Role of residual herbicides for the management of multiple herbicide resistance to ACCase and ALS inhibitors in a black-grass population

The post-emergence control of black-grass in winter cereal crops is becoming increasingly complicated due to the evolution of resistance to acetyl-coA carboxylase (ACCase) and acetolactate synthase (ALS) inhibiting herbicides. Ensuring good pre-emergence control is therefore important to protect yield and relieve the pressure on the few existing post-emergence chemical options. In this study we investigated the mechanisms of resistance in a black-grass population (Alopecurus myosuroides Huds.) from France (FR50) and assessed the usefulness of residual herbicides such as prosulfocarb, pendimethalin and flufenacet for controlling this population. The entire population contained an insensitive ALS target due to a homozygote substitution at amino acid position 574 (Arabidopsis thaliana (L.) Heynh. equivalent) conferring very high levels of resistance to selective sulfonylureas. Additionally, sixty percent of the population carried at least one ACCase substitution (codon positions 1781 or 2096). Overall higher levels of resistance were observed with clodinafop-propargyl (RF50=6.48 (4.67–9.01)), than pinoxaden (RF50=4.42 (3.12–6.26)) mainly explained by the differential response to the G2096A substitution. To our knowledge, this is the first report of a black-grass population carrying resistance mutations on the ACCase and ALS genes. Nearly all the pre-emergence treatments tested provided levels of control at above 80%. The most efficient and fast acting treatment was the tank mix prosulfocarb 2400g a.iha−1+diflufenican 80g a.iha−1+flufenacet 160g a.iha−1applied at BBCH10 that resulted in complete necrosis and absence of meristematic re-growth. Herbicide sequences, i.e. a pre-application followed by a post-application 21 days after, provided extremely good results. For instance, prosulfocarb 4000g a.iha−1+diflufenican 50g a.iha−1 followed by pinoxaden 60g a.iha−1+pendimethalin 800g a.iha−1 resulted in full control of FR50. Despite the strong resistance profile of FR50, these results showed that most herbicide combinations ensured a useful level of control at an early stage. In order to delay the onset and limit the spread of multiple herbicide resistant populations such as FR50, mixtures and sequences involving residual herbicides have an important role to play. ► First report of multiple target-site resistance to ACCase and ALS inhibitors in black-grass. ► Presence of W574L, I1781L and G2096A resistance mutations. ► Increasing levels of resistance from clethodi