Determination of glyphosate-resistant Conyza spp. in orchards and vineyards in Turkey

Conyza spp. are among the most problematic broadleaf weeds in citrus orchards and grape fields in Turkey. Glyphosate is used to control weeds, but Conyza spp. can escape the treatment and grow as a monoculture in these fields. To investigate whether glyphosate-resistant biotypes of Conyza spp. exist in Turkey, seeds of Conyza spp. were collected from 131 citrus fields and 121 vineyards with heavy Conyza infestations in the Mediterranean and Aegean regions of the country. Seeds were classified by species, and initial susceptibility screenings were conducted by applying glyphosate potassium salt at 1.323 kg a.i. ha(-1) to seedlings at the five- to six-leaf stage. Forty-five biotypes showed less than 80% susceptibility in the screenings and were subsequently used in dose-response experiments. Assays were also conducted to measure shikimic acid accumulation in resistant and susceptible biotypes after glyphosate treatment, and molecular studies were undertaken to investigate the mechanism of resistance. Among 252 populations collected from fields, 32 biotypes showed resistance to glyphosate. Molecular studies showed that target site mechanisms including mutations or expression of the EPSPS gene did not contribute to the mechanism of glyphosate resistance in Conyza biotypes from Turkey.