I spent much of this winter talking to corn and soybean growers about dicamba, new labeling and how to avoid drift issues as much as humanly possible. I frequently mentioned that grapes and tomatoes are among the most sensitive crops to dicamba, and joked that just saying, “dicamba,” may be able to induce a curl of the leaves.
Last week, I stumbled across an article from the University of Nebraska that described just how sensitive grapes and tomatoes are to dicamba. The article can be found at https://goo.gl/BTUn4M. During the summers of 2016 and 2017, test plants were sprayed with the three dicamba-based herbicides at rates of 0, 1/10, 1/50, 1/100, 1/500 and 1/1000 of the label rate. The label rate of Clarity, Engenia and XtendiMax were 16, 12.8, and 22 ounces/acre, respectively.
To help visualize the rates, 1/10 of the label rate is equivalent to three tablespoons per acre; 1/100 is equal to one teaspoon applied over a size of a football field (about one acre). The tomatoes were 10 inches tall at time of application and the grape vines were 25 inches long. Visual injuries rated on a scale of 0 (no injury) to 100 (dead plant) were collected at 7, 14, 21 and 28 days after treatment. Maximum accumulated vine length of grape, plant height of tomato and plant biomass of both species were collected at 28 days after treatment. The yields of the plants were not measured.
The general results showed that all three dicamba products had similar impact on the growth of grapes and tomatoes, and as you would expect, as the application rates increased, so did the level of damage.
The symptoms reported in grapes were in the form of leaf and stem twisting and leaf cupping. At the 1/100 label rate, there was about 35 percent visual injury at 21 days after treatment. At the 1/10 label rate, there was 64 percent visual injury at 21 days. The reduction in vine length increased from 5 percent to 40 percent with the increase of dicamba rates from 1/1000 to 1/10 of the label rate.
In tomatoes, the type of damage reported was in the form of stunting, chlorosis, callus-like formations on the stems and cupping and curling of the leaves. Injury increased from 20 percent to 80 percent as dicamba rates increased from 1/1000 to 1/10 of the label rate. The 1/100 label rate was enough to cause 50 percent injury in tomato plants. Plant height was reduced by 10 to 50 percent when going from the 1/1000 to 1/10 label rate. Some different research conducted in 2012 showed a 25 percent reduction in tomato yields at a 1/50 label rate of dicamba when applied to early vegetative stages of plant development.
What does that mean to you as a user of dicamba? Take this seriously! Be sure to check DriftWatch for sensitive crops growing near your fields. Be sure to apply the product when wind speeds between 3 and 10 mph, and not blowing toward a sensitive crop. Do not mix dicamba with anything not listed on the label.
If you are a commercial grape or tomato grower, take this seriously! Be sure your crop is registered in the DriftWatch database, so that people who are spraying dicamba know your crop is there. You can register for no cost at https://in.driftwatch.org/.
Jeff Burbrink is a Purdue Extension educator in Elkhart County. He can be reached at 533-0554 or at firstname.lastname@example.org.