News & Resources

Stay up-to-date with the latest industry news. Sign-up for alerts, tips and advice, research and industry invitations delivered straight to your inbox – Sign-Up

Featured Research

Industry Best Practice

Understanding chemical thinners and growth regulators

Chemical thinners and growth regulators can be powerful tools in managing crop load, harvest timing and quality, but as AgFirst consultant John Wilton makes clear, to use them effectively it is critical to understand how they act and which plant processes they influence.

Low rate Ethephon colour enhancement tree prior to strip picking

Compared to pest and disease control, getting to grips with chemical thinners and growth regulators in the spray program is complex.

In the case of pest control, particularly insecticides, timing of sprays is relatively simple. We have very good scouting tools and deep understanding of insect life cycles. Whether a pest has been found or the intent is to make a proactive insecticide application already identified by climatic monitoring and insect life cycle modelling, it is relatively easy to identify optimum spray timing. Often insecticide application can be in line with calendar date scheduling.

When it comes to disease management, as disease requires an incubation period before its presence is noticeable, so disease spraying tends to be calendar-based commencing as the plant is about to enter the infection window. As with insecticides there are climate modelling tools available to assist disease spray timing.

Growth regulator and thinning spray timing needs to be based on the appropriate phenological stage to achieve the intended result. Unfortunately, this is only the first step in the decision-making process because there is also the challenge of matching phenological stage with weather conditions leading to effective uptake and response within the plant. The role of growth regulators in the plant is to manipulate and modify the physiological processes within the plant determine its behaviour.

Effective use of growth regulators requires a thorough knowledge of their mode of action and also of the plant processes they influence.

A particular plant growth regulator will influence numerous plant processes depending on phenological stage at the time it is applied.

Once blackspot establishes it is there for the season and contrary to popular belief it does not stop actively producing spores. This infection probably occurred in the early bud break period. Infection starting very early in the spring will increase exponentially through to harvest. Regular fungicide spraying early in the season gives certain control.

AVG (Retain®)

The ethylene blocker aminoethoxyvinylglycine (AVG, marketed as Retain®) is an example of a growth regulator which has different roles depending on the phenological stage at the time of application.

AVG inhibits ethylene biosynthesis stopping the production of endogenous ethylene. AVG was initially developed as a replacement pre-harvest drop product for an earlier growth regulator which had been withdrawn from the market. It was very effective for this purpose on varieties prone to pre-harvest drop.

It soon emerged that in varieties such has Royal Gala which produce significant amounts of ethylene as harvest approaches AVG delayed fruit ripening making it a very useful harvest management tool. In the 20 years since its introduction to the Australasian pome fruit industries, harvest management has been AVG’s main use in our industries.

Compared to run-of-the-mill agricultural chemicals AVG was expensive, often accounting for up to half the spray program cost.

AgFirst did a lot of the early on-orchard development work to identify AVG’s responses and benefits. The main benefit for a responsive variety was harvest delay, usually around 10 to 14 days. As fruit continues to grow through the harvest period at approximately one  per cent per day, a harvest delay of 10 days represents a size lift of one count size, i.e. 10 per cent yield increase, not to mention an increase in fruit value due to larger fruit.

The data we generated over this period on application cost and potential returns indicated that diligently applied AVG would give a two to three-fold return on its cost. Sometimes returns were huge where available harvest resources were incapable of picking the crop in optimum condition for storage leaving low value process fruit the only market for the crop.

With the pome fruit industry facing uncertainty over harvest labour due to COVID-19 movement restrictions, AVG may become a very important harvest management tool to help manage our way through harvest.

Research and innovation does not stop. Once the value of managing endogenous ethylene was recognised, other potential uses for AVG were explored. New uses are continuing to evolve with the present focus on enhancing fruit set with application around blossoms on shy cropping cultivars. In some cultivars increases in fruit set and retention have been spectacular and, in some cases, made shy cropping high value varieties profitable due to the ability to consistently crop them.


Premature dormancy breaker timing has lengthened the blossom period. Here are all bud/flower stages from pre-pick right through to complete petal fall. A chemical thinner nightmare.

Ethephon is old chemistry – well out of patent protection – which is still widely used for various plant growth regulation (PGR) purposes depending on phenological stage, dose, and temperature conditions.

Cost-wise it is at the other end of the spectrum to AVG with little margin for new R&D so many of its potential PGR uses fail to appear on product labels.

I first came across Ethephon 50 years ago when it was beginning to be investigated as a potential blossom thinner, and later in the season for stimulating return bloom. Use as a pre-harvest ripening agent to advance harvest quickly followed. Later it was found to be a vigour control agent as well. Sequential low rate applications have been used for vigour control during periods of growth flushes. Where sprayed at low rates outside of the phenological endogenous ethylene production stages it does not usually stimulate increased endogenous ethylene production, so its effect is mild and short lived in the plant.

Unlike most PGRs response is rapid, usually within hours of application followed by rapid disappearance. The aggressiveness of its response is both rate and heat sensitive, and very dependent on phenological stage at the time of spraying. This means considerable skill is required in understanding all the factors impacting on its behaviour to avoid disastrous outcomes. For instance, if weather conditions go against you around time of application frost injury and increased russet levels can occur. At one time it was widely used as a blossom thinning agent, largely due to the fine R&D work carried out in Tasmania by Drs Keith Jones and Sally Bound. Initially  also used  as a blossom thinner in New Zealand, it was not long before overthinning was observed, as well as increased russet incidence in certain russet prone varieties. Over the years its use as a primary blossom thinner has declined and its used as a late blossom clean up application in combination at low rates with ammonium thiosulphate (ATS) to knock out unwanted late set flower fruit.

These Brookfield® Royal Gala have responded well to the dormancy breaker. Note the uniformity of bud break including leaf buds throughout the tree.

These Scired had a dormancy breaker applied on the same day as the Brookfield® Royal
Gala pictured above. Only the lower tree flower buds were mature enough to respond. Scired is a late budbreak cultivar so requires dormancy breaker several weeks after Royal Gala types for good response. Note the absence of budbreak in the upper tree

With increasing market requirements on crisp firm juicy fruit with high internal fruit specifications its use to enhance and advance ripening is also in decline.

Some years ago following intensive scientific literature searching I managed to turn up some indications that if applied at very low rates prior to the pre-harvest phenological endogenous ethylene production stage it could enhance red colour development provided weather conditions at the time were conducive to anthocyanin development, i.e. a period of cooler nights  about 6 week pre-harvest. When used correctly in this manner fruit colour could be advanced to bring fruit colour ahead of minimum parameters for harvest so that the majority crop could be harvested an optimum maturity for long-term storage at the beginning of the harvest window, rather than wait for colour and harvest late in the maturity window as often happened.

There are many other minor PGR uses for ethephon such as enhancing return bloom in shy cropping varieties.  Here it is often used in combination with NAA as a mid-growing season application, again at low rates.


The synthetic plant hormone 1-Naphthaleneacetic acid (NAA) is another example of a multi-use older PGR now well out of patent for which numerous uses have evolved over many years. Most uses are tied to phenological stage at time of application. It can thin fruit, act as a stop drop agent, interact positively or negatively with other PGRs to enhance or depress return bloom, as well as being phytotoxic for long periods at higher rates applied under marginal cool conditions. However, if not applied diligently, I have known its adverse effects on the tree to last up to a couple of months in some sensitive varieties.

While once an important fruit thinner, adverse effects on fruit growth, and pygmy fruit problems associated with lingering residues have contributed to a decline it is use a thinner.  Its main uses now are mid-season low rate application to enhance return bloom and pre-harvest stop drop sprays.

In recent years, there has been a move away from single higher rate sprays at the commencement of pre-harvest fruit drop to multiple low rate sequential spraying commencing well ahead of probable fruit drop. Spraying in this way has resulted in a reduction in fruit softening and less adverse maturity-related side effects.

Return bloom from proprietary BA product formulated with surfactant, plus the addition of more surfactant. The additional surfactant lead to very effective chemical thinning.

Return bloom from proprietary BA product formulated with surfactant, but no additional surfactant. Note the lack of return bloom, the result of a poor thinning response in the absence of additional surfactant leading to over cropping.


For nearly 30 years 6-BA has been a key post-bloom chemical thinner since its development as a replacement for the insecticide carbaryl. Carbaryl had a fruit thinning action but was also very toxic to insects including bees and desirable insect pest predators and therefore incompatible with integrated pest control. It was causing headaches for adjacent growers with later flowering crops requiring bees for pollination.  Once the conditions 6-BA required for effective thinning response were understood, it became the main post bloom chemical thinner for pome fruit.

Its major weakness was its very exacting weather conditions around the time of application for satisfactory thinning response and relatively short phenological stage of thinning activity.  In cooler, unreliable spring climates, opportunity for application under satisfactory conditions for good response were strictly limited and often frittered away by calendar-based pest and disease control pesticides taking precedence.  It was also discovered that suitable surfactants to enhance uptake were key to a successful thinning result.

As with most PGRs 6-BA evolved with time to be put to other uses in the orchard such as enhancing fruit firmness by increasing cell division and usually significantly increasing fruit size beyond the thinning/crop load effects.

Timing and weather conditions are critical for success with 6-BA and when suitable weather conditions occur for success, its spraying needs to take precedence over other spraying requirements.

Incidentally, the thinning activity window of 6-BA can be brought forward for some varieties (not prone to the pygmy fruit problems) with the addition of a low rate of NAA. Label optimum recommendation for 6-BA is the 10-12 mm fruitlet stage, however with NAA added it is very effective at 7-12mm.

6-BA at higher rates has a role in stimulating side shoot development to improve tree development in young trees and increase young tree cropping potential.  Fuji cultivars are particularly responsive to 6-BA for this purpose and a 6-BA thinning spray can be applied at the high label rate under optimum conditions to stimulate spur development in the second growing season.

An ineffective chemical thinner creates huge hand thinning costs. Crop load after thinning here was 542 fruit/tree in 361 clusters. Assessing thinnings on the ground is a good way to check chemical thinner results.

A really effective chemical thinning response. Little hand thinning required. 200 fruit/tree in 174 clusters.


This recently introduced post blossom chemical thinner represents a huge breakthrough in chemical thinning.  Its great benefit is that it is less weather dependent for uptake, brings new flexibility and reliability to chemical thinning programs when it is correctly applied with regard to water rates.

AgFirst conducted most of the initial field trial work with this chemistry here in New Zealand, so we now have 10 years’ experience with it.  Over this 10 year period we have compared most of the main commercial products available and found that apart from a few subtle, generally non-significant differences among them, their performance is similar when applied at the same active ingredient rate, water volumes and surfactant rates.

Under our conditions, we have found that chemical thinner formulations with factory incorporated surfactants have often had inferior performance to those where we have added surfactants proven to perform well under local conditions.  We think this is due to these surfactant-containing formulations being developed to suit milder, more humid northern hemisphere locations in which softer cuticles allow more efficient uptake than is the case in our cooler or dryer climates.

Metamitron works through its ability to suppress photosynthesis creating a carbohydrate deficiency in the tree at a period of high demand which causes excess crop to be shed.  The depth and length of this period of photosynthetic suppression determines thinning response and aggressiveness.  It takes around four or five days after application for the photosynthesis suppression to reach a level necessary for thinning response to begin and it will remain around those levels for four or five days, then slowly rising back to normal levels.

Field experience, often gained from applicator mistakes and their analysis, has enabled steady improvement in thinning consistency since Metamitron has been available for commercial use.

Important lessons include:

  • Inter fruit and active shoot growth competition is necessary for a satisfactory thinning response so over thinning of light crop trees directly due to Metamitron seldom occurs.
  • Because of the ability of trees to move photosynthates around the tree, it is necessary to obtain adequate spray coverage of at least 70 per cent of the effective tree canopy.
  • As foliage area builds up rapidly over the Metamitron spraying window, higher water rates are required for later in the application window than at the beginning.
  • Long drawn out fruitset periods are difficult to deal with. This problem can be managed by either compressing blossom periods with an effective dormancy breaker correctly applied in regard to timing or adopting a double Metamitron spray approach with the second spray about four to six days following the first one.
  • Metamitron response is directly related to concentration and coverage so if safe rates are exceeded significant over thinning will occur.
  • Coverage is very important and present field experience indicates that dilute water rates in the order of a litre of spray to 10 cubic metres of tree row volume (TRV) is about optimum.
  • When correctly applied, fruit shedding can commence as soon as 14 days after application and is usually completed by 21 days.
  • Unlike almost every other chemical thinner AgFirst have worked with, most of the crop is brought down to singles and doubles, sometimes as high as 80 per cent of the fruit in clusters of this category with singles largely outnumbering doubles. Huge savings in hand thinning costs compared to other chemical thinning chemistry.
  • Selects fruit for size and fitness and cleans fruit out of shaded areas where it is unlikely to make Class 1 quality parameters.

Initial commercial experience with Metamitron products was somewhat disappointing.  While one or two instances of over thinning occurred the majority of failure was due to inadequate spray coverage through applicators taking misguided advice on water rates believed by the merchants supplying the product to save their customers money.  There is no more expensive spray than that which fails to perform.

Climatic conditions in the week or so following application determines Metamitron thinning response.  Sunny conditions with cool nights reduce response and probably indicate the need for a double spray approach.  Cloudy days and warm nights increase response indicating a second spray is unnecessary.

Growth regulators – general comments

The PGRs discussed above are a selection of those available but they show the development history of the group as a whole.  The features are that most have a range of responses and potential uses which emerge over their lifetimes and are not necessarily known at the time the initial label claims are sought.  Research development costs are very high so only limited data and treatment conditions can be investigated for the registration data pack.

The fishhooks emerge out of the woodwork when these tools are exposed to general commercial use.  The label for a PGR needs to be viewed as a licence to use.  With time, use patterns will be refined and tailored to specific pomefruit situations.  This means considerably more understanding of their behaviour than that required for most common pesticides.

It is likely that PGRs will gain greater importance as production tools as we grapple with the problem of climate change.

These trees had a low rate Ethephon sprayed 4-6 weeks pre-harvest. This enabled the trees to be strip picked at the beginning of the harvest window.



Future Orchards is funded by Hort Innovation using the apple and pear research and development levy and funds from the Australian Government, and is delivered by APAL and AgFirst.

hort apple and pear fund logo

crop load and thinning research

Go Back to Latest News