The context
In the summer of 2024, six Ontario vineyards participated in an authorized herbicide trial. The objective was to assess efficacy as well as determine if the product fit the timing for seasonal weed and sucker management. If successful, it could replace the expensive and time-consuming manual labour required to remove suckers.
Each vineyard applied the same rate, at similar times, employing optimal sprayer settings. A few weeks after application, the researchers and registrant toured the vineyards. They were pleased with how quickly and effectively the product worked on both targets at all six locations. However, one vineyard reported visual injury on a sloped region of their operation.
This raised two questions:
- Assuming the cause was drift, and not direct overspray, why did it only happen in a specific region of a single vineyard?
- Whether drift or overspray, what is the potential for the applied rate to cause injury?
The vineyard manager and sprayer operator investigated the application equipment and found no problems with how the sprayer was calibrated or operated. Further, the nearby weather stations recorded reasonable environmental conditions. So, that seemed to discount accidental overspray and wind-borne drift.
Then we considered the topography. The level portion of the vineyard appeared undamaged, but as it began to slope downhill, we saw damage on leaves and shoots in the bottom half of the canopy. It was almost as if a stratum of herbicide stayed level as the ground fell away. We discussed temperature inversions, volatility, and sprayer wake, but nothing fit.
Then we stepped back and found ourselves looking up at the Niagara Escarpment. The Escarpment is a long cliff formed by erosion, separating the higher, level ground from where we stood below. And then we had an idea: Could the product have been lifted into contact with the canopy by a Katabatic wind?
The theory
On clear nights with calm winds, the ground cools rapidly. Air in contact with the colder ground cools by conducting heat to the ground and by radiating upwards. When this cooling process occurs along mountain slopes, or on top of a plateau, the cooling air becomes colder and denser, causing it flow downslope like water. Perhaps a layer of relatively cool Katabatic wind off the escarpment slid under the warmer layer of air in the downslope portion of the vineyard. And, perhaps, any product still suspended in the air was lifted upwards into contact with the grape canopy.
Even the coarsest hydraulic nozzle produces a population of driftable fines. These fines take a long time to fall, and some are essentially buoyant. In the following histograms, we see actual data from a nozzle rated between Medium and Coarse. The operators actually used an air-induction nozzle with a much coarser spray quality, but we’re using this data set as a worst-case scenario example. If we divide the volume produced into its constituent droplet sizes, we see that most of the volume is comprised of droplets between 150 and 250 microns.
However, droplet diameter shares a cubic relationship with volume. If we plot that same volume by number of droplets, we see the majority are between 18 and 74 microns in diameter. These very small droplets would fall so slowly that any atmospheric disturbance would displace them. Depending on the crop’s sensitivity to the herbicide, they might carry sufficient active ingredient to cause injury, assuming they didn’t evaporate to the point that they were no longer biologically active.
There are a lot of assumptions in this theory, and perhaps it’s far fetched, but it was the best we could figure. So, if those droplets were lifted into contact with the canopy, were they capable of causing injury? To find out, we conducted a simple, non-replicated bioassay.
The bioassay
On the morning of July 12, we filled a spray bottle with 50% of the field-rate (including 1% v/v MSO) and set the nozzle to the finest setting. We applied a single spritz about mid-way up the canopy of the same Riesling grapes on a VSP flat cane training system. We did this on the upwind side on both older (lower canopy) and newer growth (upper canopy). Then we performed a series of serial dilutions, halving the concentration each time, and repeating the application.
Our hope was to see a subtle response curve when we plotted concentration against tissue damage. Perhaps we’d even see a different curve for older versus newer tissue.
The vineyard manager photographed and recorded observations on an approximately weekly schedule, with a gap in observations between weeks three and six. The following images show the results of a ½ dose treatment, and a 1/16 dose treatment tracked during that period.
The results
We observed the following:
- Fruit, foliage, and shoots were injured at all doses by three days after application.
- Initial injury remained stable; no secondary injury was observed.
- The degree of injury at the lowest dose was significantly more severe than the injury observed following the original May 31st application.
- Regular vineyard operations, such as mechanical leaf removal in the fruiting zone and hedging, removed some of the damaged leaves and shoots.
- The study did not include an assessment of harvest quality.
This was severe injury, even at the lowest rate. When compared to another herbicide commonly used for perennial weed control (e.g. Ignite SN – glufosinate ammonium) the injury we saw manifested very quickly.
Recently, researchers at Cornell have been exploring the herbicide we used in this study in perennial weed and sucker control in apple orchards. They did not experience any drift issues and found it to be effective between 90-180 ml/ha (0.5-1 oz/ac) (personal communication). That’s ~4x less than the rate proposed for registration in Canada, and it suggests the herbicide in question was certainly capable of causing the damage at very low concentrations.
Ultimately, we can’t be certain how the initial off-target damage occurred, but we were able to evaluate damage potential using a rough-and-simple bioassay that any grower can try. In unusual cases of drift it’s important to know if the product we suspect is even capable of causing the damage. A simple evaluation using serial dilution and a squirt bottle can tell us if we need to look more closely, or look somewhere else to explain injury.
Thanks to Kristen Obeid, OMAFA Weed Specialist (Horticulture) and Josh Aitken, Vineyard Manager of Cave Spring Vineyard for their contributions to this work.