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

sprayers which were developed primarily for application of fungicides and insecticides.
Spraying of PBRs followed the pattern set for pesticides in general orchard
practice. By the early 1980’s it was common in Australia to apply all sprays with
an air-blast sprayer using water volumes of 1500 to 2000 L/ha, independent of
tree size. Although both Kvale (1977) and Unrath (1978) found that hand-lance
sprays were more effective than air-blast sprayers, Koen et al. (1986) showed that
this was mainly due to the higher volumes used with hand-lances. Working with
PBRs, these workers showed that the effectiveness of the bioregulator ethephon
(2-chloroethyl phosphonic acid) improved linearly as the spray volume increased
from 1000 to 6500 L/ha at constant ethephon dosage per hectare on large trees.
This confirmed work by Bukovac et al. (1986), who had shown that dose-response
was a function of carrier volume and concentration. Jones et al. (1988, 1991) also
showed that if air-blast sprayers were used on large trees then high water volumes
and lower chemical concentrations should be used. If the carrier volume is insufficient
to properly cover large trees, increasing the concentration of chemical does
not increase the response (Jones et al., 1991). Unrath (1994) confirmed this by
demonstrating decreased activity at a fixed concentration of active ingredient (a.i.)
as volume decreased.
5.1.1. Spray thinning highlights deficiencies
In apple orchards, spray thinning has been established as an accurate method of
assessing the value and efficiency of spraying systems. Chemical thinning agents
such as ethephon are polar and are not translocated within the tree (Giulivo et al.,
1981; Nir and Lavee, 1981), hence coverage is critical in achieving optimal results.
Work by Oakford et al. (1991, 1994a, b, 1995) and Bound et al. (1997b) over a
ten year period has reinforced the use of thinning as an accurate gauge of spray
application effectiveness. Wilton (1996) states that ‘. . . chemical thinning probably
has the most demanding specifications in regard to coverage of all the spraying
we do. I therefore believe that if it is possible to obtain satisfactory thinning results
with low volume spraying, it should not be too difficult to obtain very satisfactory
results for pest and disease control with low volume application techniques.’
Bukovac (1982) suggests that plant bioregulators respond differently as carrier
volume is altered, and that spray deposition is less uniform as carrier volume is
decreased with air-blast sprayers. He reports that deposit on the lower quadrant
adjacent to the spray lane is often 3–5 fold greater than in the top centre of the
same tree. With the narrow range for some growth substances between inadequate
response and phytotoxicity, such variations in spray deposits result in over-dosing
the lower portion while an inadequate dose is deposited in the tops of trees.
5.1.2. Canopy structure
Spray Technology in Perennial Tree Crops 93
Alteration of canopy structure and density can assist in improving coverage. Byers
et al. (1984) found that spray deposit increased with a decrease in tree size and
was inversely related to canopy density as indexed by light penetration. Ferree
and Hall (1980) reported the greatest spray deposit recovery from trees trained to