Air Blast Orchard Spraying (E2712)
The concept of controlling agricultural pests and diseases by spraying a control chemical is quite simple: merely apply the correct amount of the chemical evenly to the plant, leaving no area unprotected, and avoid applying the chemical to non-target areas
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The concept of controlling agricultural pests and diseases by spraying a control chemical is quite simple: merely apply the correct amount of the chemical evenly to the plant, leaving no area unprotected, and avoid applying the chemical to non-target areas.
The concept is simple, but achieving a uniform application is not. Many factors influence the uniformity of chemicals applied by spraying. The application is affected by physical factors such as nozzle type and size, application pressure and travel seed, type of chemical mix, and distance between nozzle and target. Environmental factors—i.e. temperature, wind velocity, and relative humidity—also play an important role.
Non-uniform application results in overapplying or underapplying chemicals on parts of the crop. Overapplying sometimes results in burning or damage to the plant, while underapplying or entirely missing other parts may result in crop damage or loss from ineffective pest control. Either situation is an inefficient use of chemicals.
All spraying requires care and attention so details to achieve good results, but orchard spraying is generally more challenging and difficult to perform well than boom spraying in fields. Field spraying tends to be a two-dimensional problem involving an area to be covered. Orchard spraying adds a third dimension—height—and concern for the volume in the target area (i.e. size of trees). Other significant differences are the much greater distances between nozzles and target in orchard spraying and the amount of air used to carry spray to the tree.
This bulletin discusses the factors affecting air blast spraying using hydraulic nozzles with an axial fan and shows how to set up the nozzles correctly for effective spraying. Other sprayers that use air shear nozzles, radial fans, towers and air curtains are not included here.
The Development of Orchard Sprayers
High-pressure guns were among the earliest spraying equipment used in orchard work. High pressure was required to drive the spray material from the gun uup into the tree. Guns were directed by men who “swept” the tree with the spray attempting to get good coverage. It was slow and laborious.
As high-pressure spraying continued, guns were developed with multiple nozzles. These brooms gave a much wider coverage. These were followed by sprayers with multiple nozzles on a mast or tower. Some of these sprayers had oscillating nozzles on the mast in an attempt to improve spray delivery into the tree and reduce labor requirements.
This concept eventually gave way to the development of the air blast spraying principle, in which spray delivery was assisted by a large and moving air stream.
Today, air-assisted spray delivery continues to be part of the orchard spraying technique used in virtually all modern horticultural sprayers. Some air blast sprayers make use of a very high-velocity air stream that atomizes by air shear and drives the droplets into the tree in a wedge of turbulent, expanding air. Others use a lower velocity, high-volume air stream and high-pressure atomizer and operate on the principle of displacing the air in the tree with spray-laden air.