As we are aware, optimal management of crop load is a difficult task.
Most orchardists find the spring period very stressful as they try to juggle the currently available tools for crop load management with unstable weather conditions, in an effort to maintain regular yields, consistent production of high quality fruit and high pack-out percentages.
The current recommendations—a combination of dormant pruning to remove weak spurs and unbalanced limbs to open up the tree, followed by chemical thinning during flowering and the early post-bloom period—can be successful.
However, the responses from chemical thinning are often unpredictable, leading to the need for considerable hand-thinning that is both expensive and time consuming.
The end result is often reduced fruit quality and sub-optimal return bloom the following year.
PIPS Tree Structure project
Under the umbrella of the Australian apple and pear industry five year, Productivity, Irrigation, Pests and Soils (PIPS) flagship program*, researchers from NZ, Qld and Tas are collaborating to optimise tree function by manipulating structure. This is the Tree Structure# component of the PIPS program.
The Tree Structure project is undertaking similar studies in multiple environments across Australia.
Aims
This component of the program aims to improve orchard productivity and optimise fruit quality by improving tree functional efficiency as part of the ongoing drive in development of high density orchard systems.
Proposed outcomes
Outcomes of the project will include management strategies for Australian growing environments and provision of tools and systems for manipulating tree structure that can be used to optimise orchard productivity.
Varieties and locations
Apple varieties under investigation are Royal Gala and Pink Lady™ in the Huon Valley (Tas), Stanthorpe (Qld), Lenswood (SA—year-one only) and Shepparton (Vic—years 2–5), and the new cultivar RS103-130 in Queensland.
The program has now been running for three years.
Project focus
Fruiting efficiency on different bud types was examined in year 1. Years 2–5 are focussing on comparing precision-managed and conventionally-managed trees with equivalent crop loads over a four year period.
Floral bud type and fruit quality
The objective in year one (2009–10) was to quantify the genotypic expression and regulation of fruit traits of the principal Australian-grown apple cultivars, Royal Gala and Pink Lady, in three locations with differing environmental stresses: Stanthorpe, Adelaide Hills and Huon Valley. The potential new cultivar RS103-130 was also included at Stanthorpe.
Not all buds are equal
Results from the first year studies clearly demonstrated that not all flower buds are equal when it comes to fruit size.
Axillary flower buds (on first year wood) always produce smaller fruit.
In Pink Lady, fruit size was greatest on terminal buds across all regions.
No difference in Gala fruit size was noted between spurs and terminals in South Australia, but in Tasmania and Queensland, terminals produced bigger fruit than spurs.
The greater fruit size potential of terminal floral buds appears to be associated with a greater leaf area and photosynthetic potential.
Precision management (ASE) vs conventional management
Artificial Spur Extinction (ASE) is a tree training and bud thinning method which precisely defines where and how much fruit is set on the tree.
Aim of ASE
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.
Tree setup & spur removal
The initial setup of trees for ASE involves removal of unbalanced (large) limbs and training of remaining limbs to a slightly pendulous or drooping 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 trees will be carrying smaller numbers of stronger fruit buds than a conventionally pruned tree, hence the tree can direct more energy into these buds and potentially carry a greater crop load of quality fruit.
Once set up, ASE orchards should require less annual maintenance of vegetative growth and produce more consistent fruit set each year.
Influence of tree structure on fruit set
We have now had two seasons of the ASE cropping management component of the program.
Data are still being analysed for the second season, but results from the first season are looking good.
Return bloom was adequate to provide full crop load potential even though chemical thinning was avoided in these trees.
Under conventional tree management the proportion of spur and terminal buds that fail to set any fruit can vary from 30% to 50%. In the ASE treatments, fruit set was increased in individual floral buds, and this response was seen most clearly at the lower floral bud densities.
The number of spurs not setting any fruit was reduced to as low as 10%, and the number of doubles increased.
Where to from here
If fruit quality and yield continue to be equivalent or better under the ASE production system over the remainder of the trial period, then ASE may provide a potential alternative production technique.
Using ASE to precisely adjust potential fruit numbers before bud-break will enhance fruit set on selected buds.
Once trees are set up, pruning requirements will be reduced with only light maintenance pruning each year, and hand-thinning will be reduced to breaking up clusters to singles.
Thus the need for chemical thinning will be removed.
The consequences are more reliable yields and yield-estimations early in the season, and production should be less labour intensive—a major advantage when labour is one of the biggest expenses in production!
Information on how to set up trees using ASE will be provided through the Future Orchards Program over the next couple of years.
In the interim…
Ensure your thinning program starts in the dormant period with pruning to remove unbalanced branches and weak spurs, allowing for development of new wood. Tie branches down to reduce vigour.
Use a combination of bloom and post-bloom chemical thinners as previously recommended.
Remember that the chemical thinning program needs to start early in the flowering period using the chemical/s of your choice.
Available blossom thinners are: ethephon, NAA or ATS (ammonium thiosulphate).
There are two post-bloom thinners registered in Australia: 6-benzyladenine (Maxcel, BAPSol, Exilis); and carbaryl/thiram.
Full details on these thinning agents can be found in the August 2011 issue of Tree Fruit.
Chemical thinning
Under our present tree management systems, despite the limitations of chemical thinners, a chemical thinning program produces markedly superior results to hand thinning—both economically and in terms of tree physiology.
The most effective chemical thinning programs combine blossom and post-bloom thinners.
A sequential spray program allows lower quantities of chemical to be used at each timing, thus reducing the risk of over thinning.
If the chemical thinners have been effective then all that should be required is a subsequent light hand thin to remove damaged fruit or break up any remaining bunches.
To achieve good thinning and fruit quality, all chemical thinners need to be applied at the appropriate physiological stage and under the climatic conditions which are best suited to each chemical.
A non-ionic surfactant such as Kendene (Tween 20) is recommended with all thinners, except for carbaryl/thiram.
Choice of thinning chemical is important as some cultivars do not respond well to some chemicals. The currently recommended chemical/cultivar combinations are shown in Table 1.
For more information and images, and to see Table 1, download the August 2012 issue of Tree Fruit.
