Imaging analysis method to quantify leaf deformation in response to sub-lethal rates of dicamba

Dicamba is a synthetic auxin herbicide that is prone to off-target movement, including drift and volatilization. Due to the increased acreage of dicamba-resistant soybean to control glyphosate-resistant weeds, dicamba drift injury to neighboring vegetable crops is of concern. A method to quantify leaf deformation (often referred to as leaf cupping) caused by dicamba injury was developed and compared to visual rating techniques to determine its accuracy and suitability. A second objective was to determine the relative dicamba sensitivity of several economically important vegetable crops. Soybean, snap bean, tomato, and cucumber were grown in a greenhouse and exposed to dicamba at 0, 56, 112, 280, 560, 1,120, and 2,240 mg ae ha(-1), which is, respectively, 0, 1/10,000, 1/5,000, 1/2,000, 1/1,000, 1/500, and 1/250 of the maximum recommended label rate for soybean application (560 g ae ha(-1)). Plants were evaluated visually and using an imaging analysis technique that measures the leaf deformation index (LDI) with a leaf area scanner. LDI is calculated by dividing the two-dimensional projection of the area of the leaf in its natural configuration by the area of the flattened leaf. Across all four crops, log-logistic regression analysis indicated the LDI method had lower I-50 values with lower standard error, demonstrating that the LDI method gives more precise estimates of sensitivity. This novel method provides an objective, quantitative method for measuring dicamba drift injury and determining relative sensitivities of valuable specialty crops.