Sprayer Calibration - Getting Enough Spray Into The Tree Tops

Last season I did a lot of work with sprays that give a very visual demonstration of where the spray was going in the tree. Examples of these sprays were dormancy breakers, such as "HicaneTM" blossom burner thinning sprays and brown rot sprays. In the latter instance you do not see the direct influence of the spray deposit, but the lack of disease control where spray coverage was inadequate.

A feature of many of the orchards where these particular sprays were applied was poor results towards the upper tree. This is a clear indication that most sprayers are not directing sufficient spray volume into the upper 25 to 30% of the air stream.

Studies done by J L Brann some 35 years ago in the New York apple orchards showed that for large spreading tree, 70% of the spray volume had to be directed into the upper third of the effective air stream. The majority of our more mature summerfruit tress would require at least 70% of the spray volume in the upper third of the air stream if not more. Many of these orchards almost have full canopy cover across the tree tops. Some apple blocks that have been poorly managed in the upper tree would also need most of their spray volume directed to the top third of the effective air stream.

Directing spray into the top of the tree is also critical for satisfactory chemical thinning. The normal situation with chemical thinning is to under-thin the tops and over-thin the lower tree. The proof of a good chemical thinning job is when there is uniform thinning from top to bottom of the tree.

This can only be achieved by obtaining thorough coverage of the upper tree with minimal drip or washing through the lower tree.

Droplet Size

Droplet size range is important, particularly for low volume concentrate spraying.

Coverage is dependent on droplet size with the best result coming from nozzles which produce a narrow range of droplet size. Optimum droplet size range for spraying orchard canopies is considered to be in the 70 to 250 micron range. Droplets smaller than 70 microns evaporate rapidly and do not have sufficient mass to impinge on the target so drift great distances while droplets larger than 250 microns drop rapidly to the ground, or run off the target. At least 50% of the spray volume should be in the 70 to 250 micron droplet range. The best hydraulic nozzle technology can achieve levels around 70% in the optimum range. Air Shear and spinning disc nozzles can do even better than this, making sprayers equipped with these nozzles capable of giving good results with water rates as low as 200 l/ha.

Spray Evaporation

At higher temperatures and low humidities spray droplets loose considerable volume from evaporation. Large droplets are less likely to loose water from evaporation than small droplets.

Measurement of evaporation at different relative humidities show that at levels >85% RH large droplets (250 micron mass medium diameter) will loose about 3% of their volume at 5 m distance from the sprayer, while small droplets (125 micron MMD) will loose about 6%. At low humidities of 40% RH or below, the large droplets loose about 7% of their volume, whereas the small droplets were estimated to loose about 20% of their volume through evaporation by the time they reach the upper tree.

Another US study measuring spray deposit from a sprayer nozzled at 150 micron MMD, showed similar amounts of spray material were deposited top and bottom of the tree when applied at high humidity in the early morning. This same sprayer operated in a hot afternoon with 35% RH, deposited just over one third of the amount of spray in the top of the tree that was deposited when the humidity was high. If you are forced to spray tall trees when temperatures are high and humidity is low, it is necessary to lift the droplet size of the nozzles directed towards the top of the tree. For sprayers which have turn-over nozzles switching the top two or three nozzles from low volume to high volume is worth consideration.

Dropping pump pressure and driving slower to compensate for the reduced water volume output will also increase droplet sizes and improve the chances of better coverage in the top.

Tree Row Volume (TRV)

Measurements carried out by Dr David Manktelow of Hort+Research shows that as tree height increases, the TRV to be sprayed increases. At 3.5m tree height (eg a young orchard) TRV is around 10,000 m3/ha, while at 4.5m it is almost double at around 18,000 m3/ha and at 5.5 m tree height, it is around 27,500 m3/ha. Thus as tree height goes up, water volumes must too if similar levels of spray coverage are to be achieved. Experience with dormancy breaking sprays indicate 1 l of spray to 16 to 20 m3 of canopy is necessary. Blossom burner thinning sprays, where a high level of wetting is required probably need 1 l of spray to 8 to 10 m3 of canopy.