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Chemical thinning: Can US tool benefit Aussie apple growers?

Research & Extension

Could a tool used in the US to help apple growers better manage secondary thinning to optimise fruit quality and maximise productivity be of use in Australia? PIPS researchers plan to find out.

Apple trees produce more flowers than the number of fruit that growers wish to crop. Growers need to manage this oversupply of flowers and early stage fruitlets to maximise yield and produce fruit of a consistent size.

Fruit trees utilise carbohydrates to grow fruit. Grower inputs contribute to carbohydrate production and thus, ideally, these inputs should be directed into the harvested fruit rather than thinned fruit. Through effective crop load management, carbohydrates from reserves and generated by photosynthesis can be efficiently used to grow the fruit that are carried through to maturity.

Growers use three key management strategies to set optimal crop loads: winter pruning of buds, spring chemical thinning of flowers and fruitlets (primary and secondary), and hand thinning of fruit.

It is desirable to reduce crop load early in the season for three key reasons:

1. To maximise productivity of the tree, more carbon is allocated to fruit to be carried through and less is wasted by later thinning or fruit drop.

2. To minimise biennial behaviour of the tree. For some cultivars signals from fruitlet seeds inhibit bud initiation for the next season.

3. Minimise hand thinning which is labour intensive and costly.

As part of the Hort Innovation funded ‘Productivity, Irrigation, Pests and Soils’ (PIPS) project ‘Physiological, metabolic and molecular basis of biennial bearing in apple’, secondary thinning is being investigated by the NSW Department of Primary Industries and Agriculture Victoria.

Chemical thinning carbon balance fruitlets

The carbon balance of a tree plays a big role in whether thinners targeted at fruitlets will be effective or not.

US growers benefit from service

In the United States, a service is provided to growers to assist with secondary thinning decisions. This targets the early fruitlet stage after flowers have been thinned. At this growth stage the effectiveness of chemical thinners varies based on the carbon balance (deficit or excess) of the tree and the prevailing weather conditions.

When a tree is in carbon deficit, that is the demand of the tree outstrips carbon availability, it weakens its hold on fruitlets making thinners more effective. Conversely, in carbon excess conditions, where demand is met by supply, fruitlets are retained more strongly and thinners are less effective.

Matching thinner application to the tree carbon balance may improve thinning outcomes. Weather conditions influence the photosynthesis and respiration processes and therefore also influence the carbon status of the tree and hence chemical thinner efficiency.

Cornell University has used relationships between weather variables and apple tree carbon balance to develop an advisory service to help growers make secondary thinning decisions.

This tool utilises the apple carbon model ‘MaluSim’ developed by Professor Alan Lasko. Growers can select a weather station near their orchard, input green tip date and receive a forecast for the next few days of the likely carbon status of the tree and a thinning recommendation.

Thinning recommendations for various tree carbon balance levels

Examples of weather conditions which influence the carbon balance and chemical thinning efficiency

Potential in Australia

Researchers are testing the MaluSim model under Australian conditions, with outcomes to be delivered to growers over the next 18 months.

More information

For more information watch the video ‘Understanding biennial bearing in apple – 2018 update’ on APAL’s YouTube channel.


This project has been funded by Hort Innovation using the apple and pear research and development levy, funds from the Australian Government and contributions from the state governments of Victoria and NSW.

About the authors

Rebecca Darbyshire, NSW Department of Primary Industries e: [email protected]
Dario Stefanelli, Agriculture Victoria e: [email protected]
Jens Wünsche, University of Hohenheim; and
Henryk Flachowsky, Institute for Breeding Research on Fruit Crops.

chemical thinning crop loading PIPS secondary thinning thinning vigour

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