Spray droplets map path to better protection

University of Queensland researchers have collated and mapped tens of thousands of spray droplets across grapevine canopies to help the Australian wine industry maximise coverage and reduce environmental risks associated with agricultural plant-protection products and machines.

The project is titled ‘A generic approach to improving spray coverage’ and is funded by the Australian Grape and Wine Authority.

University of Queensland (UQ) senior research fellow Andrew Hewitt and senior research scientist Chris O’Donnell are leading the project, which is in its second year.

Dr Hewitt said the spray accountancy data would support the development of modelling tools to show how sprayer setup affects spray delivery for a wide range of conditions.

It could also help address regulatory concerns related to spray drift.

“This sort of data will allow the industry to negotiate more favourable regulations in the future than would be developed without such data, as assumptions would otherwise be made for overseas worst case risk assessment scenarios,” Dr Hewitt said.

A wide number of spraying systems, already in use in the wine industry, have been tested as part of the project.

“We aimed to account for 100% of the applied spray through complex sampling with a virtual box around the grape canopy,” he said.

“Spray coverage on upper and lower leaf surfaces, as well as different regions in the canopy were sampled using a custom-made leaf washer that attaches to leaves.

“The number of droplets per square centimetre was determined using photographic paper collectors at different canopy regions.

“Tall towers measured the airborne spray in and above the canopy using monofilament line which is a very efficient collector of spray droplets. Deposition on the ground below the canopy was measured with petri dishes. Drift deposition downwind of the crop was measured using Mylar cards.”

Dr Hewitt said tens of thousands of samples were generated for three spraying systems:

  • a reference baseline sprayer with deflectors for targeting spray towards the lower canopy region and setup for the canopy height. International standards require that a reasonable worst-case baseline be established for such studies and the axial fan airblast sprayer baseline for this work was based on various grower surveys;
  • a multi-fan sprayer for directing the spray downwards and sideways toward the canopy; and
  • an electrostatic spraying system for charging droplets for attraction to the canopy.

The data is still being interpreted but Dr Hewitt said trends suggest spray efficiency varied among the application systems.

“Initial inspection of spray deposition on targets did not reveal the high coverage from the electrostatic system so a high resolution image analysis system was developed to ‘see’ the small droplets, revealing very high levels of coverage,” he said.

“All of the systems gave good spray coverage on the canopy. The main differences were in losses to the ground and as airborne spray losses.

“The electrostatic system allowed the use of a lower application volume rate, thereby reducing runoff from leaves and losses to the ground below the canopy compared to the higher volume airblast application system.”

While all systems suffered some airborne losses of sprays, Dr Hewitt said these were at low heights for the electrostatic system, which resulted in drift being contained to the first few metres beyond the vineyard.

“The multi-fan system produced an airborne spray flux cloud, which was higher than that of the electrostatic system and similar in range to that of the reference airblast sprayer but the total amount of airborne spray above the canopy was much lower with the multi-fan system, which resulted in lower drift values than the reference sprayer,” he said.

The project has also incorporated the use of a wind tunnel, located at UQ’s Gatton Campus, to allow for a large number of spraying and tank mix combinations to be screened before selecting the best ones to fully test in the field.

“Applicators have a large choice of spraying systems and tank mix components and this work helps evaluate these under controlled conditions. Ongoing testing is looking at adjuvants, which will be the main focus of upcoming field studies,” he said.

“We are finding many differences among adjuvants and initial assessments suggest that those chemistries which fit in the emulsion category rather than solution pesticide products and adjuvants help improve the droplet size spectrum for a better spray application with lower drift potential.”

The results from the first year of the project were presented at a number of regional workshops late last year, and Dr Hewitt said more are already being planned.

The next stage of the research will see calibration and dosing models tested in field studies in the upcoming season.

“We will aim to include some field laser measurement systems to assist the calibration work such as custom-made Phase Doppler Interferometer (PDI) systems for assessing the spray flux and Light Detection and Ranging (LiDAR) for assessing the canopy structure as we assess and evaluate dosing models for optimised spray delivery to each canopy type,” he said.

An electrostatic spraying system is part of a trial looking at the efficacy of Australian spray machinery and spread of products.
An electrostatic spraying system is part of a trial looking at the efficacy of Australian spray machinery and spread of products.