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

92 Sally A. Bound
5. IMPROVING THE EFFICIENCY OF SPRAYING
Proper spray application is one of the most demanding cultural practices in tree
fruit production, and often one of the most expensive. An example of this can be
found in the citrus industry where spraying costs amount to more than one-third
of the total production cost for fresh market fruit (Muraro et al., cited in Salyani
and Whitney, 1990).
Spray application is a complex process involving numerous interdependent components.
Bukovac (1985) outlined the more significant components relating to the
efficiency of sprays applied to tree fruits:
– active ingredient (chemical, physical and biological characteristics)
– formulation of the active ingredient
– characteristics of the spray solution
– droplet formation
– spray pattern characteristics
– transport of droplets to the target
– target definition and characteristics
– environmental parameters during spray application and drying
– spray droplet : surface interaction
– spray deposit formation
– penetration of the active ingredient
– translocation of the active ingredient to the reaction site.
Even as recently as the mid 1980’s, Hislop (1986) suggested that although the
prospect of improving the efficiency of spraying is bright, there is at present no
one system to match the proven reliability of the simple traditional sprayer. Bukovac
et al. (1986) also stated that current application equipment was inadequate for efficient
chemical application, and component processes were poorly understood. An
understanding of these components and their interactions is the first step in developing
truly efficient spray application systems.
5.1. High volume application
High volume hydraulic spraying has developed in a largely ad-hoc manner from
accumulated experience of actual tree spraying operations (Morgan, 1992).
Application with hand-lances offers several significant advantages, such as complete
and uniform coverage of the tree, minimal over-dosing, and the ability for the
operator to compensate for deficiencies in droplet size and delivery of droplets to
the target. Droplet size and the interaction of individual droplets with the plant
surface are not critical in high volume spraying since a thin film of spray is deposited
over the entire plant surface.
Although the move away from hand-lances to air-blast application increased
the ease and speed of spraying and was less labour intensive, non-uniform coverage
of the target with the accompanying problem of inconsistency in chemical efficacy
became an important limitation. During the 1970’s and 1980’s, orchards in countries
such as USA and Australia converted almost entirely to hydraulic, air-blast