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

98 Sally A. Bound
efficiency due to flooding of the nozzles, affecting the ability of the nozzle to atomise
the spray liquid properly. Oakford et al. (1995) suggested that airshear machines
are most effective in the range 100–400 L/ha. Although airshear nozzles are sensitive
to small changes in flow rate, air speed and fluid properties and are inherently
more difficult to control than hydraulic nozzles, the machines used in these
experiments produced highly significant results. These scientific findings have
been translated to practical use in Australia and New Zealand where airshear
machines are commonly used for sensitive operations such as fruit thinning at around
250 L/ha. Airshear technology has also provided a technological bridge for
orchardists between ULV and HV.
Many airshear sprayers are fitted with electrostatics, and Moser et al. (1984)
showed that electrostatics was an effective way to increase spray coverage. Oakford
et al. (1994b) saw no additional effect using electrostatics to charge the spray
particles, but concluded that under some conditions, electrostatics may play a role
in aiding the attachment of droplets to the target. Efforts to increase application
efficiency with electrostatic sprayers have not been as successful in orchards as in
row crops (Hogmire and Elliott, 1991).
While providing an efficient method of spray application at reduced water
volumes, the airshear system requires the purchase of new expensive machines, often
with the additional expense of a high horsepower tractor. This has meant that
uptake of this technology has not been widespread. For example, less that 10% of
Australian orchardists have chosen to make use of this expensive technology.
5.3.3. Low volume hydraulic nozzles
The performance of hydraulic nozzles has greatly improved in the last 10 years
with improved spray pattern characteristics. A change of approach in the design
of hydraulic nozzles by companies such as Delevan-Delta Inc., has led to the
development of hydraulic nozzles which are able to produce a narrower droplet
spectrum of fine droplets at lower water volumes than the traditional hydraulic
nozzles. These nozzles can now be used at lower pressures, achieving droplet sizes
of 100 to 150 µm. Examination of these nozzles fitted to a standard air-blast
sprayer has shown that they are able to operate efficiently at low pressure at volumes
as low as 200 L/ha (Bound et al., 1997b). The advantage of this technology is
that these nozzles can be fitted to existing air-blast sprayers, offering an opportunity
for orchardists to convert to low volume spray application for the cost of the
nozzles and a low-pressure gauge.
5.3.4. Reducing dosage rates
One of the benefits claimed for low-volume spraying is that chemical rates may
be reduced. Maber et al. (1984) indicate reductions of up to 50% may be possible.
However there is considerable discussion on this matter. Campbell et al. (1988) found
that low volume applications were considerably less effective in controlling both
apple scab and codling moth in an apple orchard. Cross and Berrie (1990) described
the variations found by other workers, and themselves report conflicting results when