By Dr Sally Bound
Dr Sally Bound helps us understand the research on chemical thinning of pears to help provide reliable recommendation to growers.
With little information available on chemical thinning programs for European pears or on the effect of chemical thinning agents on pear fruit quality, it is difficult for growers to put together effective thinning programs for their crops. Relying on hand-thinning for crop load management is expensive and often affects fruit size and quality because, during early fruit development, the tree puts its resources into fruit that is removed several weeks after bloom.
While a targeted chemical thinning program allows removal of excess fruit early in the season, thus optimising fruit quality, unless properly understood, the use of chemical thinners can involve a large commercial risk.
Although all available chemical thinners have some 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.
The work described in this article was undertaken with the aim of providing reliable recommendations for chemical thinning of pear cultivars. Trial work was undertaken on the cultivar Packham’s Triumph over a period of three years in both the Goulburn Valley, Victoria, and at Nubeena, Tasmania. Results from the two regions have proved to be very similar, demonstrating that results and recommendations arising from trial work undertaken in the cooler Tasmanian conditions are applicable to other growing areas in Australia.
Chemicals examined included the desiccating agents ammonium thiosulphate (ATS) and potassium thiosulphate (KTS) and the hormonal thinner ethephon applied during the flowering period and 6-benzyladenine (BA) applied as a post-bloom thinner.
Desiccants for thinning
Desiccating chemicals have a different mode of action to the hormonal type chemical thinning agents such as ethephon or NAA that have been used for many years in Australia. Desiccants act by burning the pistil (which consists of the style and stigma) of the flower, thus preventing pollination and fertilisation from occurring.
Hence desiccants need to be applied after the flower has opened so that the desiccant can reach the pistil, but before pollination because once pollination has occurred desiccants will have no effect on crop load.
When using desiccants, the time of application is critical in achieving a satisfactory level of thinning. Desiccants need to be applied when sufficient flowers have already been fertilised to give a good crop load. These earlier setting fruit are also normally larger and better quality than later setting fruit.
Both ATS and KTS proved to be effective thinning agents for Packhams, but ATS is a more aggressive thinner than KTS. As only one trial was conducted with KTS, further work is required to develop recommendations for this desiccant.
The trials conducted with ATS have shown that there is an increased thinning effect with increasing concentration, and also with number of applications. The recommendation arising from this work for use of ATS as a blossom thinner for Packhams is an initial application of 1.0 per cent ATS at around 25 per cent bloom stage, with a follow up second application from 50 per cent bloom to enhance the thinning effect. Once full bloom has been reached it is too late to apply desiccants as most flowers will already have been fertilised. If conditions are unfavourable for pollination, i.e. cool wet weather with few active bees, or a netted orchard, then spray application should be delayed to ensure adequate fruit set.
The physical mode of action of desiccants makes them less dependent on weather conditions than hormonal type thinners, however the degree of desiccation can be influenced by temperature – with higher temperatures resulting in greater desiccation. 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.
The post-bloom thinner BA also proved to be a consistent thinner of Packhams, and it can be applied as early as 10 days after full bloom (dAFB) to as late as 40 dAFB following a blossom application of either ethephon or ATS. However there was a loss in fruit size with later applications. Normally with increased thinning effect we see an increase in fruit weight/size – the decrease observed in this work with the later BA applications was most likely due to resources being directed to fruit which later drops. If thinning is completed earlier there is less wastage of resources into fruit, which will ultimately be removed by either late post-bloom thinning chemicals or hand thinning.
A range of BA concentrations from 50 through to 200 mg/L were examined and the most effective concentration range was found to be 100-150 mg/L. This is similar to the recommendation for apples.
Combining blossom and post-bloom thinners
ATS can also be effectively combined in a program with the post-bloom thinner BA. This provides more options when determining thinning programs, as either ATS or ethephon can be used as a blossom thinner prior to application of BA post-bloom. While BA can be used as a stand-alone thinner of Packhams, it is risky to rely on one spray application in case weather conditions are unsuitable for application. Two applications of BA did not increase the thinning effect, hence would not be recommended.
Impact on fruit quality
All chemicals examined maintained or improved fruit quality measured as size, firmness and sugar content, but ATS caused a slight increase in skin russet. However this increase in russet was ameliorated when BA was added to the thinning program. The negative effect of crop load on fruit firmness was also demonstrated, with fruit from trees with higher crop loads being less firm than fruit from low crop load trees.
The trials undertaken in this three year study demonstrate that there are viable options for chemical thinning of Packhams. ATS was effective as a blossom thinner and the recommendation is for two applications applied at 1.0 per cent, with the first application at 20-25 per cent bloom and the second application at 50 per cent bloom. If further thinning is required, this can be followed with a post-bloom application of 100 mg/L BA applied from 10-40 dAFB, however the earlier the BA is applied the greater the benefits on fruit size.
It should be noted that in the trials described here all chemicals were applied at high volume. If low volume controlled droplet application (CDA) technology is used for application of desiccants then the recommendation for concentration is likely to alter. These results also only refer to one cultivar, Packham’s Triumph, and caution is required if transferring recommendations to other cultivars without further scientific justification.
While this work has resulted in practical recommendations for the use of a range of chemical thinning agents to manage crop load in Packhams, there has unfortunately not been a follow-through by chemical companies to include pears on the chemical labels for these products.