into spraying techniques. Early work with spray technology identified factors causing variation. .... developed using ethephon as a thinner of apples, they often showed conflicting .... many people could be affected and often objected to spraying ...
Plant Growth Regulation 31: 173–181, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands.
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Spray application technology K.M. Jones1 , S.A. Bound1 & M.J. Oakford2 1 Tasmanian
Institute of Agricultural Science, New Town Research Laboratories, 13, St. Johns Avenue, New Town, Tasmania, Australia, 7008; 2 Department of Primary Industry and Fisheries, Grove Research Station, Pages Road, Huonville, Tasmania, Australia, 7109 Accepted 7 December 1999
Key words: airshear, air-blast, CDA, high volume, low volume, spray application
Abstract Spray application technology on perennial tree crops has been a poorly researched subject. Many of the nozzles and sprayers were originally developed for use on row crops which have an even, flat profile and spraying is downward. Some of the short-comings of these systems were exposed when tree crops were sprayed, particularly the large and dense trees of 30 years ago. Much of the variation experienced in experimental results where fruit trees had been thinned with bioregulators is likely to have been due to application methods. Air-blast sprayers using hydraulic nozzles resulted in inconsistent results causing many research workers to revert to the use of hand lances and small pumps for their trials. Translation of these results to commercial practices then became a problem, as growers were reluctant to revert to the use of hand lances for thinning, particularly as orchards increased in size. The increased emphasis on reducing the use of chemicals, including bioregulators, on food and the need to use ecologically less wasteful methods of applying them has recently promoted more research into spraying techniques. Early work with spray technology identified factors causing variation. Such factors as the range and size of spray droplets, effect of climate, use of wetters, spreaders and stickers, the carrier medium (oil, water or combination), the use of propelled air, the crop species and it’s configuration all impacted on the results.
1. Development of Australian work Our work in Australia concentrated on pome fruit thinning and attempted to improve the effectiveness of application by using some form of forced propulsion of droplets. We also examined various spray patterns and the effect of limiting droplet size to fairly narrow ranges. Initially, attempts were made to improve the efficiency of spray equipment that was in general use in the early 1980’s. These were air-blast sprayers using high pressure hydraulic nozzles which produced a droplet range of < 20 microns (µl) to approximately 500 µl. In some work where dosages and volumes were compared the results were unexpected. Jones et al. [24] showed that the volume of spray used in air-blast sprayers was a critical component in formu-
lating equilibrated dosages to large trees. This led to the common practice of using high volumes of low concentration spray to thin fruit trees. As a result up to 6,000 l/ha was used on very large trees – however this process was extremely wasteful of both water and spraying time. Controlled droplet application (CDA) technology became available in the 1970’s. CDA machines were able to produce droplets within the 60–100 µl range. This reduced both the small particles which formed the majority of spray drift, and large particles, which were associated with splash loss. Several machines had the capability of conforming closely to this range and utilised spinning discs or rotary cages to produce a consistent range of droplets. Droplet size could be adjusted by altering flow rates and pressure. This allowed the alteration of spray application
174 when needed, for instance, under warm spraying conditions in some countries it is advisable to increase droplet mean diameter to allow for evaporation effects. While spinning discs and rotary atomisers were effective they were not robust, and were subject to frequent breakdowns and damage. More robust machines were developed based on the airshear principle utilising high speed turbine fans and producing droplets in the range of 50 – 130 µl. These droplets were produced from low pressure nozzles feeding liquid into the airstream. While not producing as concise a range of droplet sizes as the spinning disc or rotary cage atomisers they were shown to be superior to those of air-blast sprayers using high pressure hydraulic nozzles by Oakford et al. [38, 40]. Experiments in Australia by Oakford et al. [40] using airshear have shown that at volumes of 200 l/ha the dosage rate of thinning chemical could be reduced by 25%. Although airshear nozzles are sensitive to small changes in flow rate, air speed and fluid properties and are inherently more difficult to control than hydraulic nozzles the machines used in these experiments produced highly significant results. These scientific findings have been translated to practical use in Australia where airshear machines are commonly used for sensitive operations such as fruit thinning. The hydraulic nozzle performance has greatly improved in the last 10 years with improved spray pattern characteristics. These nozzles can now be used at lower pressures, achieving droplet sizes of 100 to 150 µl. The advantage of these nozzles is that they can be fitted to existing air-blast sprayers. Use of these nozzles in experiments has shown that volumes of 200 l/ha were effective and that dosage rates could be reduced by 25%. The outcome of this work is that volumes of spray used in Australia for thinning applications have been reduced from 4,000 l/ha on large trees to 200 l/ha. Dosage rates of active ingredients have also been reduced by 25% without reducing efficiency. Thinning results have improved, mainly because spraying application rates per hectare have been vastly decreased. Even on large orchards bioregulators can be applied at the most effective timing for all fruit. This work has contributed to a significant reduction in the variability in the effects of applying bioregulators to tree crops. At the very least it has enabled research to focus on other causes of variability which has confounded work on tree fruit thinning in the past.
2. The recent history of spray technology Mechanisation of fruit tree spraying has slowly developed since the 1940’s. More recently increases in the cost of labour and the size of orchards has accelerated the pace of change. Ineffective spraying and wastage has alerted workers involved with spray thinning of pome fruit to spray application as a source of significant variation in results. In the 1970’s the hand lance was a common way to apply thinning sprays to pome fruits in Australia. Veinbrants [43] used this method in his thinning work where he sprayed his trees to run-off. Miller and Veinbrants [34] conducted their experiments with a hand lance and thoroughly wetted the trees in a spray adjuvant trial. Most of the Tasmanian work in the 1970’s and early 1980’s used this method of spray application [17, 18, 19, 20, 22]. In other countries Kvale [32], Unrath [42] and Williams [49] found that hand lances or guns were the most effective form of applying thinning sprays. During this period orchard spraying in countries such as USA and Australia converted almost entirely to the use of hydraulic, air-blast sprayers which were developed primarily for application of fungicides and insecticides. Compared with hand lances the air-blast sprayer was much faster and less labour intensive. Spraying of growth regulators and thinners (bioregulators) followed the pattern set for pesticides in general orchard practice. By early 1980’s it was common in Australia to apply all sprays with an air-blast sprayer using water volumes of 1,500 to 2,000 l/ha, independent of tree size. It was at this stage that we were trying to transform our thinning research in Australia to practical recommendations. Trees could be successfully thinned using a hand lance with thorough wetting or spraying to drip point [19]. On large trees this equated to a spray volume of 6,000 l/ha. However, when bioregulators were applied at a third of the volume and the same dosage per hectare with airblast sprayers, thinning was significantly reduced [24].
3. Fitting high-volume spraying into the thinning equation While there was a substantial list of publications developed using ethephon as a thinner of apples, they often showed conflicting trends, making this work difficult to interpret. Trying to apply some of the information to practical thinning was also hazardous.
175 Early work in Holland by Wertheim and Joose [47], Westerlaken [48] and Scholtens and Dijke [41] used concentrations of over 500 mg l−1 ethephon to thin apples. In Norway [31] in England [27] and in the USA [10] high ethephon concentrations were also being used. At this time the potential of ethephon as a thinner was established in Australia by Veinbrants and Hutchinson [44]. Ethephon also showed a potential to combat the problems with biennial bearing described by Edgerton and Greenhalgh [11]. Later work with ethephon [19] achieved both these objectives. Concern remained about using high levels of ethephon and these were particularly focussed on high dosages of the bioregulator reducing fruit size. Wertheim [46] and Knight [28] had shown that ethephon, used at high concentrations could thin fruit but it failed to achieve the expected size. Australian work verified this finding [19]. At this stage we were aware that our preliminary model [19] did not fully describe how the thinning potential of ethephon could be handled, with considerable variation in the results unaccounted for. Using an air-blast sprayer to conduct trials, instead of hand lances, explained some of the inconsistencies of ethephon thinning. Koen et al. [30] showed that the effectiveness of ethephon as a thinner improved linearly as the spray volume increased from 1,000 to 6,500 l/ha at constant ethephon dosage per hectare on large trees. This confirmed work by Bukovac [7], who had shown that dose-response was a function of carrier volume and concentration. It was becoming clear at this stage that inconsistencies of effects could arise unless both functions were properly integrated into spray dosage calculations. As a result it was concluded that dosage per hectare of active ingredient was a better form of calibration than concentration alone. In addition air-blast sprayers were wasteful, both of carrier water and of active ingredient. Examining methods of effectively targeting trees of various shapes and size became the priority of thinning work rather than just using various concentration treatments. Jones et al. [22, 24] showed that if air-blast sprayers were used on large trees then high water volumes and lower chemical concentrations should be used. Jones et al. [24] were able to show that increasing the concentrations of ethephon to high levels did not increase the response significantly if the carrier volume was insufficient to properly cover large trees (4 m high * 5 m length and breadth). These findings enabled thinning to be consistently achieved using relatively low concentrations of ethep-
hon in volumes capable of optimum tree cover. This also achieved the required increases in fruit size as a result of both thinning effectively and simultaneously avoiding high concentrations of ethephon. These findings on the effect of concentrations on fruit size agreed with previously published data. The wastage encountered in applying high concentrations of bioregulator to thin pome fruit was demonstrated by Jones et al. [24]. This work showed that the priority was to address application technology as the most effective way to increase thinning, not to increase the concentration of bioregulator. The situation for the fruit thinner carbaryl was not so clear, with Knight [29] finding no volume related affects and Looney and McKellar [33] getting conflicting results. Jones et al. [20] showed that carbaryl sprayed at high volume at 10 or 20 days after blossom was an effective thinner of ‘Golden Delicious’. The result of our work was that high volumes of thinning spray were recommended for large trees in Australia. In addition volumes needed to be adjusted to tree size. The exact size of trees had to embrace the principle of tree-row-volume (TRV). The concept of TRV, described by Byers [8] enabled a more accurate application by firstly deciding on the optimum volume to cover the trees and then the concentration of spray to deliver the correct dosage.
4. Reducing wastage by employing improved technology Despite all the modifications used to improve accuracy, Campbell [9] showed that wastage was a problem with high-volume air-blast spraying. Although using high volumes of spray enabled the use of lower concentrations, both the targeting and retention of spray at these volumes were inefficient. In addition to wastage, environmental problems, such as drift and run-off pollution had to be considered. Small droplets did not impact and were carried as drift to other locations affecting other crops, people, animals or watercourses. In urban or semi-urban situations, where many orcharding operations are carried out, many people could be affected and often objected to spraying operations. In addition splash and run off, caused by large droplets, could pollute water tables, streams or rivers. Carbaryl contamination has been found in streams adjacent to orchards sprayed with this insecticide (S.J. Wilson, University of Tasmania, private communication).
176 Most Western countries have adopted a much tougher line on such pollution in recent years. Addressing these problems can lead to restriction of farming operations and mandatory needs to improve spraying performance with significant reductions in pollution and wastage were required. In Australia these moves have culminated in the ‘Consumers Charter’ signed by fruit growers with consumers to reduce total chemical usage on fruit trees [1]. One of the most effective methods of doing this is to improve the effectiveness of the spray applied, thus reducing wastage. Spraying with high volumes of water is also expensive. Applications at 4,000 l/ha involve filling the standard 2,000 litre airblast sprayer tank twice per hectare and spraying can take an hour per hectare. However, spraying at 100 l/ha would allow 20 hectares to be sprayed with one tank of spray. Low volume spraying is also conducted at faster ground speeds as application rates are inversely related to speed with any given set of nozzles. Oakford et al. [40] has shown that spraying time was reduced by 60% when 100 l/ha was compared with 4,000 l/ha. There was an identified need to examine alternative systems both to improve the efficiency of bioregulator use and reduce wastage to alleviate environmental concerns. This all re-emphasises the need for good quality experimental work examining the effect of spray volume on the efficacy of bioregulators.
Working with growth regulators, Bukovac [7] showed that droplet size was an important issue in efficient performance. There are obviously inefficiencies in absorption of bioregulators into the plant system but these are minor compared with losses due to non targeting. CDA machines can limit the spectrum of water particles emitted into the air stream to a narrow range (60–130 µl). Spray droplets
