Common sowthistle (Sonchus oleraceus) and prickly lettuce (Lactuca serriola) in lentil crops of southern Australia: managing herbicide resistance and highly mobile resistance genes

Take home messages:

• Group B resistance in sowthistle and prickly lettuce in the southern region is common.
• Resistance to the imidazolinone herbicides (IMI) in these weeds is increasing and is a major issue for management in IMI-tolerant lentils.
• Resistance status of target weeds can affect control in the current and following season.
• Management of these highly mobile and prolific seed producers requires effective control and seed-set reduction both within the paddock and in surrounding areas.

Background

Common sowthistle (Sonchus oleraceus) and prickly lettuce (Lactuca serriola) have become common weeds of annual cropping in southern Australia following the adoption of reduced tillage, and are problematic in lentil crops due to poor crop competition and a lack of herbicide options (Preston et al., 2017; Wu et al., 2019). There are very few post-emergent herbicides available for the control of broadleaf weeds in lentil crops, so best practice encourages control of weeds prior to sowing or crop emergence (GRDC GrowNotes™, 2018). Further reduction of broadleaf weeds can be achieved by taking advantage of a cereal phase of rotation prior to a pulse crop, through more diverse herbicide options and increased crop competition. Seed production of both sowthistle and prickly lettuce is highly sensitive to competition, with production estimated in the tens of thousands of seeds per plant in the absence of competition (Amor, 1986; Hutchinson et al., 1984). The relatively short seed bank persistence of both species (Hutchinson et al., 1984; Weaver and Downs, 2003), particularly in reduced tillage systems where seed remains on the soil surface (Alcocer-Ruthling et al., 1992; Chauhan et al., 2006), increases the potential effectiveness of this rotation tactic.

The introduction of IMI-tolerant lentil varieties improved management options for growers by removing barriers to planting lentils where group-B herbicide residues would have otherwise caused crop damage. Recently, permits have been issued allowing limited in-crop use of IMIs in lentils for the control of annual broadleaf weeds. However, the popularisation of IMI-tolerant crop technology has increased selection pressure on weeds. Group B herbicides have a high propensity for resistance evolution and cross-resistance (Tranel and Wright, 2002) so reliance on them is risky and diverse management tactics need to be used in lentils.

Both sowthistle and prickly lettuce have evolved widespread resistance to group B sulfonylurea (SU) herbicides (Lu et al., 2007; Merriam et al., 2018). Surveys of prickly lettuce in the Mid North and Yorke Peninsula of South Australia conducted in 1999, 2004 (Lu et al., 2007) and 2019 (Merriam, unpublished data) have reported the percentage of SU-resistant populations at 66% (n=58), 82% (n=11) and 100% (n=27), respectively. Additionally, all prickly lettuce populations from the 2019 survey screened with IMIs (n=23) were resistant (Merriam, unpublished data). Resistance levels of sowthistle from across the southern region are estimated at 78% (n=355) to the SUs and 68% (n=84) to the IMIs (Merriam et al., 2018). Furthermore, 2,4-D resistance in sowthistle has been detected in the southern region (Preston et al., 2017) and glyphosate-resistant sowthistle is beginning to cause concern in the northern cropping region of Australia (MacIntosh, 2018).

Growing a cereal prior to lentils can help reduce broadleaf weed burden in a paddock however, the effectiveness of control of these species from one year to another is hampered by their ability to readily colonise from outside of the paddock. The seed of both species is highly adapted to wind dispersal (Cummins et al., 2018) and has the potential to travel long distances (Hutchinson et al., 1984; Lu et al., 2007). The prevalence of these weeds on roadsides and other uncropped areas further exacerbates the problem. The aim of this research was to determine if different herbicide management strategies and levels of crop competition within a cereal phase had a measurable carryover effect on density of these weeds in the following growing season.

Method

Two field trials were established in South Australia in 2018 at Kulpara (KYP) on the Yorke Peninsula and Roseworthy (RS2) in the Mid North. Both sites were in lentils the year preceding the trial and were sown to wheat in 2018. Treatments were applied to the 2018 crop in a split-plot design with four replicates and incorporated two levels of crop competition (achieved with seeding rates of 60kg/ha and 90kg/ha) and three post-emergent herbicide treatments (Table 1) in crossed factorial arrangement. Prior to seeding in 2018, glyphosate at 648g ai/ha and pyroxasulfone at 100g ai/ha (Sakura®, Bayer Pty Ltd, Australia) were applied across the whole trial area to control existing weeds. Plots were sown using the grower’s knife-point press-wheel seeder on 25cm row spacings on 12 May at KYP and 18 May at RS2 with a plot area of 160m2.

Table 1. Active ingredient rates, trade names and manufacturers of post-emergent herbicide treatments.

Herbicides were applied on 12 July at KYP and 1 August at RS2 using a quad bike boom sprayer equipped with TeeJet^® 110015 flat fan nozzles spaced 50cm apart and operating at 10km/h and 200kPa for an output of 58L/ha. Detailed measurements and global positioning system (GPS) coordinates were recorded at each site prior to harvest to facilitate re-establishment of the site in 2019 over the same area. This was verified using satellite imagery where available. In 2019 uniform management (common grower practice) was applied across the trial area at each site. Densities of sowthistle and prickly lettuce were assessed at key points during the 2018 season to gauge the effectiveness of treatments, and early in the 2019 growing season (prior to post-emergent herbicide application) to measure carryover effect.

Seed samples of sowthistle and prickly lettuce were collected at both trial sites for herbicide resistance screening. Methods and rates are outlined in detail in Merriam et al. (2018). Rates were based on doses determined to discriminate between resistant and susceptible biotypes and are similar to the field rate. In resistance monitoring surveys in the southern region, populations are considered resistant if percent survival is greater than 20% (Boutsalis et al., 2012).

Results and discussion

Populations of both species at both sites were identified as resistant to Group B herbicides, which is in line with regional data collected during resistance surveys (Table 2). They were also screened with glyphosate and 2,4-D, but no survivors were detected (data not shown). While resistance to both the SUs and the IMIs is very common, the incidence of SU resistance in sowthistle tends to be slightly higher within a region. In the Mid North, 90% (n=70) of sowthistle samples screened have been classified as resistant to SUs, versus 84% (n=37) classified as resistant to IMIs. On the Yorke Peninsula, 88% (n=56) of populations were classified as resistant to SUs, whereas only 67% (n=33) were classified as resistant to IMIs. A recent survey of prickly lettuce in each region found 100% of populations resistant to both herbicides, although sample sizes were small.

Table 2. Comparison of sowthistle and prickly lettuce populations from trial sites to regional averages of percent survival of Group B herbicides.

The seeding rate treatments resulted in significantly different crop establishment in 2018 at both sites, however there was no significant effect of crop establishment on weed density in 2018 or 2019 (data not shown). Initial weed densities were assessed prior to herbicide application at both sites in 2018 and showed that levels of both weeds were significantly higher at KYP compared to RS2, and that common sowthistle was more prevalent than prickly lettuce at both sites (Table 3). Results were significantly different between the two sites, so data were analysed separately for each site.

Table 3. Initial densities of common sowthistle and prickly lettuce at KYP and RS2 in 2018 post crop emergence but prior to herbicide treatment.

Weed densities were assessed six weeks after herbicide treatment application in 2018. At RS2, there was no significant effect of herbicide treatment on sowthistle density (Table 4). This could be due to variability across the plots and the low initial density of sowthistle at the site (Table 3), meaning less opportunity for the herbicide treatment to make a difference. Prickly lettuce was not detected in the RS2 trial following herbicide application (Table 4), likely due to the very low initial density of prickly lettuce at this site (Table 3). Neither weed species showed a carryover effect on density at the beginning of the 2019 growing season, evidenced by the lack of significance between treatments. This would be expected given the lack of significant treatment effect in 2018.

At the KYP site, sowthistle density was significantly correlated with herbicide treatment in 2018, with the lowest density observed under the proactive treatment and the highest density observed in untreated plots (Table 4). However, at the beginning of the 2019 season, there was no significant difference between the untreated and conventional treatment plots, while proactive treatment plots maintained a significantly lower density. The reason for the loss of significance between untreated and conventional plots from 2018 to 2019 may be due to sowthistle resistance to the residual component of the conventional treatment. The conventional treatment relies on a SU, metsulfuron-methyl, for residual control, and the sowthistle population at the site had 100% survival to SU application during screening (Table 2). Prickly lettuce density in 2018 was significantly higher in the untreated plots, but there was no significant difference between the conventional and proactive treatments. By the beginning of the 2019 season, there was no statistically significant difference between any of the treatments.

Table 4. Treatment means of weed density post-treatment in 2018 and at the beginning of the 2019 season at both trial sites.

These results suggest that carryover effects of herbicide treatment may only be significant in the following season when initial weed densities in the first year are high. Significant differences were only observed in the second year for sowthistle at the KYP site, which had the highest initial density (Table 3), nearly three times higher than the second highest density (prickly lettuce at KYP) and more than 10 times the lowest density (prickly lettuce at RS2). The data also show the potential for a relatively low weed density post-treatment to result in a high density in the following season due to prolific seed production of survivors, colonisation from outside the study area, or contributions from the soil seedbank. Although both sowthistle and prickly lettuce have relatively short seedbank persistence, in the absence of suitable growing conditions they can persist beyond a season (Hutchinson et al., 1984; Weaver and Downs, 2003), so seedbank carryover from 2018 could have made some contribution the following year. Plants growing in uncropped areas may face less intense competition and thus have potential for prolific seed production (Amor, 1986; Hutchinson et al., 1984) and significant contributions to the population in an adjacent paddock. Prickly lettuce was not detected at RS2 following herbicide treatment in 2018 but the site averaged over 10 000 plants/ha at the beginning of the following season (Table 4), suggesting that seedbank recruitment and colonisation from outside the paddock can be an important weed source.

Prices of herbicide treatments used in this study differed at around $10/ha for the conventional and $50/ha for the proactive (Brooke and McMaster, 2019). Therefore, it is important to take the initial weed densities and the resistance status of the population into account when deciding what herbicide option to utilise. High densities may justify the extra expense, especially if Group B resistance is present. Alternatively, less expensive substitutes for the proactive treatment in this study could be explored. Triathlon® (Adama Pty Ltd, Australia) also contains groups C, F, and I and would cost approximately $17/ha at the full field rate.

Conclusion

The results of this study highlight the importance of effectively controlling wind-dispersed weeds both in crop and in adjacent uncropped areas. If initial weed densities are high, as with sowthistle at KYP, the choice of herbicide in a cereal phase can have a significant carryover effect in the following season. However, even if weeds are completely controlled in-crop, they can still be present at significant densities the following year due to colonisation from adjacent areas. Since both sowthistle and prickly lettuce are prolific seed producers in the absence of competition, reducing seed set in plants growing under reduced competition should be a priority. This research also highlights the importance of understanding the resistance status of target populations to all components of the herbicide regime, as resistance to a residual component could be masked by efficacy of a non-residual component in year of application but cause problems in the following year.

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. The project is also supported by an Australian Government Research Training Program Scholarship. The author also thanks project supervisors (Dr Christopher Preston, Dr Gurjeet Gill and Dr Jenna Malone) and University of Adelaide Weed Science Research Group colleagues Ben Fleet, Jerome Martin and David Brunton for technical assistance.

Useful resources

Merriam et al (2018):

Extent of herbicide resistant common sowthistle in southern Australia - Sciences Adelaide

References

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Contact details

Alicia Merriam, PhD candidate
Waite Campus, University of Adelaide, PMB1 Glen Osmond SA, 5064
alicia.merriam@adelaide.edu.au
0475 593 149
@alicia_merriam