam (RF = 2.3–3.1) was noted in all LOLMU populations, but the Pro-197-Gln or Pro-197-Leu ALS mutation (mostly in homozygous status) was identified in LOLMU-001, LOLMU-002 and LOLMU-004 only. Additionally, despite no known ACCase mutations in any LOLMU populations, LOLMU-002 survived pinoxaden and propaquizafop application (RF = 3.4 or 1.3), and LOLMU-003 survived pinoxaden (RF = 2.3), suggesting the possibility of non-target-site resistance mechanisms for ACCase and/or ALS resistance in these populations. Different resistance levels, as evidenced by a reduction in growth as dose increased above field rates in ALOMY and LOLMU, were due to variations in mutation rate and the level of heterozygosity, resulting in an overall resistance rating of low to moderate. This is the first study confirming cross- and multiple resistance to ACCase- and ALS-inhibiting herbicides, highlighting that resistance monitoring in A. myosuroides and L. multiflorum in Ireland is critical, and the adoption of integrated weed management strategies (chemical and non-chemical/cultural strategies) is essential.
Highlights
Black-grass (Alopecurus myosuroides) and Italian ryegrass (Lolium multiflorum) are the most problematic grass weeds threatening the sustainability of crop rotations dominated by autumn-sown cereals across north-western Europe [1]
We found four A. myosuroides (ALOMY-001 to ALOMY-004) and four L. multiflorum (LOLMU-001 to LOLMU-004) populations, with a high herbicide survival rate in the fields, and growers had reported control failures or difficult-to-control with acetyl CoA carboxylase (ACCase)- and/or acetolactate synthase (ALS)-inhibiting herbicides at recommended field rate, that were used in this study
Using glasshouse experiments and molecular diagnostics to detect target-site resistance (TSR) mechanism, we confirmed the first cases of cross- and multiple resistance to ACCase- and ALSinhibiting herbicides in four suspected resistant populations of A. myosuroides (ALOMY-001 to ALOMY-004) and L. multiflorum (LOLMU-001 to LOLMU-004) from Ireland
Summary
Black-grass (Alopecurus myosuroides) and Italian ryegrass (Lolium multiflorum) are the most problematic grass weeds threatening the sustainability of crop rotations dominated by autumn-sown cereals across north-western Europe [1]. They are difficult to control in early-sown winter cereals established after non-inversion tillage, due to the high seed retention of freshly shed populations, and as peak seed germination between. As A. myosuroides and L. multiflorum are genetically variable obligate outcrossers, with high fecundity and rapid seedbank turnover [6,7], they are at high-risk of evolving resistance from continuous herbicide use [8,9].
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