Evaluation of herbicides to manage herbicide-resistant corn poppy (Papaver rhoeas) in winter cereals

Corn poppy is the most important broad-leaved weed in winter cereals in southern Europe. It is an increasing problem due to the appearance of herbicide-resistant biotypes. Experiments were conducted in north-eastern Spain with three corn poppy populations to determine the level of resistance to 2,4-D and tribenuron using seed-germination tests. Field experiments were conducted at sites from where the tested populations had been collected to test the effectiveness of various herbicides applied pre-emergence (PRE) or post-emergence (POST) in winter cereals. In the seed-germination tests, two populations were found to be 2,4-D-resistant and one resistant to both 2,4-D and tribenuron. In field trials, the efficacy of 2,4-D at 900 g a.i. ha−1 ranged from 25 to 76% of reduction of plant densities, depending on season and population, indicating that all populations were 2,4-D-resistant in field conditions. Moreover, at 15 g a.i. ha−1 tribenuron, one of the three populations (CU2) was only reduced by 65% in two seasons, indicating also the presence of resistance to tribenuron in this population in addition to 2,4-D. Florasulam at 6.25 g a.i. ha−1 + 2,4-D at 300 g a.i. ha−1 resulted in poor control (75–85%) in CU2 suggesting possible cross-resistance to this triazolopyrimidine. On average, all the other herbicides provided 90–100% control of corn poppy. Only PRE herbicides and the mixture 150 g a.i. ha−1 bromoxynil + 150 g a.i. ha−1 ioxynil + 450 g a.i. ha−1 mecoprop in POST performed consistently well (>90% control). The results of this study indicate that 2,4-D and tribenuron-resistant corn poppy populations in winter cereals can be controlled by application of PRE or POST herbicides with alternative modes of action.