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

96 Sally A. Bound
Table 2. Spray volume factors in deciduous fruit trees (Boucher, 1999).
Foliage density Spray volume factor (SVF)
Dormant trees 75
Low density (early season sprays) 100
Medium density 125
High density (mid to late season sprays) 150
SVF equals the number of litres of spray retained by 1000 m 3 of TRV sprayed to
run-off and varies depending on the density of the foliage within the tree canopy.
Dormant trees have a lower SVF than trees in the middle of the growing season
(Table 2).
The method of calculation of TRV used in the USA has been questioned by Wilton
(1996) and Manktelow and Praat (1997a, b). They suggest that the US-TRV system
doesn’t work in New Zealand because New Zealand trees are more triangular in
shape than the rectangular US trees. The US system of calculating TRV multiplies
tree height by maximum spread and divides this by row spacing and, according
to Manktelow and Praat (1997b) over-estimates spray volumes required in New
Zealand canopies by up to 70%. By either halving the TRV estimated from a
rectangular profile or measuring canopy spread at half-metre height intervals, adding
together the stack of smaller rectangles to give a whole tree TRV, they have improved
spraying efficiency.
The use of the TRV method to calibrate commercial air-blast sprayers has been
demonstrated to reduce variability in thinning (Herrera-Aguirre and Unrath, 1980).
Byers et al. (1984) concluded that adjustment of the chemical application rate for
an orchard using TRV estimates may greatly reduce the variability in chemical
deposits and subsequent responses observed. However, despite the evidence showing
the benefits of the TRV system and active promotion over the last 20 years, the
majority of orchardists have resisted its uptake due to its complexity.
Furness et al. (1998) have proposed a simpler method of calibration and chemical
labelling which is based on a unit canopy size and length of row. This unit canopy
row (UCR) for fruit trees and grapevines is defined as 1 m high × 1 m wide ×
100 m of row length (100 m 3 of foliage). They claim that this method is a simple
alternative to the TRV method which has been rejected by orchardists. The UCR
system has been assessed in both Australia and New Zealand and has achieved
excellent control.
5.3. Towards Low Volume application
In low volume spraying, the entire plant surface is no longer completely wetted,
as in high volume application. Rather, a number of discrete droplets are deposited
per unit area to provide adequate coverage. The key to successful low volume
application lies in using nozzles which deliver droplets in a narrow range of sizes
between 100 and 170 µm in diameter.