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

According to Furness (2000), the number of droplets per unit leaf area (contact
density), size and concentration required for good efficacy against pests and diseases
with most agricultural chemicals is not known. However, based on experience,
optimal densities for most contact chemicals in dilute high volume applications have
been determined as at least 200 droplets per cm 2 with fine droplets, or 25–50 droplets
per cm 2 or better with medium sized droplets on all parts of the plant.
4.6. Dosage
Dosage describes the amount of chemical deposited per unit area of target and is
the result of the interaction between application rate, coverage and the canopy
being sprayed. If droplet size and density is known, the dose of chemical reaching
the target can be calculated. Furness (2000) describes how this can be done.
When the droplet spectrum is narrow, the calculation of dosage is relatively
easy. However, where a wide spectrum of droplet sizes is produced, such as in
the output from hydraulic nozzles, it becomes difficult to deliver specific chemical
doses to the target. For example, the dosage variation between a 5 µm and a
500 µm droplet is 100 3 , i.e. one million.
Good coverage of a sub-lethal dose, or poor coverage of a high dose can both
fail to provide control. The rate of chemical applied per hectare, or unit volume
of water, does not automatically determine the dosage achieved. Poor sprayer
set-up will give poor dosage and coverage. Thus poor control is achieved, even
with high application volumes or chemical rates.
4.7. Timing and weather
Spray Technology in Perennial Tree Crops 91
Timing is a critical consideration in spraying. Any spray application can only be
fully effective if it is aimed at the most vulnerable stage of the pest, weed or
disease or, in the case of PBRs, the optimum physiological stage of the tree.
Weather conditions before, during and after spraying influence spray coverage,
retention and efficacy. Bound et al. (1997a) have described the importance of
temperature, both during and several days after spraying, when applying the PBR
6-benzyladenine. Wind speed is a key weather factor. Spraying in moderate to strong
wind conditions (> 10 km/hr) is inadvisable, but spraying in light or still wind
conditions is often more dangerous. Under these conditions a thermal inversion
may cause spray to become trapped. This pesticide cloud is not dispersed and diluted
as would normally occur, but stays together in a concentrated form that can move
considerable distances. Rain too soon after application will either wash off the
chemical or cause re-activation, which may lead to phytotoxicity, while spraying
after a prolonged wet period, or under slow drying conditions may result in fruit
russeting.