Developments In Pesticide Application Technology In the Orchard

The application of pesticides has been of concern for many years, particularly methods of reducing drift and improving deposition. The majority of growers use traditional airblast sprayers fitted with hollow cone or air shear nozzles, many growers choose not to replace mechanically reliable sprayers and so a large number of orchard sprayers are in excess of ten years old. Many airblast sprayers are too big for modern planting systems, the fan diameter is too large and the volume of air created is too great for the target canopy. Frequently, manufacturers build larger machines with a greater amount of air than is required for modern apple plantings, they are frequently designed for nut trees. Some growers buy them in the mis-guided belief that more air is better than too little air. The ideal air volume should match the tree canopy volume. Progress lies in the direction of more efficient application of power through a better understanding of the factors involved in getting the pesticide from the tank to the plants. 

Deposition within the canopy is so important, good coverage throughout the tree is required if satisfactory insect and disease control is to be carried out. Spray drift is also an important and costly problem facing fruit growers. Drift results in damage to susceptible off target crops, environmental contamination to watercourses and an unintentionally reduced rate of application to the target crop, thus reducing the effectiveness of the pesticide. Pesticide drift also affects neighbouring properties, often leading to public outcry. 

Traditional airblast sprayers direct the air from a single axial flow fan, mounted directly behind the sprayer, in an upward and outward direction. Axial fans are designed to move large volumes of air at low pressures. In order to accommodate varying crop canopies, e.g. as the season progresses, many modern sprayers are fitted with adjustable pitch propellers to provide variable airflows. Traditional advice to growers has been to use an adjustable deflector plate, fitted at the top and base of the air outlet to direct the air towards, and confine it to the target canopy. 

Sprayer design, deposition and drift

Even when sprayers are calibrated regularly their accuracy cannot always be relied on. Across the world surveys of the mechanical condition and the accuracy of sprayers show that many sprayers leave much to be desired. A combination of inaccurate speeds, worn nozzles, unsuitable filters and inaccurate gauges caused the problems. Survey and clinic results show how badly maintained many sprayers are on farms and how their operators require training.

The principal involved in traditional orchard spraying is to create enough air from the airblast sprayer to shift and replace the air within the canopy with pesticide-laden air from the sprayer. The leaves have a tendency to shingle in the airflow, altering airflow characteristics and reducing penetration and deposition. Tree canopies vary along the row, sometimes trees are absent presenting no resistance to air movement, resulting in air traveling through the target row and away. When applying pesticides growers know that small or fine/medium droplets give the best coverage, as large droplets (in excess of 300 mm) will run off the leaf onto the ground. Good coverage is critical for all contact pesticides. A traditional airblast sprayer sends air upwards above the canopy, carrying with it a plume of pesticide droplets. Unfortunately small or fine droplets will drift if they don’t become attached to the target leaf, insect or apple. Directed deposition is needed if pesticide is to be applied to the target zone. 

Airflow

Trials with various types of airblast sprayers were conducted at Cornell University to study how changes in fan speed affect air speed, volume and direction. Indoor trials were conducted using a Gill sonic anemometer (Gill industries, Hampshire, UK) to determine airflows.

It is clear that on the left side of an airblast sprayer, the peak of the air stream centre moves down with increasing distance away from sprayer centre. The rate of descent increases with decreasing air intake, resulting in relatively low velocities in the spray target zone on the left of the sprayer compared to the right side. On the right side, the general trend showed a slightly rising stream due to the counter-clockwise direction of the fan. The air velocity on both sides was highest at maximum fan speed, velocity decreasing when decreasing the fan speed. With the help of air velocity patterns we can see the effect of changes in air output.

Field trials were conducted in an orchard, (20 feet rows, 11 feet trees) using an AgTec P300 (AgTec, Minnesota) sprayer fitted with airshear nozzles operating at two fan speeds, 2076 rpm (540 rpm PTO) and 1557 rpm (405 rpm PTO). Drift was detected using Water sensitive cards (Syngenta, North Carolina) and analyzed using DropletScan (WRK, Cabot, AR) image analysis software. The methodology and results are published in detail, Landers and Farooq (2004). At a fan speed of 2076 rpm, drift was detected up to 80 feet from the target row where 10% card coverage occurred. Reducing fan speed by 25%, resulted in considerably less drift, with card coverage at 20 feet and 40 feet from the target row being 16% and 0.20% respectively. Reducing fan speed increased droplet size from 351 microns VMD at 2076 rpm to 460 microns VMD at 1557 rpm. (Note, no spread factor has been taken into consideration).

Nozzle orientation

In the 2004 growing season, a MIBO vertical patternator was used to evaluate the effect of nozzle orientation on spray deposition, results show great variability in spray pattern. Nozzles set in the “typical growers” pattern, i.e. pointing radially outwards resulted in a large quantity of liquid being blown above the target row. The quantity overshooting the target varied according to tree height, canopy density and size/speed of the fan on the airblast. Different types of airblast sprayer also behaved differently. We tested 20 sprayers at growers orchards and noted the great inbalance on distribution between the left and the right-hand side of the sprayer due to the airflow.

The best spray pattern for most conditions tested occurred when the right hand side nozzles were pointing horizontally to counteract the upward movement of the air from the fan. Best results occurred with the left side nozzles pointing 45^o upwards to counteract the downward direction of the air from the fan. The results show the importance of correct nozzle orientation if pesticides are to be applied effectively onto the target. It should be noted that each sprayer design will vary, due to fan size and air volume, so no “blanket” recommendation can be made.

Correct adjustment of top and base deflector plates on traditional airblast sprayers should also be carried out to direct the air towards, and confine it to the target canopy. Variable pitch blades (if fitted) must be adjusted to vary the amount of air being delivered into the target or alternatively the speed of the fan should be reduced by slowing down the PTO to match the developing canopy.

Conclusions

1. Airblast sprayers, as used in orchards, create a large plume of pesticide spray due to the use of large capacity fans. The resultant plume of pesticide spray frequently misses the target canopy and is accelerated upwards into the air or through the target canopy.
2. Spray drift is inevitable with crop spraying, even when growers follow best management practices. Research since the mid-1960s indicates that pesticide spray droplets will be transported by wind currents for distances ranging from a few feet to many, many miles.
3. Improved designs of sprayers which direct spray into the canopy, increases deposition and reduces drift, but does not eliminate drift completely.
4. The results have shown potential for improving deposition and reducing spray drift by carefully adjusting nozzle orientation. Nozzle orientation needs to be adjusted independently and in consideration of airflow rate and direction on two sides of the air blast sprayers.

Acknowledgements

I wish to thank Muhammad Farooq, Bruce Wadhams, Eric VanHemel, Gary Wood and Rob Lasher for their technical assistance. Funding for the projects mentioned in this paper was provided by the NYS Apple Research and Development Program, the Viticultural Consortium-East, Lake Erie Regional Grape Program, Grape Production Research Fund, New York Wine Growers, the Wine and Grape Foundation and NYSERDA.

Reference

Landers, A.J. and Farooq M. 2004 Reducing spray drift from orchards – a successful case study. NY Fruit Quarterly 12 (3) Autumn 2004 pp 23-26

Key:
ID = Lechler Air Induction; AD = Lechler Drift Reducing; TD = Agrotop by GreenLeaf
DG = Drift Guard by TeeJet; AVI = Albuz Air Induction

Related Links:

• BBA website