Production Practices and Quality Assessment of Food Crops. Vol. 1

Spray Technology in Perennial Tree Crops 85
Traditionally, very large spray volumes have been common when treating orchards
or vineyards. Volumes of 2,000–5,000 L/ha for deciduous fruit and 10,000–30,000
L/ha for citrus have been considered standard. There were two reasons for these
large volumes. Firstly, it was generally accepted that large spray volumes would help
to achieve the required thorough coverage of leaves and stems. Secondly, when using
spray guns, only the large drops have enough inertia to reach their target site.
However, these high volumes resulted in considerable wastage of the spray through
runoff and drift.
Over the last 50 years there has been a trend towards reducing the volume of
liquid applied, necessitating the application of discrete droplets. A range of low
volume (LV) and ultralow volume (ULV) sprayers have been developed for pesticide
application in orchards. Gunn (1980) reports that the early LV and ULV sprayers
were able to demonstrate a degree of pest control, but were unable to give adequate
control when subjected to severe pest/disease pressures, particularly with the control
of red spider mite (Oligonychus ulmi) or apple mildew (Podophaera leucotricha).
However with the move towards Controlled Droplet Application (CDA) technology,
Bals (1976) demonstrated that targets could be sprayed more efficiently by selecting
optimum droplet size and density for maximum retention and coverage. CDA spray
heads can be fitted to air-blast sprayers to give a diverging spray pattern, or on a
vertical boom to give a converging spray pattern as described by Oakford et al.
(1994a).
Another development over the last 10–15 years includes the tower sprayer which
uses an air curtain and rotary atomiser. This system allows horizontal penetration
into the canopy which is preferential to the vertical penetration from an air-blast
sprayer (Landers, 1999). Tunnel sprayers, developed in both Europe and the USA
during the 1990’s, offer advantages in trellised and pedestrian orchards (orchards
with dwarf trees). The use of a spray collection device at the base of the tunnel
results in the ability to recirculate spray with subsequent savings in pesticide and
a reduction in drift. The use of tunnel sprayers however is limited due to the restricted
tree size and shape on which it can be used (Cross and Berrie, 1995).
With the increasing urban encroachment on orchards and the need to reduce spray
pollution from drift and runoff, there has been a worldwide push to increase the
efficiency of spray usage. The need to reduce chemical usage on food has given
added impetus. In Australia these moves have culminated in the ‘Consumers
Charter’, signed by fruit growers with consumer bodies, to reduce total chemical
usage on fruit trees. Oakford et al. (1995) have pointed out the considerable
influence that spray technology could have in achieving this goal.
3. MECHANISMS OF DROPLET PRODUCTION
Liquids can be atomised or broken into droplets by a number of methods: hydraulic
pressure, centrifugal energy, airshear, kinetic, thermal, and electrodynamic methods.
In agricultural spray systems, hydraulic energy (as in conventional nozzles) and
centrifugal energy are the most common atomising methods. In addition to breaking
up the spray liquid into droplets, the functions of spray nozzles include control-