While knowledge of chemical thinning and tree response has come a long way over the last 25 years, using chemicals to control crop load is a fine balancing act, particularly with the unpredictable spring weather conditions experienced in most Australian apple growing regions. This unpredictability combined with environmental concerns associated with some thinning chemicals means that growers need to find alternatives to chemical thinning.

A research group in France, led by Pierre-Éric Lauri, studied the difference between regular and biennial bearing cultivars in the mid 1990s and observed that regular bearers have high natural spur extinction. Following on from these studies, Dr Stuart Tustin and his group in New Zealand further developed the concept of Artificial Spur Extinction (ASE) for management of crop load.

The Productivity, Irrigation, Pests and Soils (PIPS) programme has demonstrated the application of ASE under Australian growing conditions, comparing the impact of ASE with conventional pruning management on fruit set response, yield and fruit quality.

Results from this work were extremely positive, however because chemical thinners were not included in this study several questions have arisen:

1  Is ASE effective on cultivars with an extreme biennial bearing tendency, such as Fuji?

2  Do ASE managed trees respond to chemical thinners?

3  Can ASE technology be successfully merged with chemical thinning to optimise yields and fruit quality?

4  Can hand thinning be reduced or eliminated by a combination of ASE and chemical thinning?

5  How does ASE technology compare directly with best practice chemical thinning programmes in terms of yield, pack-outs, and the cost to benefit ratio?

A preliminary one-year study on Gala in 2013/14 suggested that a chemical thinning program may not be needed in ASE managed trees, but to answer these questions we have established a two-year study on the cultivars Gala and Fuji which commenced in 2015.

In each cultivar, we will compare conventional management with ASE management and we will apply a chemical thinning programme to half of the conventionally managed trees and half of the ASE managed trees.

What is ASE?

Artificial Spur Extinction (ASE) is a crop load management method using bud thinning techniques to precisely define where and how much fruit is set on each limb of the tree. The aim of ASE is to promote the vigour and performance of floral spurs, stimulate spur strength and improve fruit quality and regularity of production.

The initial setup of trees for ASE involves removal of unbalanced (large) limbs and training of remaining limbs to a near horizontal position, which optimises fruiting whilst restricting excessive vegetative growth.

Floral spurs are then selectively removed while trees are still dormant to precisely define the density and location of potential fruit on the tree. This means that at bud burst, ASE-managed trees commence spring growth in an already significantly ‘crop thinned’ state, carrying fewer but stronger flower buds than conventionally managed trees. This reduces competition for tree resources, enabling optimisation of crop load and maximising fruit quality potential.

Hand thinning is simplified because spacing, position and number of clusters are already set by the bud thinning process – all that is required is to break up fruit bunches and remove fruit with defects.

Trial design and treatments

In the study reported here, treatments were ASE or conventional hand thinning plus or minus a chemical thinning regime. Floral buds were thinned to six buds per cm² of limb cross-sectional area (LCSA) on the ASE trees in late August just prior to bud break.

A full chemical thinning programme, using the bloom thinners Ethrel^® (ethephon) and NAA and post-bloom thinner Maxcel^® (BA), was undertaken following the program normally used for each block. All trees were hand-thinned nine weeks after full bloom (FB), following natural fruit drop, to six fruit per cm² of LCSA. Fruit was harvested at normal commercial harvest.

Influence of tree structure on flower number and fruit set

In ASE managed trees, the number of flower buds (clusters) is reduced prior to bud burst, resulting in fewer buds than in conventionally managed trees.

Conventional trees carried approximately double the number of flower buds at bud break compared with the ASE managed trees. A high proportion of clusters set fruit in these trials – the result of an unusual season.  However the addition of chemical thinning had no effect on ASE managed trees, while in conventionally managed trees, the proportion of clusters set was significantly reduced in both Gala and Fuji, with the greatest effect in Fuji.

Although varying from year to year, under conventional tree management it is not uncommon that 30 to 50 percent of spur and terminal buds fail to set fruit. Weather conditions in 2015 led to an extremely high fruit set in the Huon Valley where the trials were located. However, the proportion of buds setting multiple fruit was higher in ASE treatments than in the conventionally managed trees.

Even though the number of flower buds was reduced in the ASE managed trees, the percentage of fruit set was higher than in conventional trees. This is explained by a high percentage of clusters setting fruit with a higher proportion of multiple fruit set in each cluster. These results are in agreement with our earlier work showing that most floral buds set fruit under ASE management, most likely due to stronger buds and less competition for resources.

Yield efficiency (kg of fruit per cm² trunk cross-sectional area) was improved by over 20 per cent in the ASE managed trees in both cultivars. Chemical thinning had no effect on yield efficiency of ASE managed trees, but in the conventional trees chemical thinning slightly improved efficiency in Gala but not Fuji.

Fruit was larger in the ASE than in the conventionally managed trees in both Gala and Fuji. Although all trees were hand-thinned to the same crop loads in early December, initial fruit set in the conventional trees was reduced by chemical thinning but there was no corresponding increase in fruit size as would be expected with a lower crop load. So while these trees were carrying less fruit in the period between fruit set and hand-thinning, as both ethephon and NAA are known to impair fruit size, even if thinning occurs, it appears that the chemical thinners may have negatively affected fruit size.

Fruit shape (measured as fruit length to diameter ratio) was influenced by the crop load management regime in Gala but not Fuji. Shape was significantly improved with ASE management compared with conventional and, similarly to fruit weight, it appears that the addition of chemical thinners to these trees negatively affected Gala fruit shape; ethephon in particular is known to flatten fruit.

Cytolin^® is commonly used to improve fruit shape and size in Gala, so if the improved fruit shape observed in ASE managed trees is shown to be consistent from year to year, then the application of Cytolin may no longer be necessary.

Conclusions

The lack of effect of chemical thinning in further reducing crop load on ASE managed trees demonstrates that ASE is a suitable tool for managing crop load without additional chemical thinning. There is also the added negative impact of chemical thinners on fruit size and shape.

Although this study will continue for the coming season, providing another season of data, including a cost-benefit analysis, early results indicate the potential of ASE to supersede and eliminate the present requirements for chemical thinning to regulate biennial bearing and crop loading.

Using ASE to manage crop load has the added advantage that the achievement of target crop loads is no longer dependent on unpredictable weather conditions during the flowering and post-bloom periods when thinners are normally applied.

In addition, bud position is optimised in ASE, fruit is well spaced and light distribution into the canopy is enhanced. With this technology growers are also able to set their trees up to carry a pre-determined crop load with reasonable accuracy, thus enabling improved management of fruit size.

Implementation of ASE and a move away from chemical thinning, combined with simplified hand-thinning and more even fruit maturity will reduce both time and cost to the grower, but will require a paradigm shift.

The chemicals Ethrel^®, Cytolin^®, NAA and Maxcel^® are registered for use on some apple varieties in Australia. The list of registered products and permits for apple production are available on the Australian Pesticide and Veterinary Medicine Authority website.

Acknowledgements

Thanks to Scott Price, manager of Calvert Brothers Rookwood orchard at Ranelagh, for use of his orchard and provision of trial trees. The PIPS 1 project team of Stuart Tustin, Ben van Hooijdonk and Ken Breen (Plant & Food Research, NZ); Simon Middleton, John Wilkie and Heidi Parkes (DAF Queensland); and Dugald Close who paved the way for this work with the initial study comparing ASE and conventional management.

This project is funded by Horticulture Innovation Australia Ltd, using the apple and pear industry levy paid by growers and matched funds from the Australian Government.

Source: Sally Bound, Tasmanian Institute of Agriculture; APAL