First Report on Assessing the Severity of Herbicide Resistance to ACCase Inhibitors Pinoxaden, Propaquizafop and Cycloxydim in Six Avena fatua Populations in Ireland

 
Vijaya Bhaskar Alwarnaidu Vijayarajan  ,* , Patrick D. Forristal 1, Sarah K. Cook 2, Jimmy Staples 1, David Schilder 1, Michael Hennessy 1 and Susanne Barth 1
1 Crop Research Centre Oak Park, Teagasc, Carlow R93 XE12, Ireland; Dermot.Forristal@teagasc.ie (P.D.F.);
Jimmy.Staples@teagasc.ie (J.S.); David.Schilder@teagasc.ie (D.S.); Michael.Hennessy@teagasc.ie (M.H.); Susanne.Barth@teagasc.ie (S.B.)
2 Crop Protection Department, Weed Biology, ADAS Boxworth, Boxworth CB23 4NN, UK; sarah.cook@adas.co.uk
* Correspondence: Vijaya.Bhaskar@teagasc.ie; Tel.: +35-359-917-0248

Received: 25 August 2020; Accepted: 7 September 2020; Published: 10 September 2020

Abstract: In response to growers reports of poor weed control, resistance to ACCase inhibitors pinoxaden, propaquizafop and cycloxydim was investigated in populations of six wild oats, Avena fatua, collected from cereal-dominated crop rotations in Ireland. Glasshouse assays confirmed reduced sensitivity to all three ACCase inhibitors in four of the six populations, R2 to R5. R1 was cross-resistant to pinoxaden and propaquizafop and R6 was resistant to propaquizafop only. Dose-response studies confirmed significant differences in the severity of resistance amongst these populations (p < 0.05). For pinoxaden, the ED50 or GR50 resistance factor (RF) of R1, R3 and R5 were between 11.6 and 13.1 times or 25.1 and 30.2 times more resistant, respectively, compared with the susceptible populations. For propaquizafop, the ED50 and GR50 RF of R1, R2, R3, R5 and R6 were between >7.8 and >32 or 16.6 and 59 times more resistant, respectively. For cycloxydim, only R5 had both high ED50 and GR50 RF values of >43.2 and 98.4 respectively. In R2, although the ED50 values to both pinoxaden and cycloxydim and additionally, R3 to cycloxydim, were above recommended field rates, their GR50 values remained below, suggesting a shift towards cross-resistance. While R4 was the only population, where both ED50 and GR50 for all ACCase inhibitors remained below recommended field rates, they would not give effective control at these rates, strongly indicating evolving resistance. This is the first study reporting variable cross-resistance types and levels to ACCase inhibitors in A. fatua from Ireland.