Glyphosate Resistant Horseweed: Identification, Confirmation and Control in Nursery Crop Production

Christina Jennings, Tennessee State University


Glyphosate resistant horseweed (Conyza canadensis) is a significant issue in nursery crop production that has not been studied extensively. The objectives of the research were to identify glyphosate resistant and susceptible horseweed populations, test viability of horseweed seeds, evaluate a leaf dip assay for rapid detection of resistance, and conduct whole plant and pre-emergent herbicide assays for controlling horseweed. Glyphosate resistant and susceptible horseweed populations were identified, and seeds were collected and used in germination assays to test viability. Two whole plant assays were conducted by growing horseweed to the rosette stage (Plant Assay #1) or three different growth stages (seedling, rosette, and bolting; Plant Assay #2) and treated with glyphosate at varying rates. Two leaf dip assays were performed [Leaf Assay #1 (rosette plants) and Leaf Assay #2 (bolting plants)] by placing horseweed leaves into varying glyphosate rates. Two pre-emergent assays were conducted by sowing horseweed seeds into a substrate treated with pre-emergent herbicides. Seed germination assays showed resistant seeds had a greater germination rate (70.2 and 75.6%) compared to the susceptible seeds (52.2 and 46.4%). Plant Assay #1 susceptible shoot fresh weight was lower than resistant in all glyphosate treatments. Plant Assay #2 growth stage shoot fresh weight decreased by 62.7% (bolting), 82.8% (rosette), and 99.6% (seedling) from the control. Susceptible survival rate at the highest rate was 10% (bolting and rosette) and 2.6% (seedling). Leaf Assay #1 resistant plants at the high rate (69%) had greater leaf health than the susceptible low rate (67%). Leaf Assay #2 displayed no defined differences between resistant and susceptible leaf health. In the pre-emergent assays, several herbicides provided control of horseweed seed germination. Flumioxazin (SureGuard) was most effective for controlling horseweed seed germination, while pendimethalin (Pendulum AquaCap) provided poor control. In conclusion, glyphosate resistance is readily inherited in horseweed seeds and growth stage impacts glyphosate efficacy in susceptible plants. Leaf dip assays can be used for rapid detection of glyphosate resistance but only with tissue from young plants. Pre-emergent herbicides are very effective at preventing horseweed germination and should be adopted for managing glyphosate resistant horseweed in nursery crops.

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Recommended Citation

Christina Jennings, "Glyphosate Resistant Horseweed: Identification, Confirmation and Control in Nursery Crop Production" (2019). ETD Collection for Tennessee State University. Paper AAI22584971.