A resistance update on broadleaf weeds in South Australia and Victoria

Take home messages

• Herbicide resistance is most prevalent in wild radish, Indian hedge mustard (IHM), sowthistle (milk thistle), prickly lettuce (whip thistle) and fleabane.
• The most common resistance is to Group 2 herbicides.
• The highest incidence of resistance is in sowthistle and prickly lettuce.
• Integrate herbicides with alternative modes of action such as Group 13, 14 and 27 in cropping programs.

Broadleaf weed species in broadacre cropping across southern Australia

Herbicide resistance has been detected in several broadleaf weed species across southern Australian cropping paddocks (Table 1). Wild radish, indian hedge mustard, sowthistle (milk thistle), prickly lettuce (whip thistle), wild turnip and fleabane are the most prevalent broadleaf weeds with resistance. Herbicide resistance also has been confirmed in another six broadleaf weed species (Table 1).

Table 1: Occurrence of broadleaf weed species confirmed herbicide resistant from southern Australia. Source of data: CropLife Australia.

Content last updated: July 16, 2020

Resistance has been quantified via random weed surveys (funded by GRDC) involving the collection of weed seeds from plants present in randomly chosen paddocks at harvest (Figure 1, Table 2).

Figure 1. Survey area across south-eastern Australia for 2013-2017. Each point represents a field where one or more of the three broadleaf weed species were collected. The different symbols and colours represent different weed species present: wild radish (yellow circles); IHM (red triangles); sowthistle (blue squares); turnip weed and IHM (blue circles); wild radish and sowthistle (orange circles); IHM and sowthistle (pink circles); and all three species (green circles). Figure courtesy of John Broster.

Wild radish

In wild radish, Group 2 herbicides sulfonylurea resistance was more prevalent than resistance to imidazolinone (IMI) herbicides. A significant percentage of samples from SA exhibited resistance to 2,4-D (39%), with fewer from Victoria (7%). Almost all of the samples in SA were collected from the south-east and in Victoria from the south-west. No resistance to atrazine or diflufenican was detected.

Indian hedge mustard (IHM)

Similar to wild radish, greater resistance was detected to sulfonylureas than IMI herbicides. One third of the samples from Victoria also exhibited resistance to 2,4-D compared to only 3% from SA. In contrast to wild radish, resistance to atrazine and diflufenican was also detected in both states.

Sowthistle

Over three quarters of the samples from both states exhibited resistance to sulfonylurea herbicides. Although IMI herbicides were not tested, other trials have indicated that there is a strong correlation between sulfonylurea and IMI herbicide resistance in sowthistle. A recent survey identified 78% of populations resistant to sulfonylureas and 68% resistant to IMI herbicides (Merriam et al. 2018). A low percentage of samples from SA (6%) and Victoria (3%) exhibited resistance to 2,4-D. No resistance to glyphosate was detected.

Table 2: Extent of resistance to various herbicides in three broadleaf weed species from surveys conducted 2013-2017. The number of samples for each species is in brackets ( ) next to the species name. Samples were considered to be resistant if >20% of individuals within that population survived application of the herbicide in a pot trial.

A dash indicates herbicide was not used on this species.

Prickly lettuce

Prickly lettuce is not collected in the pre-harvest surveys as it has not set seed. Directed surveys were conducted in SA in 1999 and 2004 that showed sulfonylurea resistance in prickly lettuce was already high with 66% of populations from 1999 and 82% of populations from 2004 resistant to sulfonylureas (Lu et al. 2007). In 2018, 30 populations were collected from four regions in South Australia. Every population exhibited resistance to both sulfonylureas and IMI herbicides, showing a continued increase in Group 2 herbicide resistance.

Fleabane

Seeds of fleabane from a survey of 89 agricultural locations in 2014 across north-east Victoria were collected (Aves et al. 2020). Of these, 40% exhibited resistance to glyphosate, 100% resistant to sulfonylureas, but there was no resistance to Group 4 herbicides or paraquat. Testing of suspect samples sent by agronomists to commercial testing services have identified resistance to paraquat (Table 1).

Wild turnip

This species was only included in earlier surveys. Wild turnip was collected from 31 paddocks in the 2012 SA Mallee survey, with 23% and 16% of samples exhibiting resistance to sulfonylureas and IMI herbicides, respectively. No resistance to atrazine, Brodal® or 2,4-D was detected.

Resistance in broadleaf weeds:

Resistance to the Group 2 sulfonylurea (for example, metsulfuron), imidazolinone (for example, Intervix) and triazolopyrimidine (for example, Eclipse®, Crusader®) is common. Resistance in broadleaf weeds is almost exclusively due to target-site resistance. The target site of Group 2 herbicides is the enzyme, acetolactate synthase (ALS). A number of point mutations at eight different sites on the ALS gene have been recorded in resistant individuals of various species (Tranel et al. 2022). The most common site for mutations is Proline-197, with eleven different amino acid substitutions reported at this site across 40 different weed species. Different amino acid substitutions at each of the sites can provide different cross resistance patterns across the chemical families within Group 2 herbicides and the resistance phenotype for a substitution can vary between species (Tranel et al. 2022). Resistance can vary between weak resistance with substantial biomass reduction to no reduction at all, even at rates magnitudes higher than recommended label rates. Cross pollinating species such as wild radish can easily accumulate multiple mutations in the same plant increasing the level of resistance further.

Group 4: In SA and Victoria, resistance to Group 4 herbicides has been detected in wild radish, IHM and to a lesser extent in sowthistle (Tables 1 and 2).

Group 5: and 12: Resistance to atrazine and diflufenican has only been detected in IHM from the random weed surveys and not in wild radish. However, resistance to both atrazine and diflufenican in wild radish from SA and Victoria has been confirmed by commercial testing services.

Group 9: Out of the broadleaf weed species presented here, resistance to glyphosate has only been detected in fleabane in SA and Victoria, more common from horticulture. Glyphosate resistance in other broadleaf weeds has not been detected in SA or Vic, whereas in NSW and Qld, glyphosate resistance in sowthistle has been confirmed for over a decade.

Minor weeds: In a few cases, resistance has been detected in several other weed species, such as bedstraw (Table 2), other than the main weed species discussed here. Factors that are responsible for the low prevalence of resistance in these species includes a narrower distribution resulting in fewer individuals exposed and an inherently lower frequency of resistance in the species.

Combatting herbicide resistance in broadleaf weeds.

Where there are multiple herbicides within a group (such as Group 2 sulfonylureas and IMI’s), testing for resistance can help identify those herbicides that are still effective. It is also useful to test for resistance to other mode of action herbicides such as Group 4 (2,4-D), Group 5 (atrazine) and Group 12 (diflufenican), to identify herbicides that still work.

For some herbicide groups where weak resistance is common, such as Group 4, resistant populations may be controlled by products containing more than one mode of action herbicide, such as Group 6 (bromoxynil) or Group 12 (diflufenican, picolinafen). Identifying such mixtures that are still able to control the resistant population is helpful for management.

In the past few years, herbicides with alternative modes of action that control broadleaf weeds have become available such as Group 14 (G) and Group 27 (H). The option to use herbicides pre-emergent at sowing such as Voraxor® (Group 14) and Callisto® (Group 27) can increase the flexibility of broadleaf weed control in cereals. A continuing challenge has been in controlling resistant broadleaf weeds in broadleaf crops where IMI herbicides have been widely used. Herbicide introductions such as Overwatch® (Group 13, selective in canola) and Reflex® (Group 14, selective in pulses) allow a greater range of modes of action for control of some broadleaf weeds. Using these alternative herbicides will also help to extend the life of the more commonly used products.

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the author would like to thank them for their continued support.

References

Aves C, Broster J, Weston L, Gill GS, Preston C (2020) Conyza bonariensis (flax-leaf fleabane) resistant to both glyphosate and ALS inhibiting herbicides in north-eastern Victoria. Crop and Pasture Science 71, 864-871.

Lu Y-Q, Baker J, Preston, C (2007) The spread of resistance to acetolactate synthase inhibiting herbicides in a wind borne, self-pollinated weed species, Lactuca serriola L. Theoretical and Applied Genetics 115, 443-450.

Merriam AB, Boutsalis P, Malone J, Gill G, Preston C (2018) Extent of herbicide resistant common sowthistle (Sonchus oleraceus) in southern Australia. 21st Australasian Weeds Conference. Sydney, Australia, September 2018, pp. 16-19.

Tranel PJ, Wright TR (2002) Resistance of weeds to ALS-inhibiting herbicides: what have we learned? Weed Science 50, 700-712.

Contact details

Peter Boutsalis
University of Adelaide
Waite Campus, Glen Osmond SA 5064
peter.boutsalis@adelaide.edu.au