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

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

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air-blast sprayers in an attempt to improve coverage in dense canopies in grapevines<br />

has been shown to actually reduce chemical deposition (Pergher <strong>and</strong> Gubiani, 1995).<br />

Despite all the modifications used to improve accuracy, wastage is still a problem<br />

with high-volume air-blast spraying (Campbell, 1985). Spray drift pollution from<br />

hydraulic air-blast sprayers, caused by small droplets <strong>of</strong> < 50 µm was described<br />

by Moore (1990), Walklate (1991) <strong>and</strong> Hobson et al. (1993). According to Fox et<br />

al. (1990) this drift can be deposited up to 100 m from the air-blast sprayer source<br />

<strong>and</strong> is a major problem in urban <strong>and</strong> semi-urban areas.<br />

The high proportion <strong>of</strong> large droplets produced by traditional hydraulic pressure<br />

nozzles used in high volume air-blast spraying means that there is considerable<br />

wastage <strong>of</strong> spray, apart from drift, through either run<strong>of</strong>f or deflection <strong>of</strong> large droplets<br />

from the target. Splash <strong>and</strong> run<strong>of</strong>f can pose a larger problem than drift. As well<br />

as leading to possible phytotoxicity through an accumulation <strong>of</strong> spray liquid at<br />

the leaf tip, run<strong>of</strong>f also leads to soil contamination. Douglas (1995) reported on<br />

investigations that found that only 14% <strong>of</strong> a 10,000 L/ha application, a common rate<br />

in citrus orchards, remains on the tree; the rest is lost as drift <strong>and</strong> run<strong>of</strong>f. Active<br />

ingredients have been detected in both ground water <strong>and</strong> streams in horticultural<br />

areas in Australia as a result <strong>of</strong> this (Jones et al., 2000).<br />

Praat et al. (2000) report that canopy development has a major influence on spray<br />

drift, with 25 times less drift from a fully foliated canopy compared with a dormant<br />

canopy. They also found that the proximity <strong>of</strong> the sprayer relative to the edge <strong>of</strong><br />

the sprayed block was an additional major factor influencing spray drift.<br />

5.2. Tree row volume<br />

Spray Technology in Perennial Tree <strong>Crops</strong> 95<br />

Most chemical applications in tree crops, particularly with air-blast sprayers, have<br />

been made on the basis <strong>of</strong> a specified rate per hectare regardless <strong>of</strong> tree size or<br />

spacing, or foliage density. However it is illogical to apply the same amount <strong>of</strong><br />

chemical to small trees as to large trees or to disregard the planting density or foliage<br />

density. Byers et al. (1971) described the concept <strong>of</strong> tree-row-volume (TRV) which<br />

is based on volume <strong>of</strong> crop foliage rather than l<strong>and</strong> area. TRV is the volume occupied<br />

by the foliage <strong>of</strong> the crop <strong>and</strong> is calculated from the height <strong>and</strong> width <strong>of</strong> the tree<br />

<strong>and</strong> the total length <strong>of</strong> row in a hectare. The TRV concept proposed by Byers et<br />

al. (1971) has reference to a ‘st<strong>and</strong>ard’ apple canopy, which has been widely accepted<br />

in the American literature to consist <strong>of</strong> trees 6.1 m tall, 7.0 m wide, planted at<br />

10.7 m row spacings (designated as the United States TRV system, or US-TRV).<br />

Successful pest <strong>and</strong> disease control has been achieved on such trees using dilute<br />

spray volumes <strong>of</strong> 3740 L/ha (400 US gallons/acre) <strong>and</strong> this was used as a ‘base spray<br />

volume’ for US-TRV coverage estimates (Manktelow <strong>and</strong> Praat, 1997b). The US-<br />

TRV calculation assumes that a row <strong>of</strong> trees can be described as a rectangular box<br />

<strong>and</strong> the volume occupied by canopy per hectare is calculated on that basis.<br />

Manktelow <strong>and</strong> Praat (1997a) suggest that seasonal growth in mature, slender spindle<br />

blocks can give a 30% increase in TRV from dormant to full leaf. Wilton (1996)<br />

suggests that TRV needs to be calculated two or three times during the season <strong>and</strong><br />

spray volumes adjusted accordingly to allow for tree growth. Boucher (1999) discusses<br />

the use <strong>of</strong> the spray volume factor (SVF) to adjust TRV calculations. The

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