Effectiveness of alternative herbicides on three Conyza species from Europe with and without glyphosate resistance

Abstract

Glyphosate has been applied in European countries for over a decade between rows in olive groves and grape vineyards to control Conyza species [hairy fleabane (C. bonariensis), horseweed (C. canadensis) and Sumatran fleabane (C. sumatrensis)], however poor control has been observed in recent years. Glyphosate susceptible (GS) or resistant (GR) populations were assayed in each species. In addition, Conyza spp. control with alternative herbicides (alone or in mixture with glyphosate) over two years was also assessed. The GS populations of the three species were controlled with glyphosate field doses (1080 g ae ha−1). The GR hairy fleabane, horseweed and Sumatran fleabane populations were 15.0, 15.7 and 19.8 times more resistant, respectively, than their respective GS population. The shikimic accumulation of GS populations was 4–6 times higher compared with the GR Conyza populations, confirming the glyphosate resistance of the latter ones. The increase in the glyphosate dose did not control the GR Conyza populations, despite providing a higher dry growth reduction. Glufosinate and flazasulfuron, alone or mixed with glyphosate, were the effective options to control GR and GS populations of hairy fleabane and Sumatran fleabane. However, the GR horseweed population might have evolved multiple resistance to glyphosate and flazasulfuron in Hungary. The other herbicides (PSI, auxinic and PPO) showed an additive effect together with the control provided by glyphosate in the GS and GR populations; however generally, these herbicides could be applied alone at the rosette stage. Effective herbicides with modes of action different from glyphosate, except flazasulfuron for controlling horseweed, should be used to delay the selection of herbicide resistance in perennial crops in Europe.

Introduction

Conyza species [hairy fleabane (C. bonariensis), horseweed (C. canadensis) and Sumatran fleabane (C. sumatrensis)] are common weeds invading a great variety of agronomic crop systems worldwide (Travlos and Chachalis, 2010, 2013), due to their rapid adaptation to undisturbed (non-tillage) and plant-free soils (Brown and Whitwell, 1988). Conyza species have evolved resistance to herbicides in many countries (100 cases), and are listed among the most problematic weeds (Matzrafi et al., 2015; Heap, 2018). Glyphosate is responsible for 13, 42 and 8 cases of resistance in hairy fleabane, horseweed and Sumatran fleabane, respectively, reported in annual and perennial crops, orchards, forests, pastures, roadsides, railways, industrial sites and nurseries around the world (Heap, 2018).

Glyphosate [N-(phosphonomethyl)-glycine] is the world's most successful post-emergence and non-selective herbicide (Duke, 2018). In Spain it has been widely used to control weeds in citrus orchards, olive groves, grape vineyards, and others perennial and annual crops (Gonzalez-Torralva et al., 2014, González-Torralva et al., 2010), as well as in path borders, railway lines, recreation areas and derelict sites (Urbano et al., 2007). This herbicide is absorbed through leaves and other young-green tissues, and translocated via phloem into meristematic tissues (Preston and Wakelin, 2008). Glyphosate is a potent inhibitor of 5-enolpyruvylshikimate 3-phosphate synthase (EPSPS) by blocking the biosynthesis of phenylalanine, tryptophan, tyrosine and others aromatic compounds in susceptible plants (Maeda and Dudareva, 2012).

Glyphosate-resistant weeds are able to survive glyphosate exposure due to target-site (mutations in the gene encoding the EPSPS or gene duplication) (Heap and Duke, 2018) or non-target-site (degradation to non-toxic compound, impaired translocation, poor absorption and/or vacuolar compartmentation) (Ghanizadeh and Harrington, 2017) resistance mechanisms, alone or in association (Sammons and Gaines, 2014). The resistant weeds will not be killed by using glyphosate alone.

The herbicide combinations with different modes of action, either in sequential application or tank-mix, can contribute to improve the control of resistant weeds (Tornisielo et al., 2013), and additionally, may also avoid or delay evolution of resistance (Ganie and Jhala, 2017). Improvements in weed control have been observed in glyphosate-resistant species by applying glyphosate tank-mix combinations (Riley and Bradley, 2014), in some cases, the control was improved by 25–30% compared to glyphosate alone (Eubank et al., 2008).

The objectives were to characterize glyphosate resistant Conyza species, collected in perennial crop systems from different European countries, and to propose alternative chemical strategies for their management.

Section snippets

Plant material and experimental conditions

Seeds of Conyza species (hairy fleabane, horseweed and Sumatran fleabane) resistant (GR) and susceptible (GS) to glyphosate (Table 1), were planted in plastic containers (10 × 10 cm x 6.3 cm) filled with peat moistened and covered with parafilm. Plastic containers were kept in a controlled condition room [28/18 °C (day/night), 16 h photoperiod, 850 mmol m−2 s−1 light density and 60% relative humidity] until germination. Seedlings were transplanted into 250 cm3 pots (1 plant pot−1) filled with

Confirmation of glyphosate-resistant populations in Conyza spp.

Dose-response assays revealed variable glyphosate susceptibility between GR and GS Conyza populations, both in terms of PM and DWR. All GS populations died at the dose recommend to the farmers in European countries in olive groves and vineyards (1080 g ha−1). On the other hand, the GR populations were not injured at this dose. The resistance indexes (RI=R/S) based on the LD50 (15.2, 12.6 and 8.1) and DWR50 (15.0, 15.7 and 19.8) values were higher in the GR than the GS populations of hairy

Conclusion

Dry weight reduction estimates, weed survival and shikimate accumulation confirm the resistance between Conyza species populations collected from those sites with glyphosate history applications (GR) compared to sites never treated (GS). One horseweed population might have evolved multiple resistance to glyphosate and flazasulfuron in Hungary. Diquat, flazasulfuron (except for the GR horseweed population), glufosinate and pyraflufen-ethyl, herbicides with different mechanisms of action (PSI,

Acknowledgements

This work was funded by the Asociación Agroquímicos y Medio Ambiente (Spain). Dr. Phillip Villani (The University of Melbourne) revised and corrected the English language used in this manuscript.

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