Understanding the potential for resistance evolution to the new herbicide pyroxasulfone: field selection at high doses versus recurrent selection at low doses

Pyroxasulfone is a new pre-emergence herbicide that provides effective control of Lolium rigidum, including populations with evolved resistance to multiple herbicide modes of action. Prior to its commercialisation, the potential of resistance evolution was examined with two separate studies: (i) a field screening with a high pyroxasulfone dose causing mortality >99.999% of 100 million L. rigidum herbicide-susceptible individuals to establish the frequency of major gene resistance mechanisms and (ii) a 3-year low-dose recurrent selection experiment of a herbicide-susceptible (S) and a multiple herbicide-resistant (MR) L. rigidum population. The field screening indicated that no major-effect resistance genes were present in 100 million L. rigidum individuals. By contrast, pyroxasulfone resistance was obtained by recurrent low-dose pyroxasulfone selection of multiple herbicide-resistant L. rigidum. The multiple-resistant MR population showed a clear capacity to evolve pyroxasulfone resistance with >30% plant survival at 240 g ha)1 (2.4-fold the recommended rate) after three generations of recurrent pyroxasulfone selection. For the first time, information regarding the potential for resistance evolution is available prior to herbicide commercialisation. Persistent pyroxasulfone use at low dose has the potential to rapidly lead to herbicide resistance evolution in L. rigidum field populations. Effective stewardship programmes should be developed to encourage pyroxasulfone use at the full label rate to minimise the possibility of rapid low-doseinduced resistance evolution and to ensure pyroxasulfone sustainability.

Weed Research, 52: 489-499