Abstract
Glyphosate-resistant (GR) Kochia scoparia has evolved in dryland chemical fallow systems throughout North America and the mechanism of resistance involves 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene duplication. Agricultural fields in four states were surveyed for K. scoparia in 2013 and tested for glyphosate-resistance level and EPSPS gene copy number. Glyphosate resistance was confirmed in K. scoparia populations collected from sugarbeet fields in Colorado, Wyoming, and Nebraska, and Montana. Glyphosate resistance was also confirmed in K. scoparia accessions collected from wheat-fallow fields in Montana. All GR samples had increased EPSPS gene copy number, with median population values up to 11 from sugarbeet fields and up to 13 in Montana wheat-fallow fields. The results indicate that glyphosate susceptibility can be accurately diagnosed using EPSPS gene copy number.
Highlights
Kochia scoparia (L.) Schrad. is a competitive weed that can cause substantial yield loss, and is a problem weed in sugarbeet and chemical fallow [1, 2]
The objectives of this study were to a) compare the level of glyphosate resistance in K. scoparia collected from sugarbeet fields and wheat-chemical fallow fields; b) determine whether GR K. scoparia from sugarbeet fields has the same mechanism of resistance as identified in dryland chemical fallow-based systems; and c) quantify the effect of enolpyruvylshikimate-3-phosphate synthase (EPSPS) copy number on whole-plant response to glyphosate across numerous (65) K. scoparia accessions collected from sugarbeet fields from Colorado, Wyoming, Nebraska, and Montana and 12 accessions collected from wheat-chemical fallow fields from Montana
The median LD50 values for both sets of K. scoparia were greater than the standard field use rate of 840 g ae ha-1, this is not necessarily an indication of a high level of resistance
Summary
Kochia scoparia (L.) Schrad. is a competitive weed that can cause substantial yield loss, and is a problem weed in sugarbeet and chemical fallow [1, 2]. K. scoparia has protogynous flowers in which the stigmas usually emerge one week before pollen is shed and are receptive to foreign pollen which can promote outcrossing between plants in close proximity [3]. It produces copious amounts of pollen for extended periods of time, which is generally an indication that the species is naturally highly outcrossing [1]. In Wyoming, K. scoparia densities as low as 0.2 plants m-1 of crop row reduced sugarbeet root yield by 18% [5]. The outcrossing nature of K. scoparia combined with prolific seed production results in genetically diverse populations that facilitate the evolution of herbicide resistance [6]
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