Characterization of clopyralid resistance in lawn burweed (Soliva sessilis)

Abstract

Soliva sessilis is a troublesome annual weed species in New Zealand turfgrass. This weed has been controlled selectively in New Zealand turfgrass for many years using pyridine herbicides such as clopyralid. However, in some golf courses, the continuous application of pyridine herbicides has resulted in the selection of S. sessilis populations that are resistant to these herbicides. This study focuses on a clopyralid-resistant population of S. sessilis collected from a golf course with a long history of clopyralid applications. The resistant phenotype of S. sessilis was highly resistant to clopyralid (over 225-fold). It was also cross-resistant to dicamba, MCPA and picloram but not mecoprop. The level of resistance to dicamba was high (7-14-fold) but much lower (2-3-fold) for both MCPA and picloram. The phenotype was morphologically distinct from its susceptible counterpart. Individuals of the clopyralid-resistant phenotype had fewer lobes on their leaves and were slightly larger compared to the susceptible phenotype. Resistant individuals also had a larger leaf area and greater root dry weight than the susceptible plants. An evaluation of internal transcribed spacer (ITS) regions confirmed that clopyralid-resistant phenotypes are conspecific with S. sessilis. In summary, the cross-resistance to several auxinic herbicides in this S. sessilis phenotype greatly reduces chemical options for controlling it; thus, other integrated management practices may be needed such as using turfgrass competition to reduce weed germination. However, the morphological differences between resistant and susceptible plants make it easy to see, which will help with its management.

Fig 1

Fitted clopyralid dose-response curves for two S. sessilis populations, the resistant population OR and the susceptible population OS in (a) the first and (b) second dose-response experiments. The percentage of survival of treated plants was used to produce the fitted curves. Vertical bars represent ± standard error of the mean.

Fig 2

Fitted dose-response curves for two S. sessilis populations, the resistant population OR and the susceptible population OS in (a) first and (b) second picloram dose-response experiments, (c) first and (d) second MCPA dose-response experiments, e) first and (f) second dicamba dose-response experiments, and (g) first and (h) second mecoprop dose-response experiments. The percentage of survival of treated plants was used to produce the fitted curves. Vertical bars represent ± standard error of the mean.

Fig 3

Variation in leaf morphology at (a) seedling (b) the 2–3 leaf and (c) the 5–6 leaf stage between clopyralid-resistant and clopyralid-susceptible plants.

Fig 4. Sequence alignment of the internal transcribed spacer 1 (ITS1) regions of clopyralid-resistant population (OR), S. sessilis, S. anthemifolia, and S. mutisii.

Hyphens (-) denote alignment gaps and asterisks donates residues conserved in all sequences.

Fig 5. Sequence alignment of the internal transcribed spacer 2 (ITS2) regions of clopyralid-resistant population (OR), S. sessilis, S. anthemifolia, S. mutisii and S. stolonifera.

Hyphens (-) denote alignment gaps and asterisks donate residues conserved in all sequences.

Fig 6

The predicted internal transcribed spacer 2 (ITS2) secondary structure of (a) clopyralid-resistant population (OR), (b) S. sessilis, (c), S. anthemifolia, (d) S. mutisii and (e) S. stolonifera. The four helices are labelled I–IV. The secondary structures were predicted and assessed using the ITS2 database web server (http://its2.bioapps.biozentrum.uni-wuerzburg.de).