The corn and soybean planting season has been compressed significantly due to the frequent rain showers received during late April and May. Many growers across Indiana were rushing to get crops in the ground or burndown weedy fields during the small windows of suitable field conditions during May. With that said, we noticed that the total number of herbicide drift samples received at the Purdue Plant and Pest Diagnostic Lab (PPDL) has increased substantially for the month of May when compared to previous years (Table 1), especially when compared to 2020 and 2021. This high number observed in 2022 is likely the result of spray operations that were made during windy weather conditions.
| Table 1. Total number of herbicide injury plant samples received by May 31 at the Purdue Plant and Pest Diagnostic Lab over the last 5 years. | |
|---|---|
| Year | Number of Samples |
| 2018 | 36 |
| 2019 | 35 |
| 2020 | 21 |
| 2021 | 19 |
| 2022 | 44 |
Synthetic auxin herbicides (2,4-D and dicamba) are the most common cause of off-target herbicide injury that we see with burndown herbicide applications. These herbicides can cause injury at very low exposure rates and affect many broadleaf plant species (including trees, ornamental, and garden plants). Symptoms of exposure include mostly leaf curling and stem twisting and affect mostly the newer tissues or growing points (Figure 1). Contact type herbicides such as paraquat (Group #22) and the PPO-inhibiting herbicides (Group #14: saflufenacil, flumioxazin, sulfentrazone, and fomesafen) are also often associated with herbicide drift concerns. Exposure to these herbicides will result in small and round necrotic spots on those leaves that are exposed to the spray droplets (Figure 2). See the publication “Diagnosing Herbicide Injury on Garden and Landscape Plants” for more information on herbicide injury diagnostics.
Figure 1. Synthetic auxin injury on tree leaves. (Photo Credit: PPDL)
Injury symptoms caused by drift of contact type herbicides. (Photo Credit: PPDL)
Herbicide applications under high wind speeds (above 10-15 miles per hour) can result in herbicide drift. This process can move herbicide particles hundreds of feet away from the application target and onto susceptible vegetation. Synthetic auxin herbicide molecules (2,4-D and dicamba) can also volatilize and move off-target for miles under air temperature inversion conditions. Air temperature inversions are more common within 1 to 2 hours after sunrise and before sunset or whenever wind speeds are below 3 miles per hour and air masses are stable (whenever the weather is “foggy” or smoke gets trapped near the soil surface). Pesticide applicators need to be aware of their surroundings and understand the environmental conditions that favor herbicide off-target movement to minimize this risk moving forward this season. Proper sprayer operation, nozzle selection, and the use of drift-reducing agents can also reduce the risk of pesticide drift. For more information on herbicide drift and volatility check out this article from our colleagues at the University of Missouri: “Off-target pesticide movement: a review of our current understanding of drift due to inversions and secondary movement”.
