Herbicide resistance in Conyza bonariensis (L.) Cronquist (flaxleaf fleabane) populations from northeast Victoria and its management in mixed farming systems

Abstract

Conyza bonariensis (L.) Cronquist is a global weed and considered one of the most problematic species in modern agriculture. As a species it has developed resistance to herbicides of nine different active ingredients globally including 5-enolpyruvylskikimate 3-phosphate synthase (EPSPS) inhibitors, photosystem I (PSI) electron diverters, photosystem II (PSII) inhibitors and acetolactate synthase (ALS) inhibitors. Examination of 88 C. bonariensis populations collected across northeast Victoria identified that 40% of populations (or plants in specific populations) were resistant to 1080 g a.e. ha⁻¹ glyphosate. Multiple resistance was found to glyphosate and the ALS inhibitors chlorsulfuron, metsulfuron-methyl and sulfometuron-methyl in five of the nine populations fully characterised. This is the first reported case of multiple resistance to EPSPS- and ALS-inhibiting herbicides in C. bonariensis. Nine populations collected as part of a resistance survey conducted across northeast Victoria showed varying levels of glyphosate resistance; glyphosate susceptible (GS) biotypes DL4, IR7 and IR11; low level glyphosate-resistant (Gr) biotypes DL3, DL13 and IR14 with Resistance Indices (RI’s) between 2.3 and 2.8; and high level glyphosate resistance (GR) biotypes DL19, IR5 and IR10 with RI’s over 6. Results of laboratory evaluation for herbicide translocation demonstrated that this was not involved in the resistance present in these populations. GR, Gr and GS populations showed differential accumulation of shikimate suggesting insensitive EPSPS may be involved in the resistance found in these C. bonariensis populations. Sequencing both genomic DNA and plasmid DNA identified Pro106-Thr and Pro106-Ser mutations, these mutations have previously been found to confer glyphosate resistance. As these mutations occurred in all three population groups, therefore (an) other mechanism(s) must be contributing to the resistance. Future investigation focused on expression of EPSPS and ABC transporter genes may provide greater insight into the mechanisms conferring resistance in these C. bonariensis populations. C. bonariensis is a successful ruderal invader common on irrigation channel banks in Victoria and New South Wales, Australia. Options approved for herbicide control on channel banks are limited and field experimentation conducted in New South Wales over two years demonstrated that there are no effective herbicide control options for managing the weed in these sites. The lack of effective herbicide options highlights the need for further research into both new herbicides and non-chemical control options. Little is known about the use of defoliation as a strategic management tool of C. bonariensis and information available shows variable results. Field experiments were therefore conducted in Dookie, Victoria and Goolgowi, New South Wales to investigate using defoliation in conjunction with herbicide applications. Greatest control over the two experiments was provided by the sequential applications of paraquat + diquat applied 5-10 days after defoliation; and MCPA + dicamba applied 8-9 days prior to defoliation. These experiments demonstrated effective control could be achieved by the use of defoliation with herbicide application in a double-knock system. In a region where widespread resistance to EPSPS and ALS inhibitors has been demonstrated, additional strategies for management of C. bonariensis are critical.