Water & Environment, Mount Pleasant Laboraties, Launceston, Tasmania 7249, Australia. Key words: crop regulation, modelling, pome fruit, thinning. 1.
Plant Growth Regulation 31: 75–84, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands.
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Modelling thinning of Pome fruits K.M. Jones1 , S.A. Bound1 , M.J. Oakford2 & P. Gillard3 1
Tasmanian Institute of Agricultural Science, University of Tasmania, New Town Research Laboratories, 13 St Johns Avenue, New Town, Tasmania 7008, Australia; 2 Department of Primary Industries, Water & Environment, Grove Research Station, Pages Road, Huonville, Tasmania 7109, Australia; 3 Department of Primary Industries, Water & Environment, Mount Pleasant Laboraties, Launceston, Tasmania 7249, Australia
Key words: crop regulation, modelling, pome fruit, thinning 1. Introduction Thinning pome fruits has a history of unpredictability. Despite the publication of many scientific papers on this subject the use of bioregulators to thin fruits was rare before the 1970’s. In the early 1980’s attempts were made to examine simple interactions between components affecting thinning. One of the first interactions to emerge was the important one between the concentration of the active ingredient used for thinning and the timing of the spray application. Later, other components such as cultivar, rootstock, climate factors, pruning, spray technology, dosage, age of tree, vigour, last year’s crop, the required size of fruit and interaction between two or more bioregulators emerged as sources of variation. Although some researchers made an effort to standardise application conditions most experiments had substantial components of unaccounted for variation. In any multivariate situation it is not easy to apportion the causes of variation among the various components. Nevertheless, unless this is done it is difficult to see how predictable outcomes can be achieved. Modelling offered a practical and effective method of partitioning variation and quantifying its components in a rational manner. Although it was comparatively easy to build a conceptual model, quantifying the components proved more difficult. The basis for any model must be the severity of thinning required, which itself is based on crop loading. The Australian model is based on the relationship between fruit size and fruit numbers per tree. Although the concept was understood for most of this century it was well illustrated by the regressions of Forshey (10).
Understanding the implications of crop loading, expressed as either number of fruit per cm2 trunk cross sectional area (TCSA) or per 100 blossom clusters, is the basis of thinning. To construct a model, data banks must accommodate the crop loading changes with age of tree, cultivar, rootshock, pruning severity and tree vigour. Considerable amounts of verified information are necessary to construct these data banks. These start as simple two-way tables involving one variable per table and are integrated into a computerised model. The basic program deals with the blossom (primary) thinners, ethephon and naphthalene acetic acid (NAA) which are currently registered in Australia. Subsequent programs deal with post bloom (secondary) thinners. The program then selects a pathway through the interacting factors such as cultivar, tree age, rootstock, and tree vigour. The program asks 15 questions about the particular set of trees and modifies the basic treatment with the selected bioregulator depending on the answers given. The program also allows a revision of all modifications made to give the final recommendation. Provisions can be made to store and print data. This allow for a review of the position when decisions are made the next year. The value of this type of model is that the software is easy to use. It has been tested in practical conditions in Australia and has performed much better than most horticultural advisers and is now considered to be ‘best practice’ in the country. There are many lessons to be learnt from the process of model building. The gaps in our knowledge of thinning have been exposed and areas needing further research revealed. The process is ongoing and annual reviews need to be undertaken. New cultivars, new concepts and new bioregulators need to be incor-
76 porated when sufficient verified evidence is available. Although the principles remain the same, adaptations would be necessary for different areas of the world where emphasis on specific factors could be critically different.
2. Correlating scientific findings on thinning apples as a model The early timing of apple thinning with bioregulators in Australia has become more widely used and successful. Fruit size has been greatly improved and there are bonuses of better keeping quality and regular cropping. To attain this objective, thinning must be predictable or recommendations lose credibility and are not used. The challenge at present in Australia is to remove 90% of potential fruit with bioregulators during the period from blossom to four weeks later without overthinning. To do this by hand-thinning is neither practical nor economic. Reliability in thinning has been achieved by channelling research findings into a predictive model of fruit thinning. The basis for the model is a mathematical matrix based on optimum crop loading. We have used data from our experiments over the last 20 years to construct a computer databank. This information consists of data sets which are complex and multifactorial. They include combinations of prime factors that influence thinning such as cultivar, rootstock, choice of primary thinners, age of trees, fruit size required and the size of trees. Several modifiers, such as last years crop, are used to more accurately calculate the thinning recommendation to produce an optimum outcome. The system is computerised and is a pathway analysis able to deliver best available advice for orchardists. It can also quickly identify areas needing further research and the exact data necessary to enhance the model.
3. Using a model to build predictive systems Although thinning with bioregulators is now regularly practised by most Australian commercial orchards a major factor determining confidence in the practice has been the predictability of results. Unpredictability gives rise to risk and lack of confidence and hence declining use of bioregulators to thin fruit. Most orchardists are concerned about overthinning, whereas the more common underthinning is equally
as counter productive. Both lead to mistrust of the current recommendations and any research that promoted them. Recommendations need to be based on rigorous scientific findings and must be repeatable. Research has often involved only one or two aspects of the problem, leaving other important factors unexplored. Consequently, application of findings leads to inconsistencies and many countries are still relying on hand thinning. Notable exceptions to this are the USA and Australia. Although our climates differ widely, aggressive thinning policies have been adopted in both regions. Williams (35) has advocated these aggressive policies for Washington State. Stover (33) reported the development of a computerised expert system to reduce the uncertainty of chemical thinning in America. Our development of a model of bioregulator thinning has produced an expert system which gives predictable outcomes. The objective was to use existing data to act as a basis for a systematic approach through modelling. Much information was available and a lot of scientific data which had remained unused, mainly because it could not stand alone as the basis for a strategy, could when combined with other data play a useful part in verifying specific details of the overall system.
4. The modelling processes Many factors contribute to variable thinning outcomes. Jones et al (12) produced a simple model exploring the interaction of concentration of ethephon used and the timing of the spray application for ‘Golden Delicious’. A more elaborate model was developed (29) to more accurately define this thinning interaction between ethephon concentration and spray timing for this cultivar. Other factors were found to cause variations in response. These factors and their inter-relationship were illustrated (30) in the first attempt to explain thinning relationships in a non-quantitative form (Figure 1). This conceptual model illustrated the multifactorial situation and attempted to show how interactions could influence the outcome. Quantitative data on climatic effects made a contribution. It was demonstrated (29) that ethephon activity exhibited a linear temperature dependence in thinning apples, and climate, particularly humidity, could affect NAA activity (16).
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Figure 1. A conceptual model of the thinning process.
78 Spray application technology, which will be discussed separately in this edition, was identified as an important component of variation in the thinning response. The importance of spray application on thinning responses was brought to light in 1982, indicating the potential to reduce the considerable variation that occurred (6). Thinning response of large apple trees increased with larger volumes of spray when using air blast sprayers (23, 30). In describing the conceptual model of fruit thinning several fields of research needing attention were indicated (30). This was the first time the need for a fully integrated approach to thinning was demonstrated. The concept, initially qualitative, soon became quantitative as values were fitted to the model subcomponents. At this time the effect of blossom density on thinning was assessed, as were the modifications made by pruning. It was found that thinning was more difficult when the number of flower buds were reduced by pruning (15), agreeing with earlier contentions (36). This provided a useful measurement of flower bud density effects on the predictability of thinning. Ethephon was influencing thinning strategies through it’s effect of promoting return bloom beyond that expected from thinning (34, 8). Biennial bearing makes predictable thinning strategies difficult to achieve. The flower promoting effect of ethephon on cultivars and rootstocks grown under Australian conditions was confirmed (16, 17, 18). Some explanation of this effect had been shown where the vegetative growth regulation and flower promoting effects of ethephon are closely linked (17, 18). This reduction of vegetative growth and flower promotion was not observed with the other major blossom thinner NAA which can increase vegetative growth (14). This finding corroborated other evidence of the difference in action of the two bioregulators and strongly indicated separate treatment and development of specific pathways for ethephon and NAA in the model (11). The availability of two quite different bioregulators was based on early work (12, 13). An important starting concept is the reconciliation of the cultivar – rootstock combination with the primary thinner. Generally, in Australia red apples, particularly those that tend to be biennial, such as red ‘Delicious’ or ‘Fuji’ are treated with ethephon. Regular bearing green apples, such as ‘Golden Delicious’ or ‘Granny Smith’ are preferentially treated with NAA.
5. Building the framework of the thinning model To achieve the desired crop loads at different tree ages, a knowledge of the impact of crop loading on varying ages of trees was required. In Tasmanian work with red ‘Fuji’ (19, 20, 21, 22) optimum crops for various ages of trees on different rootstocks were identified. Several key issues emerged. Work with ‘Fuji’ and other cultivars, particularly ‘Golden Delicious’ and red ‘Delicious’, have indicated variables which can impact on bioregulator thinning. An important variable is the cultivar itself. There is conclusive evidence that different cultivars need quite different treatment (19, 25, 26, 27). All these variables must be accommodated to produce reliable and predictable outcomes. A considerable amount of information is also necessary to target optimum crops per tree in order to produce the crop reduction needed to give the required fruit size. Thinning strategies in Australia in the 1970’s consisted of waiting to assess the strength of natural set before attempting to thin. Usually NAA or ethephon at high concentrations were used at 14 to 28 days after full bloom (dAFB). This was often followed with later thinning sprays of the persistent pesticides carbaryl and thiram up to 60 dAFB. Subsequent work pointed to the greater effectiveness of earlier application of the thinners ethephon and NAA or early hand thinning (12, 19, 23, 24, 1). Environmental concerns in many countries, including Australia, have focussed on a reduction in chemical use, particularly of those chemicals perceived to be dangerous or persistent in plants. In our model we have attempted to apply the principal of limited chemical use to our strategies. We have also focussed on the use of bioregulators such as NAA, ethephon and benzyladenine (BA) rather than persistent chemicals such as carbaryl and thiram (2, 4). In this we have the advantage of having researched early thinning strategies in which sequential programmes of low bioregulator doses are preferred (11). Timing of thinning was shown to be a critical issue in determining fruit size and the number of fruit per tree that could attain this size. The concept of applying a bioregulator once at high dosage rates was found to be not only environmentally unacceptable but also to entail a high risk of overthinning, hence this was not commercially sustainable. The model facilitates the use of the best practice available at the time, which currently is the sequential use of low dosages of bioregulators. Confidence in changing to early timing of thinning can only be maintained if recommendations consider
79 all the factors involved and can process them into easily understood instructions. This was demonstrated in the construction of a working model for thinning (28).
to enable encoding of the information into the model. Separate models are required for each primary thinner, or combinations, that may be used. 7.2 Cultivar
6. Constructing the model The computer model formalises the decisions made in thinning preparation as a pathway form of reasoning often used by expert advisers (Figure 2). The framework of the model is based on optimum crop loading for a widely varying set of circumstances. The relationship between crop loading and fruit size, demonstrated in 1976 (10), showed that there was a negative linear relationship between numbers per tree and average fruit size. The relationship between crop numbers and fruit size requirements for each cultivar, rootstock and age combination has to be established as an integral part of model construction. Tables have been incorporated for each of the six major components of the model and the interactions between them. The data for these was accessed from our own and other scientific, published work on thinning. Fine tuning of the detail of the relationships was accomplished by the authors of this paper as more data became available. The result is computer software in the form of an expert system that can be used by operators with basic computer skills. This enables each block of fruit, in each orchard, to access best thinning practice without relying on unsustainable, unskilled or semi-skilled advice. Practices in Australia are changing quickly and some of the larger farm businesses have already obtained and successfully used early versions of this system.
The next factor influencing thinning practice is the cultivar involved. We have shown that thinning strategies for one cultivar cannot be superimposed on another (19, 25, 26, 27). Each cultivar, and each selection within the cultivar, needs separate consideration. The necessity for research on each cultivar is evident when comparing the thinning of genetically diverse cultivars such as ‘Jonagold’ and ‘Gala’ (26). Recent work with ‘Pink Lady’ has shown that strategies for thinning this cultivar need to be quite different from one of it’s parents, ‘Golden Delicious’ (27). Work with ‘Fuji’ has shown that earlier work on ‘Golden Delicious’ or red ‘Delicious’ could not be used in thinning ‘Fuji’ (19). 7.3 Rootstock Rootstocks have a direct effect on thinning outcomes. Trees on vigorous rootstocks are usually easier to thin than trees on less vigorous stocks (19). Three rootstock categories (see Figure 2) have been programmed into the model: the categories are vigorous (e.g. seedling, M25), medium (e.g. MM106) and dwarfing (e.g. M9). There is also the cultivar – rootstock interaction to consider and this has been included. To fulfil requirements of the model, considerable amounts of data were needed to modify outcomes accurately for the large number of combinations. In practice these were limited to the most probable combinations used in Australia. Interstock information is now becoming available. These data will form part of future models when sufficient work is undertaken to understand the relationships with such factors as rootstock and cultivar.
7. Major components of this model 7.4 Age of tree 7.1 Primary thinner The first decision to be made before the trees blossom is the choice of primary thinner. In Australia at present only NAA and ethephon are available as primary thinners. Alternative blossom thinners are based on the principal of burning the style and stigma of the flower. These damaged flowers produced infertile fruitlets which subsequently abscise. Research is proceeding and these will shortly become available (37, 5). However, several years data will be necessary
Optimum crop loading per tree, for a given fruit size, has been entered into the model for each cultivar, on each type of rootstock and for each year from age one to mature trees aged 10 years or over. Crop loading increases as trees increase in age and size but rates of increase are conditioned by cultivar and rootstock. Physical capabilities of various sizes of tree to grow optimum crops are also balanced by knowledge of biennial bearing. Limiting young trees to less than 6 fruit per cm2 TCSA has prevented biennial bearing in
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Figure 2. A quantitative model of the thinning spur
81 most cultivars. Considerations of larger fruit size or vegetative growth rate of young trees could limit the crop to 3 fruit per cm2 TCSA. 7.5 Size of tree Our work on spray technology has shown that tree size and shape is a critical issue in correct spray application (31, 32). The model can extrapolate the concentration of spray to be used and, given the type of sprayer being used, the correct spray volume. This ensures that the recommendation is applied as a dosage term, which is a combination of concentration and volume of spray used and referred to in our conceptual model (30 and Figure 1). The objective here is to deliver the same dosage to all flowers or fruit on the tree with minimal wastage. Correct spray application must be factored in to prevent under or overthinning or zonal inconsistencies within the tree. It was also shown that reduction of wastage can be a factor of using the correct spray volumes for the tree size and spraying machine used (3); if the factors are equated properly dosages can be reduced. 7.6 Mean fruit weight required Three mean fruit weight categories are accommodated in the matrix, large, medium and small. Generally fruit required by the domestic Australian market and for export to Asia is 150 g/fruit or larger (medium category). Recently this has increased to larger fruit (250 g) for some markets, which becomes the large category. There are still some markets in Europe and Asia that will take fruit as small as 100 to 120 g/fruit and this forms the small category. Reducing mean fruit size requirements will increase crop loading significantly. Some cultivars, such as ‘Fuji’ and spur type red ‘Delicious’ react to heavy cropping by becoming biennial. ‘Granny Smith’ and, ‘Golden Delicious’ do not show such an unfavourable reaction to heavy crops in Australia. For both ethephon and NAA a base dosage of bioregulator is calculated by the model. This is selected by the croploading weighting of the multidimensional data banks major components. These cover commonly used combinations of cultivar and rootstocks and a wide range of tree ages and sizes.
8. Modifying factors There are three factors which have further impact on thinning strategies. These can modify the effects of the major components. 8.1 Last year’s crop Bennial bearing exhibits the extremes which can be faced by thinning practice, particularly if regular and fairly aggressive thinning programs are not followed. The current year’s thinning practice must be reconciled with last year’s crop. A previously heavy crop can influence the production of flowers in the current year but can also influence the potential of those flowers to set (7). Heavy previous crops should reduce the recommended base dosage while lighter, early thinned crops should increase it. This is further modified by the current years flower bud density which is known at the time thinning decisions have to be made and can be entered into the model. 8.2 Rate of growth Vigorous vegetative growth of trees can reduce fruit set in both young (17) and mature trees (18). Three categories of vigour have been established for all trees. High vigour will reduce the base dosage rate and lower vigor increase it. 8.3 Pruning severity Severe pruning can remove significant numbers of competing flower buds rendering the remainder more difficult to thin (15). Removing competing flower buds before thinning can be an advantage in producing large fruit, particularly if the weaker flower buds are removed by selective pruning. Light pruning is favoured for larger crops of small fruit. The model will reconcile the required fruit size with pruning severity and modify the base rate accordingly.
9. Recommendations A recommendation is based on reconciling the croploading with the 6 factors and 3 modifiers. This establishes, then modifies, the dosage of the bioregulator to be used. The revision chart shows how the recommendations were arrived at. It also shows how the major factors and modifiers contribute to the result. There are provisions made to revise the input. If the
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Figure 3. Temperature and timing adjustment for ethephon application at 150 mg/L at FB.
recommendation varies from what is expected the revision chart allows the sequence to be studied in detail and gives an opportunity to revise base levels and modifications if they are considered to be inaccurate. Use of the model will be enhanced by accurate records, particularly of orchard performance. It will also, over a period, encourage more accurate record keeping, particularly of the performance of each block of fruit of single age, cultivar and rootstock. Substantial knowledge of each of the factors or modifiers, in all blocks of fruit, will enhance the information available to the model and improve the outcome or recommendation. This knowledge is often found wanting in many Australian orchards. Using the model should improve the discipline of individual block recording as the outcomes become more obvious and predictable.
10. Climate Weather has an important impact on the effectiveness of thinners. Temperature at the time of spraying can be critical. The thinning effectiveness of ethephon has been shown to increase as the ambient temperature rises (9, 14). NAA is also affected by both temperature and humidity (16). Higher temperatures also increase the thinning effect of BA (4). Calibrations of application rates for various climate conditions are applied prior to the final recommendations. Tables of options can be given when interactions between factors occur.
In the case of ethephon, the timing of the spray and ambient temperature interact significantly during the blossom period where apples can be thinned with this primary thinner (12, 14). The full combinations of options for this period plus ambient temperatures from 14 to 24 ◦ C are given. An example of this is shown in Figure 3. This gives the actual quantity of 48% active ingredient ethephon (in ml) to be mixed in 100 litres of spray water to apply the equivalent of 150 mg/L ethephon at FB and 19 ◦ C.
11. Spray application Spray application of the recommended rate is equated with the tree size, type of sprayer and volume of spray to be used. Effectiveness of thinning can be critically affected by the practical application of spray, particularly wastage from non-targeting. Final calculations for spray preparation depend on integrating these factors correctly and delivering the recommendations as ml of bioregulator per 100 L of water.
12. Conclusions The model provides the framework and a method of transferring thinning research to a useable extension tool. Based on the latest verified information it can deliver best practice to orchardists. Gaps in knowledge
83 are quickly identified. Research can be prioritised to accommodate these deficiencies. Additional cultivars, new concepts or thinners can be added to the model. As work on BA has resulted in registration in Australia (2, 4) it has recently been incorporated in the Australian model. At present the model accommodates ‘Golden Delicious’, ‘Granny Smith’, spur red ‘Delicious’, non-spur red ‘Delicious’ and ‘Fuji’. The aim is to add ‘Gala’ and ‘Jonagold’ to the model by 2000. ‘Pink Lady’, ‘Sundowner’ and ‘Braeburn’ could be added to the model by 2002. The concept of very early blossom thinning is well advanced. Research on blossom burners or desiccants has been underway in Australia for 8 years. While a number of desiccants have been found to be unsuitable for Australian conditions two are currently being developed and should be registered by 2000. There is another longer term advantage in the model. In Australia and USA, and to a lesser extent in Europe, researchers have been involved in extension activities at thinning time. This is because orchardists want to apply the latest scientific findings as soon as possible. They also want to access findings of new cultivars, new thinners and new ideas. Reliable data are not easy to access and often have not been adequately correlated. Research institutes are probably the best organisations to deliver recommendations based on complex, interactive factors and to do the research to construct reliable, predictive models. The model is able to deliver research recommendations to all orchardists who acquire the up-to-date software. The major feature is that it is a tool that can be used in the absence of the expert researcher. It also allows the researcher time to modify the model and to do the work for these modifications.
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Bound SA, Jones KM and Oakford MJ (1997) Post-bloom thinning with 6-benzyledenine. Acta Horticulturae 463: 495– 499 Bound SA and Jones KM (1997) Investigating the efficacy of endothal as a chemical thinner of red ‘Delicious’ apple. Journal of Horticultural Science 72, 171–177 Bukovac MJ (1982) Low-volume application of plant growth substances to fruit trees. In 21st International Horticultural Congress, Hamburg, Federal Republic of Germany. Abstracts Vol. 1, No. 1062 Chan, BG and Cain JC (1967) The effect of seed formation and subsequent flowering in apples. Journal of the American Society of Horticultural Science 80: 90–96 Ebert A and Bangerth F (1981) Relations between the concentration of diffusible and extractable gibberellin-like substances and the alternate-bearing behaviour in apple as affected by chemical fruit thinning. Scientia Horticulturae 15: 45–52 Flore JA and Bukovac MJ (1982) Factors influencing absorption of 14C (chloroethyl) Phosphonic acid. Journal American Society Horticultural Science 107: 965–968 Forshey GC (1976) Factors affecting the thinning of apples. Plant Sciences Bulletin 64: 1–7 Jones KM (1996) Thinning apples in Tasmania. Department of Primary Industries and Fisheries, Tasmania Jones KM, Koen TB and Meredith RJ (1983) Thinning ‘Golden Delicious’ apples using ethephon sprays. Journal of Horticultural Science 58: 381–388 Jones KM, Koen TB and Meredith RJ (1984) Thinning ‘Golden Delicious’ apples using ethephon sprays. Australian Journal of Experimental Agriculture 24: 453–456 Jones KM and Koen TB (1985) Temperature effects on ethephon thinning of apples. Journal of Horticultural Science 60: 21–24 Jones KM and Koen TB (1986) Manipulation of blossom density and the effects of ethephon thinning on ‘Golden Delicious’. Acta Horticulturae 179: 653–657 Jones KM, Koen TB, Longley SB and Oakford MJ (1988) Thinning ‘Golden Delicious’ apples with naphthalene acetic acid in relation to spray concentration, volume and time of day. Journal of Horticultural Science 63: 1–4 Jones KM, Koen TB, Wilson SJ, Oakford MJ and Longley SB (1989) A re-examination of the relationship between vegetative growth and flower bud initiation in apples. Acta Horticulturae 239: 363–366 Jones KM, Koen TB, Oakford MJ and Longley SB (1989) Using ethephon and daminozide to regulate growth and initiate flower buds on bearing Red ‘Delicious’ trees. Acta Horticulturae 240: 185–188 Jones KM, Koen TB, Oakford MJ and Bound SA (1989) Thinning red ‘Fuji’ apples with ethephon and NAA. Journal of Horticultural Science 64: 527–532 Jones KM, Koen TB, Oakford MJ and Bound SA (1990) Thinning red ‘Fuji’ at two timings. Journal of Horticultural Science 65: 381–384 Jones KM, Koen TB, Bound SA and Oakford MJ (1991) Some reservation in thinning ‘Fuji’ apples with naphthalene acetic acid (NAA) and ethephon. New Zealand Journal of Crop and Horticultural Science 19: 225–228 Jones KM, Koen TB and Oakford MJ (1991) How volume of spray affects the thinning of red ‘Delicious’ with ethephon. New Zealand Journal of Crop and Horticultural Science 19: 31–36 Jones KM, Bound SA, Koen TB and Oakford MJ (1992) Effect of hand thinning on the cropping potential of red ‘Fuji’ apple
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Oakford MJ, Jones KM, Bound SA and O’Rielly L (1994) A comparison of air-shear and electrostatic spray technology with a conventional air-blast sprayer to thin apples. Australian Journal of Experimental Agriculture 34: 669–672 Oakford MJ, Bound SA, Jones KM and O’Rielly L (1995) Use of airshear technology to reduce chemical spray rates for thinning apples. Australian Journal of Experimental Agriculture 35: 789–794 Stover E (1992) New software programme for chemical thinning. Goodfruit Grower 45: 4 Williams MW (1972) Induction of spur and flower bud formation in young apple trees with chemical growth retardants. Journal of the American Society for Horticultural Science 97: 210–212 Williams MW (1987) Aggressive chemical thinning programmes suggested this year. Goodfruit Grower 38: 8–11 Williams, MW and Edgerton LJ (1981) Fruit thinning of apples and pears with chemicals. Bulletin 299, United States Department of Agriculture. Williams, MW, Bound SA, Hughes J and Tustin S (1995) Endothall: A blossom thinner for apples. Hort Technology 5: 257–259
