Glyphosate [N-(phosphonomethyl)-glycine] is a systemic, nonselective, postemergence herbicide that controls more weed species than any other herbicide (Duke, 2018; Heap and Duke, 2018). It has been used to control annual, perennial, and biennial species of grasses, sedges, and broadleaf weeds (Dinelli et al., 2006). Glyphosate inhibits the enzyme 5-enolpyruvlshikimate-3-phosphate synthase (EPSPS), which catalyzes the reaction of shikimate-3-phosphate and phosphoenolpyruvate to form 5-enolpyruvil-shikimate-3-phosphate (Fernandez et al., 2015; González-Torralva et al., 2012). Inhibition of EPSPS prevents the biosynthesis of phenylalanine, tryptophan, tyrosine, and other aromatic compounds in sensitive plants (Amaro-Blanco et al., 2018; Tahmasebi et al., 2018). In Turkey, glyphosate is the most widely used herbicide and is registered on more than 70 crops, including peach (Torun, 2017). In the past 5 years, the total amount of glyphosate sold in Turkey was ≈1.1 million kg of acid equivalent (Ministry of Agriculture and Forestry, 2018).
The application of glyphosate in crop and noncrop areas has resulted in decreased efficacy on several populations of three widespread species of the genus Conyza (Amaro-Blanco et al., 2018). These species include C. bonariensis (hairy fleabane), C. canadensis (horseweed), and C. sumatrensis [Sumatran fleabane (Syn. C. albida)]; there are at least 13 hairy fleabane, 42 horseweed, and 8 Sumatran fleabane cases of resistance reported in field crops, orchards, forests, pastures, urban areas, and nurseries around the world (Heap, 2018; Mylonas et al., 2014). Several glyphosate-resistant Conyza species have been reported in European and Mediterranean countries including France (Fernandez et al., 2015), Spain (Amaro-Blanco et al., 2018), Greece (Margaritopoulou et al., 2018), and Israel (Matzrafi et al., 2015). These species are native to the Americas (Amaro-Blanco et al., 2018) and considered as invasive and troublesome species in many parts of the world (Matzrafi et al., 2015). They are common weeds in orchards, row crops, roadsides, abandoned fields, and wasteland (Amaro-Blanco et al., 2018; Sansom et al., 2013) and occur in more than 70 countries (Holm et al., 1997). Currently, these Conyza species have become established in new territories including the Mediterranean basin (Amaro-Blanco et al., 2018) and are invading a variety of cropping systems (Tahmasebi et al., 2018).
In 2015, peach growers in Çanakkale Province of Turkey complained about a lack of glyphosate control of Conyza species. To date the only report of poor Conyza species control with glyphosate in Turkey was reported in citrus orchards in Adana, Mersin, and Hatay of Mediterranean region (Dogan et al., 2016). No research has been conducted to confirm and determine the level of resistance in these populations.
There are ≈56,000 ha of cherry, apple, pear, peach, and nectarine orchards in the Çanakkale Province in northwestern Turkey, which is considered one of the most important fruit and vegetable production areas in Turkey (TUIK, 2018). Currently, C. sumatrensis is considered as the most common troublesome weed in these orchards. Because of the poor control of C. sumatrensis with glyphosate, the objectives of this study were to confirm and identify the level of glyphosate resistance in C. sumatrensis and to determine the effect of chlorsulfuron (an acetolactate inhibitor) and metribuzin (a photosynthetic inhibitor) on glyphosate-resistant populations, which some farmers use to solve the problem despite their not being registered for use in orchards.
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