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

educing dosage rates to one quarter of the full rate for a range of fungicides,
insecticides, plant growth regulators and nutrients. Oakford et al. (1995) demonstrated
that reducing chemical dosage rates of PBRs to 25% of the full rate is too
low, however they achieved good results with a 25% reduction of the chemical
dose rate, which is equivalent to 75% of the full rate. Work by Bound et al. (1997b)
with low volume, low pressure nozzles fitted to a standard air-blast sprayer confirmed
that chemical dosage rates were effective at 75% of the full rate, but a
50% reduction in dosage reduced chemical efficacy.
5.3.5. Advantages of low volume spraying
Low volume spraying represents substantial benefits over traditional HV methods.
The reduction in spraying time by up to 60% (Oakford et al., 1995) means significant
savings to orchardists and allows timing of sprays to be optimised.
Environmental issues are becoming more prominent, and any methods which reduce
both atmospheric and ground water pollution require serious consideration. Hence,
the ability of low volume sprayers to reduce spray wastage and pollution from
drift and runoff are of major importance.
5.4. Spray control systems
Spray control systems are based on sensing the presence or absence of target plants
and controlling the sprayer output in an on/off manner. Giles et al. (1987) have
reported spray volume savings ranging from 28–35% in peaches and 36–52% in
apples. Developments in spray control systems have progressed to the stage where
sprayer-mounted laser or sonar sensors spot the tree and translate its profile to an
on-board computer (Rigo, 1995). Sonar sensors use ultrasonic sensors to image
the tree, while laser sensors utilise a single laser to target trees on both sides of
the sprayer. As well as turning on the sprayer, the computer controls each of the
jets so that only the correct column of spray is applied. This system is capable of
saving up to 60% of orchard spraying costs (Anon, 1995).
5.5. Air speed
Spray Technology in Perennial Tree Crops 99
The importance of the airstream in conveying the spray to the target has been clearly
demonstrated (Randall, 1971). Working with air-blast sprayers he demonstrated
that higher volumes of lower velocity air produced better coverage than lower
volumes of higher velocity air for a given energy input, and concluded that the nature
of the airstream produced has a significant effect on coverage. Furness and
Pinczewski (1985) found that converging air-jets resulted in greater uniformity of
droplet number per cm 2 on grape and citrus foliage compared with diverging airjets
when low volume sprays were applied. They discuss the energy savings and
advantages of multiple head air assisted spray machines, and concluded that turbulent
air improves the uniformity of spray coverage and improves spray penetration
into dense plant canopies. According to Furness (1997) the nature of the airstream
is far more important than droplet spectrum and nozzle type in improving spray