Managing avocado tree balance by pruning

Transcription

1 1 INTRODUCTION Managing avocado tree balance by pruning Alvaro Vidiella, NZ Avocado October 2014 The best practice guidelines presented in this document are a summary of the knowledge currently available on how to use pruning to balance avocado trees. The preparation of this document has involved growers, contractors, consultants, and scientists that have participated in the development of different pruning methods aimed to balance avocado trees. Not much scientific work has been done on this subject, so much of the information presented in these guidelines is based on anecdotal evidence rather than on replicated trials. Two five year projects led by NZ Avocado have started in New Zealand in 2014 that will help us answer some of the many questions we still have about pruning to balance avocado trees. This document will be updated on a regular basis to include the knowledge derived from those projects. Pruning to balance trees has been practiced by some growers in the Far North for almost 15 years now and in the Bay of Plenty for almost 10 years (Elmsly, 2010). In recent years, more and more growers and rural professionals claim that pruning is playing a very important role in the balance of our orchards, and the interest in balancing trees by both, flower pruning and structural pruning, has grown exponentially. Flower pruning gained interest from late 2000s and its understanding was promoted through industry activities (Dixon, 2008; Mitchell, 2009; Elmsly, 2011). Parallel to this, structural pruning has been discovered by our industry after years of considering that the only effective strategy to canopy management was to make room in the orchard for larger trees by thinning and to buy larger Hydraladas as the trees grew bigger. In the last years, with the practice carried out by growers and contractors in our orchards, our understanding of the role of flower pruning and structural pruning in tree balance has evolved. Many growers and contractors now consider a correct strategy to carry out structural pruning at times and in ways in which it has an effect on tree balance, and use flower pruning when it is clear that structural pruning has not achieved this goal. This doesn t mean that we already know how this is done for certain, but, with time, the industry has developed tools that we are confident that will play an important role in the mitigation of irregular bearing. This document attempts to describe what we know about these tools. Of course, even though pruning could be considered to be the most effective method to balance avocado trees, it is by no means the only management practice that needs to be considered when aiming to balance avocado trees. What is suggested in these guidelines should be considered in the context of all management practices that are known to influence total yield and its consistency. 2 IRREGULAR BEARING = ALTERNATE BEARING + EXTERNAL STRESSES Irregular bearing results from the combination of alternate bearing and additional external stresses. Alternate bearing is part of the ecological behaviour of many perennial plants including avocado trees. Alternate bearing trees seem to have decided that what is best for the continuity of the species is to use all available resources to produce as much fruit as it can in any particular season, often to the detriment of fruit production the following season. Avocados, as good examples of alternate bearing trees, tend to follow a natural cycle of high production (exhausting) seasons and low production (recovery) seasons. However, this alternate bearing behaviour is altered, and usually exacerbated, by more or less frequent external stresses that can result in a low (or nil) production season. In most cases these external stresses are environmental in nature, such as frost. In conclusion, alternate bearing, in combination with these external stresses results in irregular bearing. 1

2 What can we do to mitigate irregular bearing? We basically need to do two things. We need to manage our trees to balance them and we need to avoid as much as possible any external stress that can drastically reduce the production of our trees. To have balanced trees will mainly require: maintaining the plant nutrients at the right levels, maintaining the right soil conditions to maximize root health, controlling pests and diseases, following an adequate canopy management strategy, and, of course, balancing competition of the different seasons crops that the tree is carrying (e.g., in spring: flush, flowers and mature fruit). Avoiding external stresses mainly requires identifying the main risks present in the orchard and implementing all possible protection measures to avoid them. Some of the main risks that have been identified by industry professionals in New Zealand orchards are frost, drought during key phenological moments, unsuitable environmental conditions for pollination, heavy continued rainfall, high winds, high pest or disease pressure, and cold temperatures during flower and fruitlet development. This document deals mainly with how to identify unbalanced trees and with how to manage these trees to help them reach their balance. 3 BALANCED AND UNBALANCED TREES To produce high crops in successive years, an avocado orchard must have a high proportion of balanced trees. A balanced tree carries a balanced ratio of reproductive structures (flower buds, flowers and fruit) and vegetative structures (mainly flush). For example, in early summer a balanced tree will have produced a sufficient crop, will have enough flowers to potentially set a sufficient crop for the next season, and will produce sufficient flush to support the flowers for the following crop (Figure 1). 2

3 Figure 1. This young tree produced the equivalent of 15 ton per hectare in 2013, has a fruit load sufficient to produce again 15 t/ha in 2014, and has grown enough new flush to secure good flowering intensity for 2014, which would eventually be the 2015 crop. It is obvious that at orchard level it is highly desirable to have balanced trees and that at industry level it is highly desirable to rely on a high population of balanced orchards. Industry data show that there are a number of balanced orchards spread through NZ avocado growing regions (Figure 2. A.), but orchards that have highly unbalanced trees are a large majority (Figure 2. B). Figure 2. A. Average tons per hectare of export and local market fruit of the orchards with highest yields and lowest Irregular Bearing Index (IBI) in the different growing regions of NZ. The graph shows average data for 31 orchards from the Bay of Plenty, 14 orchards from Far North, 12 orchards from Whangarei, and 2 orchards from the rest of New Zealand. B. Average tons per hectare of New Zealand avocado orchards. Avocado trees carry several crops at any time during its annual phenological cycle. A balanced tree is a healthy tree able to sustain crops in adequate proportions for the current and future seasons (Table 1). 3

4 Balanced tree Table 1. Simultaneous crops carried by a balanced avocado tree at different times. Spring year 2013 Summer 2014 Autumn 2014 Spring year 2014 Spring year 2015 Crop 2013 Sufficient mature fruit Crop 2014 Sufficient flowers Sufficient fruit set Sufficient fruit load Sufficient mature fruit Crop 2015 Sufficient flush Sufficient flower induction Sufficient flower buds Sufficient flowers Sufficient mature fruit Crop 2016 Sufficient flush Sufficient flowers Crop 2017 Sufficient flush An unbalanced tree has flowers, fruit and flush in a proportion that is inadequate to support consecutive sufficient crops. In Table 2 and Table 3 two extremes of unbalanced trees are described. Table 2 An unbalanced tree with an excessive number of flowers in spring has the potential to set an excessive crop for the following season, which will lead to the production of insufficient flush and, consequently, to the presence of insufficient flowers in the following spring: Unbalanced tree Spring 2013 Summer 2014 Autumn 2014 Spring 2014 Spring 2015 Crop 2013 Insufficient mature fruit Crop 2014 Excessive flowers Excessive fruit set Excessive fruit load Excessive mature fruit Crop 2015 Insufficient flush Insufficient flower induction Insufficient flower buds Insufficient flowers Insufficient mature fruit Table 3 On the contrary, a tree with little flowers in spring will usually set an insufficient crop for the following season, but will most likely produce a large amount of flush that will lead to excessive flowering the following season: Unbalanced tree Spring 2013 Summer 2014 Autumn 2014 Spring 2014 Spring 2015 Crop 2013 Excessive mature fruit Crop 2014 Insufficient flowers Insufficient fruit set Insufficient fruit load Insufficient mature fruit Crop 2015 Excessive flush Excessive flower induction Excessive flower buds Excessive flowers Excessive mature fruit As can be seen from the tables above, we are talking about a cyclic process. In most blocks, most of the trees coincide in their balance status. In any particular block most trees tend to be either balanced or unbalanced. However, it is common to have trees with different balance status in any particular block (Figure 3). Figure 3. In this orchard most of the trees are balanced. However, there are some trees that are not balanced. The tree on the left has little fruit and a large amount of flush, while the tree on the right has a very large amount of fruit and very little flush. For this reason, it is necessary to consider that although in a block most trees will be in a similar balance situation, it is important to pay attention to each individual tree to manage it according to its condition. 4

5 4 PRUNING TO BALANCE AVOCADO TREES From the moment that we notice that the tree will carry a large crop it will be very important to ensure that the tree is in the best possible situation to carry that crop. Basically, among other things, it will be very important that plant nutrients are kept at the right levels, that soil moisture is managed adequately, and that pests and diseases are controlled correctly. If this is the case and the tree canopy has the right size and shape for the tree spacing of the orchard, we could then focus on balancing competition of the different seasons crops that the tree is carrying by pruning. An unbalanced tree has an inadequate proportion of reproductive and vegetative structures. Pruning can help balance a tree by modifying these proportions by removing a portion of the organs that are in excess at key phenological moments. Pruning has been trialled to balance avocado trees for a very long time with different degrees of success. There are references that go back to the first half of the 20 th century. Today, a number of growers in NZ seem to be achieving high proportions of balanced avocado trees in their orchards using pruning as their main tool. For example, Mary and Warwick Coles have been doing structural pruning in their Bay of Plenty 2,5 ha orchard for more than 10 years (Figure 4). They remove a significant amount of fruit every time they prune. They think that pruning has played an important role in making their orchard a very regular and highly producing orchard, with an Irregular Bearing Index (IBI) of 15 (out of 100, very low) and a an average yield of 17 t/ha. Figure 4. Warwick Coles indicating where he would be cutting to remove some fruit. Ian Fulton believes that flower pruning in the last years in his Far North 8.3 orchard has helped him to achieve his average yield of 13 t/ha and IBI of 13. He removes excess flowers from the areas in the canopy where flower intensity seems to be excessive by cutting about 2/3 of each panicle, back to 3 to 4 cm diameter wood. Pruning to balance trees can be classified in 3 different categories: structural pruning, flower pruning and fruit pruning. 4.1 Flower pruning Flower pruning is commonly referred to as the act of eliminating part of the inflorescences of the tree by cutting them off. In most cases flower pruning is aimed to reduce the tree s setting capacity, reducing the risk of setting an excessive crop and inducing flush growth during spring. 5

6 This type of pruning is the most commonly used form of pruning to balance avocado trees in our orchards. There seems to be consensus about flower pruning being an effective way of avoiding an excessive fruit set and obtaining good flush growth to sustain a return flowering the following spring. Farre et al. (1987) observed that flower pruning done in Spain in March (equivalent to September in New Zealand) did not reduce the crop the following year and reduced alternate bearing drastically. However, no further scientific evidence has been produced in the last years about its effectiveness or about the right timing of pruning or of the right intensity to obtain best results. Most of the practice at the moment is based on anecdotal evidence. One quite clear case of the effectiveness of flower pruning can be seen at an orchard in Katikati where half of a large tree was flower pruned in early spring 2012, leaving a vertical division between the pruned and the unpruned parts of the tree (Figure 5). Pruned Not Pruned Figure 5. Michael Dillon flower pruning half of the canopy of the tree on the right in spring Both halves set large amounts of fruit that spring, but flush growth was much more intense in the pruned part. In spring 2013 a slightly larger amount of fruit was harvested from the unpruned half, but, while the pruned part had practically no flowering and fruit set, the pruned half set a similar crop to the previous one as seen in the photos of Figure 6 taken in April

7 Pruned Not Pruned April Pruned Normal crop Not Pruned No fruit April 2014, detail of the canopy Figure 6. Tree that had half of its canopy flower-pruned in Spring Advantages of flower pruning: It induces flush development before flower induction (i.e., before the time buds can receive the signal to become floral buds, thought to happen from mid-summer to mid-autumn in NZ). This increasing the chances of having a reasonable flowering intensity the following spring with the potential of leading to a significant return crop. The flush growth will cover the fruit, protecting it from sunburn and frost. It reduces the amount of fruit that the tree sets which is also thought to yield larger fruit at harvest. It reduces tree resource depletion. 7

8 Original cut Figure 7. Branch flower pruned in spring showing fruit wet and flush development the following May. (Photo Jerome Hardy) Flower pruning cuts The types of cuts usually done when flower pruning vary mainly depending on the diameter of the cuts. Some growers do few larger cuts targeting a position near the base of the branch that is carrying the panicles while others tend to make a large number of small cuts eliminating just the apical panicle. A survey carried out in 2010 concluded that cuts 4 to 6 cm in diameter were more effective than thinner cuts in producing return crop (Elmsly, 2010). Most growers seem to aim for cuts with a diameter of about 3 to 6 cm, removing several panicles with every cut. 8

9 Original cut Figure 8. Effect of early flower pruning with cut done on a 2 to 3 cm diameter branch Flower pruning intensity If too little is cut there will be no response and if too much is cut the current season crop may be compromised. The amount of flowers removed by flower pruning will depend on how unbalanced the tree is, being higher in trees carrying an exceptionally intensive flowering. Growers usually eliminate from 30% to 50% of the flowers of the tree, though it is difficult to measure this in the orchard, and there is no scientific information about the effects of different flower pruning intensities on trees with different balance condition. However, growers are usually surprised at the large amount of flowers on the orchard ground after flower pruning (Mitchell, 2009) Flower pruning timing The time to prune trees with excess flowering potential goes from the autumn previous to flowering, runs through the flowering period, and ends a short time after flowering, during fruitlet development. In a survey carried out in 2010 the timing that seemed to yield the best results was flower pruning by the first week of September (Elmsly, 2010). Some contractors recommend that the flower pruning is initiated by early September and that it is finished by the end of October. Others extend this period from the beginning of August to the end of November. The main factors that need to be considered when deciding when to flower prune are: Effect on the following season s flowering: the earliest the flower pruning is done, the more intense the effect on the following season s return bloom. Most contractors and experienced 9

10 growers consider that flower pruning from right before flowering until shortly before the end of flowering has the largest effect on the following season s flowering intensity. Risk of frost damage: a late frost can reduce significantly the flowering potential of an avocado tree. Consequently it is usually recommended to wait until the frost risk has passed, or the effect of frost on the flowering potential has been determined before flower pruning. Risk of low fruit set: In orchards where fruit set has been low in previous seasons, flower pruning towards the end of the flowering period or even a bit later, once fruit set has been defined, should be considered. However, the benefits of flower pruning when it has been delayed to this point are not clear. 4.2 Structural pruning Structural pruning consists in the removal of a significant part of the canopy of the tree to shape and size the tree. Consequently, while the main objective of structural pruning is to shape and size the tree, it will inevitably have an effect on tree balance. The main differences with flower pruning are that structural pruning cuts are usually done on larger diameter branches, eliminating relatively large limbs, and that this pruning is done at any time of the year, not necessarily close to or during flowering. By removing a proportion of the canopy, resources in the main branches and root system will be allocated to a smaller canopy volume which means that each remaining canopy element will have a greater chance of receiving a larger amount of resources. On top of this, structural pruning also usually results in an increase in light reaching the remaining canopy elements. As a result, remaining canopy elements will usually be stronger after pruning and more likely to produce stronger flush, leaves, flowers and fruit. Structural pruning is carried out by many New Zealand avocado growers following a wide range of methods. Anecdotal evidence about the effectiveness of structural pruning to balance trees is becoming more abundant each season. Even though a trial carried out in Katikati in the early 90s showed more regular production in trees that had been pruned compared to unpruned trees (Thorp and Stowell, 2001), there is very little more available scientific information to confirm the role of structural pruning in avocado tree balance. On top of this, there is very little objective information about the actual pruning methods being followed and about their advantages and disadvantages. One of the projects started by NZ Avocado in 2014 is aimed to gather and validate this information which will help growers make informed decisions about structural pruning. The pruning trial that NZ Avocado carried out from 2011 to 2014 on 10 orchards showed that in some of the orchards of the experiment, removing 30% of the canopy in autumn while working on its structure seemed to lead to more balanced trees. However, the results were not conclusive and further work is being carried out with two new projects that will help recognize the effect of structural pruning on tree balance. Structural pruning is in many cases enough to eliminate sufficient flowering potential and balance an avocado tree (Figure 9). However, in many occasions this pruning needs to be complemented with a lighter prune during flowering. To this end, the spring following the structural pruning intervention many growers assess the flower intensity of their trees and proceed to flower prune if they consider that the flower intensity is still excessive. 10

11 Figure 9. Photo taken in July of a tree that had been just pruned going into an on year, in a Katikati high producing orchard where the aim is to balance the trees with only structural pruning. Some growers consider structural pruning in late summer-early autumn to be as effective as flower pruning. On top of this they think that structural pruning does not have some of the inconveniencies of flower pruning, like that flower pruning produces a denser canopy that reduces access and that it needs to be done in a relatively short period of time, at a time when most of the contractors are busy picking. Some growers consider that structural pruning reduces the risk of having low fruit set in a marginal spring compared to flower pruning where the strongest flowers of the tree, the apical flowers of the largest panicles, are removed. However, some consider that after flower pruning there is always enough strong flowers to set a crop even in marginal conditions. Unfortunately we have no data that can corroborate one or the other hypothesis. Some contractors claim that they have been able to balance trees doing structural pruning every 2 to 3 years and doing flower pruning in the in between years on the trees that seem to require it. There are several structural pruning methods at different developmental stages in our industry. To implement any of these methods it is important to first determine the objectives of the pruning strategy, starting with what is the aimed size and shape of the trees. This is determined by the expected final spacing of the trees, combined with the light harvesting strategy of the orchard and by the management conditions of the orchard, i.e., the way spraying, picking and other activities are carried out in the orchard Structural pruning intensity related to tree balance In the pruning trial carried out in 10 orchards by NZ Avocado from 2011 to 2014, two pruning intensities, 15 and 30% of annual canopy removal, were tested. In most orchards only 30% seemed to have some effect on the balance of the tree. Contractors and growers involved in the trial were of the opinion that it would have been better to have a comparison between 30 and 50% of canopy removal. 15% was not sufficient to show any effect on tree structure and tree balance. 30% showed some effect but in many cases seemed to be insufficient. 11

12 4.2.2 Structural pruning timing related to tree balance In New Zealand structural pruning is done at different times during the year depending on the fruit load of the trees, on when the fruit is harvested and on availability of labour and machinery to do the job. Most of the structural pruning in NZ orchards is done during autumn and winter. However, in some regions some growers are pruning their trees during spring. Some growers prune twice a year, doing most of their pruning in autumn, approximately removing about 70% of what will removed that year, and removing the remaining 30% in spring. Autumn and winter pruning As mentioned before, the majority of NZ growers are doing most of their structural pruning during autumn and winter. The main inconvenience of this method is that at this time of the year trees usually carry large amounts of fruit in the areas that are usually targeted to be pruned. Growers that have been pruning for several years and are convinced of the advantages of their pruning strategy have become used to losing some fruit with the expectation of controlling size and shape of the trees and of achieving a more consistent production. They believe that losing this fruit is necessary to have less but stronger flowers and more flush the following spring. Spring pruning The time that is considered best for pruning avocado trees in many avocado producing countries is spring, in particular immediately after an early harvest and before flowering. In situations when this window between the end of harvest and flowering exists, most of the pruning is targeted to this period. This practice avoids fruit loss and increases the effect of pruning on balance by removing flowering capacity at the time considered as ideal. Leonardi (2005) reported a higher response in shoot growth in trees pruned right after harvest, several months prior to flowering in subtropical southeast Queensland in Australia, compared to shoot growth of unpruned trees and of trees pruned later. From anecdotal experience it is considered that in NZ conditions the regrowth stimulated by pruning will have a very good chance of flowering the following spring if pruning is done before the end of November. However, in NZ conditions this timing is not always possible since trees can carry fruit throughout the year. Consequently, it is usually not practical to prune in spring when the trees are loaded with valuable fruit even during flowering. Some growers claim that they are being successful at solving this problem by strip picking or harvesting as much fruit as possible from the trees or limbs that need to be pruned before flowering. 4.3 Fruit pruning It is has been observed that thinning fruit at early stages of fruitlet development in Hass increases the final fruit size (Lahav, 1969), but has no effect if it is done too late (Kohne and Schutte, 1991). However, if there is little information about how flower pruning and how structural pruning contribute to balance the trees, there is even less information about the effects of thinning fruit on tree balance, even if the role of thinning fruit on the balance of the tree has been discussed as early as in the 1930 s (Hodgson, 1934). When we have a tree with too much fruit, it seems logical that by removing the excess fruit we could contribute to its balance, as occurs in other horticultural crops. Of course, it is much more difficult to thin fruit in avocados because they are such big trees. However, many experienced growers, contractors, and consultants think that removing some of the fruit in a heavy set could have some positive influence on fruit size and leave the tree less exhausted, with a better chance of having a return flowering able to set a significant crop, at least, with a better chance of ending up with a healthier tree. On top of this, it has been pointed out that removing excess fruit might help to bring 12

13 on more flush which will protect fruit from sunburn and frost. As a matter of fact, there are some growers (and some of them that have been pruning for a long time) that have started to thin fruit by pruning fruit off from the areas of the trees that have set excessively. However, the benefits of fruit pruning, if any, will need to be assessed carefully to prove its value. Removing fruit from avocado trees has its risks. Fruit drop in late summer and early autumn could reduce substantially the amount of fruit of the trees even after thinning, which could compromise the tree s final load. On top of this, if the fruit is removed by pruning, a part of the foliage will be removed with the fruit, reducing the photosynthetic capacity of the tree (Figure 10, bottom image). In New Zealand, fruitlet pruning to thin the number of fruitlets at early stages of fruit development to induce flush development in unbalanced trees has been practised by far less orchardists than flower pruning. Therefore even anecdotal experience of this practice is very limited and usually refers to few trees in an orchard. A trial conducted by NZ Avocado and Plant and Food Research has been running from the end of 2013.: o o o o Preliminary analysis of the data shows: more vegetative growth (flush), larger fruit, and more or the same amount of fruit in the trees pruned in December than in the trees not pruned. The trees pruned in February had less vegetative growth, larger fruit and less or the same amount of fruit than the control, unpruned trees. The orchard was heavily frosted in May and August which will make it difficult to see if there was any effect on the return flowering and return crop. Starch samples have been collected from the trees and return flowering is being assessed. A new season of this project has started in a new orchard in spring The orchard is carrying what seems to be an excessive flowering. The trees were divided in 4 groups: flower pruned in mid October; flower pruned in late November; fruit pruned in late December, and unpruned. Little is yet known about the intensity and timing of fruit removal to have better results and about how to do it in a cost effective manner. However, the following can be said about this practice in New Zealand orchards. o o Intensity of fruit removal: Depending on how unbalanced a tree is, pruning intensity usually varies from 10 to 40% of the flowers and 20 to 60% of the fruitlets of the tree. The tree must be able to support the development of the fruit that remains on the tree. In a block of mature healthy trees planted at 10x10 m spacing, expected to produce 20 tons/ha: Each tree will have to hold an average of 200 kg of fruit. At an average weight of 240 g per fruit (count 23) 200 kg of fruit on a tree would represent 850 fruit per tree. To account for possible drop of up to 30 to 50% of the fruit in late summer and autumn, an average of 1200 to 1600 fruit should remain on a tree to end with the target 850 fruit. Method of fruit removal: Until now, fruit pruning has been done by cutting the tips of branches with high fruit loads (Figure 10). This is preferably done on branches with yellowish leaves, with no flush, or on lower branches holding fruit that will touch the ground as it develops (Figure 11). 13

14 Figure 10. Top, full view and detail of avocado tree with excessive fruit set. Bottom, a tree with excessive fruit set after it has been fruit pruned. 14

15 o Figure 11. Type of branch targeted with fruit pruning. Timing of fruit removal: In the case of heavy fruit set, the sooner the tree is pruned the larger the chance of flush development and, eventually, flower induction. Flower induction in NZ avocado trees is supposed to happen along a period of time that may start as early as before February, and may continue until as late as March (Dixon et al., 2006). However, the factors that influence flower induction in an orchard are still unknown. Anecdotal experience shows that the production of flush that carries flowers the following spring on branches pruned in late summer is possible but very variable. On top of this, the chances of fruit set on this lately developed flush are believed to be low. Trials done overseas show influence of 100% fruit removal on next season s flower intensity as shown in Table 4. Table 4. Effect on return flowering of removal of 100% of young fruit at different times during summer and autumn in an orchard in California. The months in blue are the equivalent for the tree cycle in the southern hemisphere (Schaffer et al., 2013). 15

16 Dec. Jan. Feb. March April May June July If pruning is done too late for flower induction to occur, it would however be beneficial by inducing flush development that would eventually cover the developing fruit, protecting it from sunburn (Figure 12). Figure 12. Photos taken in early May in an orchard where some trees were fruit pruned in late December. Left: tree with excessive fruit load that was not fruit pruned. Right: tree that was carrying excessive fruit set and was fruit pruned in late December. There is still a significant amount of fruit on the tree that was fruit pruned, but the fruit is underneath the new induced foliage. 5 OTHER MANAGEMENT PRACTICES DIRECTED TO BALANCE THE TREES 5.1 Nutrition Nitrogen is thought to induce flush development. Consequently, the application of the correct amount of nitrogen soon after flowering in trees with high flower intensity is thought to favour flush development. How much to apply? Unfortunately, the knowledge based on scientific evidence available on what is the correct amount of nitrogen to be applied in a NZ avocado orchard is not large. In general terms it can be said that the nitrogen application in trees with heavy fruit set and low flush development could be increased in as much as 30 to 60% during the 6 to 8 weeks following flowering compared to the amount that would be applied to a tree that is actively 16

17 flushing. However, any particular recommendation should be made by an expert that is familiar with the characteristics and history of the orchard. How to apply? o o Frequency: Definitely with nitrogen smaller and more frequent applications are better than bulk applications Method: The best method to apply nitrogen is by combining the frequent applications with irrigation (and rain events if there is no irrigation system). This way it is guaranteed that each fertilizer application is incorporated into the soil by a subsequent irrigation event (or rain event). In this context, fertigation systems are becoming more common in avocado orchards in New Zealand. In this systems, fertilizer and water go together into the soil, increasing the efficacy and efficiency of the fertilizer application. 5.2 Early harvest: Early harvest has been trialled to mitigate alternate bearing for a long time (Hodgson, 1948). When dealing with overloaded trees one of the first things we need to ensure is that the fruit due to be harvested this season is harvested as soon as possible, hopefully before flowering. It is not always possible to harvest all the fruit from the trees early enough, but it is highly desirable to remove as much fruit as possible and as early as possible from the trees with higher crop load or from the areas of the trees that are carrying the largest load. 5.3 Chemical fruit thinning: Experiments were carried out from 2006 to 2011 by NZ Avocado applying ethephon, an ethylene producing compound, to trees with high flowering intensity (Dixon et al., 2008 and unpublished data). The product was applied to single branches and whole trees. The product proved effective at thinning flowers and reducing initial fruit set, however final production was also reduced and the variability in the response was very large. The risk of reducing the amount of fruit of the tree below the desired level, even by eliminating all fruit, was very high. The product acted very different depending on the temperature at which it was applied, of the area of the tree on which it was applied, and on the balance status of the branches and whole trees. To be effective, the circumstances in which chemical thinning is effective would need to be determined with accuracy and it could only be applied in blocks or trees in which these circumstances would be the same across the block. 6 REFERENCES 1) Dixon A.J., Mandemaker T.A., Elmsly T.A., Dixon E.M., Reduction of initial fruit set through the use of a chemical fruit set thinner ethephon. NZAGA Annual Research Report, 8: ) Dixon A.J., Smith D.B., Greenwood A.C., Elmsly TA., Putative timing of irreversible commitment to flowering of Hass Avocado trees in the Western Bay of Plenty. NZAGA Annual Research Report, 6: ) Elmsly T., Flower pruning survey results. Avoscene, September issue: 40. 4) Farre J.M., Hermoso J.M., Pliegi F., Effects of pre-bloom pruning on leaf nutrient status, growth and cropping of the avocado cv Hass. South African Avocado Growers Association Yearbook, volume 10: ) Fisher E., Vidiella A., Flower pruning update: industry view. Avoscene, Spring issue: ) Hodgson R.W., To thin or not to thin, the avocado grower dilemma. California Avocado Association Yearbook, volume 19: ) Lahav E., Localization of fruit on the tree, branch girdling and fruit thinning. The Volcani Institute of Agricultural Research Section B. Avocado. Pp

18 8) Kohne J.S., Schutte J.M., Increasing fruit size. Southa African Avocado Growers Association Yearbook, volume 14: 38 9) Leonardi J., New Strategies and tools for avocado canopy management. NZ and Australia Avocado Grower s Conference. 10) Mitchell N., Flower pruning looks promising. Avoscene, March issue: ) Schaffer et al., The Avocado, Botany, Production and Uses: ) Thorp, T. and Stowell, B., Pruning height and selective limb removal affect yield of large Hass avocado trees. HortScience 36(4):