Getting the most out of chemical thinning

Responses from chemical thinning can be unpredictable, making optimal management of crop load a difficult task. Here Dr Sally Bound helps navigate through the different options available to find a perfect match for you orchard.

Trying to juggle the different ways to manage crop load with unstable spring weather conditions is a fine balancing act. However, combining the following can help maximise fruit quality and pack-out while maintaining consistent yields:

  • Dormant pruning to remove weak spurs and unbalanced limbs to open up the tree and reduce excess wood and bud load;
  • Followed by the use of a structured chemical thinning program during flowering and the early post-bloom period; and
  • A final tidy-up hand thin.

An understanding of the many interacting factors influencing fruit thinning will aid in developing a successful thinning program. To maximise the benefits of any thinning program, good overall tree management is important – the greatest benefits are achieved where good management of all aspects of fruit production are employed. For example, where an orchard is subjected to water stress or where nutrition is lacking, then thinning is unlikely to substantially increase fruit size or quality.

Early crop load management is important

Figure 1: The relationship between crop load and fruit firmness in ‘Delicious’ apple.

Figure 1: The relationship between crop load and fruit firmness in ‘Delicious’ apple.

Apple trees experience a natural ‘December’ drop, but this is insufficient to achieve optimum crop loads, fruit size and quality, or to prevent biennial bearing. Flowers for the following year are initiated during spring and trees carrying heavy crop loads over this period will have less flower buds the following spring. Hence to ensure regular bearing and optimise fruit quality we need to remove more than 90% of the potential fruitlets within six weeks of bloom.

The first six weeks after flowering is also when cell division occurs in the developing fruitlets, and maximising resources during this period of fruit growth will maximise both fruit size and internal quality. Leaving excess fruit on the tree during this period is a waste of the tree’s resources, as growth is put into fruit that is later removed. It is more productive to channel this energy into fruit that will remain on the tree through to harvest. This is particularly important in drought years when water resources are scarce.

Figure 2: The relationship between crop load and fruit sugar content in ‘Fuji’ apple

Figure 2: The relationship between crop load and fruit sugar content in ‘Fuji’ apple.

The negative relationship between crop load and fruit size is well known, but high crop loads can also have a negative effect on fruit firmness (Figure 1) and sugar content (Figure 2). Hence strategies such as the use of substantial removal of flower buds during dormant pruning, and application of blossom thinners early in the flowering period, provide an excellent chance of maximising fruit quality, and thus returns to the grower.

Stages in the thinning program

1. Pruning

Pruning during the dormant period should be considered the first stage of any thinning program. Well sized, high quality fruit are associated with open, healthy trees.

In apples, the best fruit occurs on younger spurs (2-4 year old). Canopies need to be opened up and trees kept young by appropriate pruning so that older spurs are removed and fruiting occurs on young wood. Adequate light is also important for production of good quality fruit and initiation of strong flower buds – this is especially important in netted canopies. Heading back of limbs should be avoided as this tends to invigorate the tree, instead larger unbalanced branches should be removed completely. Aiming for 6-7 branches per metre of tree height will open up the tree, resulting in improved bud initiation and better fruit quality as well as better spray penetration and access for harvesting. Tying down upright branches will also help to calm trees.

The timing of pruning also has an effect on fruit quality. Tasmanian studies have shown that delaying dormant pruning until spring adversely affects fruit skin finish, sugar content and fruit shape. Summer pruning is a common practice in Australian orchards to remove the current season’s growth to prevent shading of the fruit and improve fruit colour. However, this practice reduces fruit size and sugar content and produces flatter fruit – probably due to reduced photosynthate production resulting from the reduction in the leaf to fruit ratio caused by summer pruning.

2. Chemical thinning

A program combining both bloom and post-bloom chemical thinners will give the most reliable results. Remember that the chemical thinning program needs to start early in the flowering period using the chemical/s of your choice. We are fairly lucky in Australia with a choice of chemicals available. Registered blossom thinners are: ATS (ammonium thiosulphate), ethephon, and NAA. Lime sulphur is also effective as a blossom thinner. There are two post-bloom thinners registered in Australia: 6-benzyladenine and carbaryl/thiram.

Blossom thinning

Regardless of whether thinning is undertaken by hand or with the aid of chemical thinning agents, the thinning process should start early in the flowering period.

Ammonium thiosulphate (ATS): ATS works by burning the style and stigma of the flower, thus preventing pollination and fertilisation. While leaf damage does occur with desiccants, the degree of damage that occurs when using the recommended rates does not affect fruit development, size or quality. It should also be noted that the degree of desiccation can be influenced by temperature – with higher temperatures resulting in greater desiccation.

Application timing: Time of application is critical in achieving a satisfactory level of thinning (Table 1). The chemical must be applied when sufficient flowers have already been fertilised to give a good crop load. Multiple applications are recommended. In cultivars with an extended flowering period, such as ‘Gala’, three applications may be necessary.

Spray volume: ATS should be applied at high volumes, ensuring thorough wetting of the trees. Ensure the sprayer is properly calibrated to give even coverage over the whole tree. Low volume concentrate spraying of desiccating chemicals is not advised as this is likely to result in an extreme desiccating action, causing severe burning of foliage and death of buds.

Ethephon: Ethephon can be a vigorous thinner; completely removing weak spurs or depleting fruit positioned low on the tree.

Complete fruit removal: While ethephon thins effectively at around 40 dAFB (days after full bloom) it does not improve fruit size and can cause yellowing of the fruit at harvest. However, if the crop is damaged, eg. by hail, application at 40-50 dAFB can remove the damaged crop. This not only saves hand removal of the crop, but has a positive effect on return bloom.

Advantages: Improves return bloom.

Disadvantages: Ethephon has a tendency to flatten fruit. It can also depress fruit size if used at higher rates or too late. Ethephon is not effective in cooler temperature conditions.

NAA: While NAA can thin most cultivars between FB (full bloom) and 21 dAFB, the earlier it is applied the better the response in fruit size. NAA promotes vegetative growth that can be advantageous in green apples such as ‘Golden Delicious’ or ‘Granny Smith’, but can be a distinct disadvantage in red apples where extra vegetative growth shades the fruit, inhibiting red colour production.

Interactions: NAA interacts with other plant bioregulators containing the gibberellins GA4+7. Hence it is not compatible with formulations such as Cytolin® when applied at the normal recommended rate. However, if the rate of NAA is reduced to 3-4 ppm then a Cytolin / NAA program works well.

Disadvantages: High concentrations and/or late applications can depress fruit size as well as producing pygmy fruit and reducing seed numbers which can affect fruit quality. The efficacy of NAA is affected by temperature and humidity and rewetting with light rain or dew often causing over-thinning. As NAA is most effective under humid conditions, it can cause fruit russeting.

Post-bloom thinning

6-benzyladenine (BA): There are several products registered in Australia with the cytokinin 6-benzyladenine as the active ingredient [CyLex & Maxcel (Valent BioSciences), BAPSol (Gro-Chem NZ), Exilis (Fine Agrochemicals Limited)]. BA is a post-bloom thinner and works most effectively following treatment at flowering with one of the blossom thinners ATS, ethephon, or NAA.

Application conditions: BA is temperature dependent, the warmer the temperature the more effective it will be. Ideally it should be applied on a rising temperature curve, with maximum daily temperatures in excess of 15°C. Look for the best available weather during the recommended application period.

Advantages: If used correctly, BA is a consistent thinner. It is not persistent or toxic, and is OK for IPM programs. BA increases fruit size independently of the thinning effect and may also increase fruit firmness.

Disadvantages: Temperature dependent – needs warm to hot conditions.

Carbaryl / Thiram: Carbaryl is regarded as a mild thinner and usually only removes the slower growing fruit within bunches. In Australia carbaryl is used as a post-bloom thinner, normally in a tank mix with thiram.

Carbaryl can also be used on trees where use of a primary thinner is not warranted, either because the trees are young or because of sparse blossom buds.

Application conditions: Carbaryl is temperature dependent, requiring warm dry conditions for effective thinning. The warmer the temperature the greater the thinning effect. Cool or damp conditions following application can result in fruit russet.

Disadvantages: Carbaryl is toxic to bees and beneficial species important in IPM programs. It is also toxic to mammals, and has been found in waterways. Carbaryl can leave residues on fruit, and its use is banned on fruit exported to some countries.

Chemical Concentration Time of application
ATS 0.75 – 1.5% v/v 20% and 80% bloom
ethephon 30-50 ppm for younger trees
100-150 ppm for mature trees
Balloon blossom to 7 dAFB
naphthalene acetic acid (NAA) 4-5 ppm for easy to thin cultivars
up to 12 ppm for difficult to thin cultivars*
FB to 5 dAFB
benzyladenine (BA) 150 ppm applied as a fine mist (9L/ha in 1200L/ha) Fuji & Gala     15-22 dAFB
Red Delicious  10-20 dAFB
Golden Delicious  10-20 dAFB
Lime sulphur 2% solution, applied to runoff 20% and 80% bloom
carbaryl recommended label rate 14 -60 dAFB
Repeat at 7-10 day intervals as required
thiram recommended label rate As for carbaryl
Table 1: Available chemical thinning agents and recommended concentrations and application times. Note FB is ‘full bloom’ and dAFB is ‘days after full bloom’.
*2 sequential sprays may be required – the 1st spray applied at FB and the second 3-5 dAFB

Dormancy breakers

As dormancy breakers often compress the flowering period, making thinning chemicals more effective, caution is advised with thinning chemicals when dormancy breakers such as Dormex® or Waiken™ have been applied. In this situation careful fruitlet counts should be undertaken after the application of primary thinners to determine whether further thinning is necessary. If using ATS, only one application may be required to reduce the crop load to a suitable level.

Netted orchards

Fruit set in trees under hail netting tends to be lower than uncovered trees. Due to the lower light levels, chemical thinning of netted trees often induces greater fruitlet drop, hence care should be taken to avoid over thinning. While low blossom trees under nets may not require the application of chemical thinners, trees with medium to heavy blossoms will still benefit from chemical thinning to minimise or eliminate the need for follow-up hand thinning.

Benefits of chemical thinning


Always follow the label instructions and seek local advice on your chemicals!

Misuse of some fruit thinners can cause crop damage.

All chemical thinners have some disadvantages, however despite these limitations, 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 Kendeen 20 will enhance the efficacy of ethephon, NAA and BA. 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 2.

Delicious Golden Delicious Fuji Granny Smith Gala Pink LadyTM Sun-downer
– primary thinner
– primary thinner
 –  –  –  √
– primary thinner
 √  –
– 3-7 d after NAA @ FB
 –  √  –
– 5-7 d after NAA @ FB
 √  –  –  –
 Benzyladenine (BA)
– secondary thinner
 –  –  –
– secondary thinner
 √  √  √  √  √
Table 2: Recommended chemical/cultivar combinations.

Economics of thinning

Economics strongly favour an aggressive thinning approach based on knowledge and records. Under-thinning is costly, resulting in small fruit size, poor fruit quality, extensive hand thinning and inadequate flower bud initiation for next year. Over-thinning is not a common occurrence, even with an aggressive approach, and even if it does occur there will be no hand thinning expenses, fruit quality is normally high and return bloom the next year is assured. Under-thinning has no such bonuses!

There is no simple answer to the question “How do I thin my crop?”. The two most important factors are application timing and ensuring that conditions are suitable for the particular chemical at time of spray application. Discuss possible strategies with your neighbours and local grower group. If you have a program that works, stick to it!

Future directions – a new tool to replace chemical thinning

Crop load management is the single most important practice determining the annual profitability of apple orchards, yet it is the most difficult. While our knowledge of chemical thinning and tree response has come a long way over the last 25 years, using chemicals to control crop load will always remain unpredictable. Hence we need to look for other ways to manage crop load. As early as 1995, Pierre-Éric Lauri and his research group studied the difference between regular and biennial bearing cultivars, and observed that regular bearers have high natural spur extinction. The New Zealand research group led by Dr Stuart Tustin has followed up on these studies and further developed the concept of Artificial Spur Extinction for management of crop load (see ‘Artificial spur extinction – a new crop management tool’, Australian Fruitgrower, June 2014).

As part of the national HAL-funded PIPS (Productivity, Irrigation, Pests and Soils) Tree Structure program, field trials duplicated in Tasmania, Victoria and Queensland have been comparing the impact of Artifical Spur Extinction with conventional pruning management on fruit set response, yield and fruit quality. The project team is in the process of summarising the data from this exciting project. However, results indicate the potential of Artificial Spur Extinction to supersede and eliminate the present requirements for chemical thinning to regulate biennial bearing and crop loading. This exciting work is likely to result in a shift in contemporary orchard system production approaches and technologies.

About the author

Dr Sally Bound is a Senior Research Fellow at the Perennial Horticulture Centre, Tasmanian Institute of Agriculture, University of Tasmania.

By |August 26th, 2014|Vigour, crop load and thinning|

About the Author:

Senior Research Fellow
Tasmanian Institute of Agriculture
03 6226 2958
Email Sally.