Horticultural crop response to synthetic auxins

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT REQUEST OF AUTHOR.] Rising adoption of dicamba-tolerant soybeans increases the potential exposure of grapes to dicamba, where off-target injury may occur via particle or vapor drift. In 2017 and 2018 at three locations in Missouri, research in production vineyards focused on the single-season effects of dicamba on hybrid grapes (Vidal blanc). During flowering and early fruit set, established grapes were exposed to low rates of dicamba, delivered as a spray solution (36 or 72 ppm) or by vapor from treated soil. The severity of dicamba injury (leaf cupping and feathering) was similar at 2 of 3 site years, with greater injury related to particle versus vapor drift of dicamba. Across all site years, season-long shoot growth was reduced up to 97 percent following exposure to particle drift. Minimal shoot inhibition resulted from dicamba vapor. At harvest, impacts of dicamba on grape yield were variable. At one site, grape yield was reduced 20-48 percent from dicamba vapor compared to respective controls, with evidence for increased sensitivity during flowering. In one site year, the final sugar content of berries at harvest was reduced by dicamba as particle drift, up to 19 percent. Grapes are highly sensitive to dicamba, with visual symptoms extending throughout the growing season. Impacts of dicamba on berry sugar and yield may result from early season injury.--Chapter 2 abstract.