Discrimination of ACCase-inhibiting herbicides-resistant Digitaria ciliaris populations with three diagnostic bioassays

Abstract

Background: Diagnostic bioassays are used to screen the suspected R population. They are conducted at a single herbicide dose and evaluated at a specific time after treatment that can differentiate resistant from susceptible population. Objective: Three different bioassays were evaluated to assess the detection of acetyl CoA carboxylase-inhibiting herbicides resistance in D. ciliaris. Method: Increasing herbicide rates were used to evaluate the three bioassays for differentiating R from S populations. Results: R1 and R2 differed from S in all employed bioassays. In the Agar-based gel box box assay, the S biotype had greater plant damage at the lower herbicide concentration relative to the R biotypes 3 DAT but differences between R and S decreased over time. In the leaf flotation assay, R biotypes floated at the lower concentration on the surface, whereas the leaves of S biotypes failed to float. For the electrical conductivity assay, the S biotype contained high electrical conductivity due to the high leaching of electrolyte into the water across all four herbicides tested than the R biotypes. Conclusion: While these assays were able to separate R and S biotypes, the level of resistance difference for any assay was no greater than 40% depending on rating data and exposure dose. While a statistical separation could be achieved using a rate response regression analysis for these bioassays, our data highlights the challenges associated whether these methods could provide an obvious difference at any single rate or rating data to be used as a consistent, effective first-phase resistance screen.