A South African perspective on chemical thinners

Fresh Quarterly interviews a deciduous fruit expert. By Wiehann Steyn.

Professor Karen Theron has been with the Department of Horticultural Science at Stellenbosch University since 1985. She currently holds the Chair in Applied Preharvest Deciduous Fruit Research. For the past eight years, Theron has participated in the European Fruit Research Institutes Network (EUFRIN), an organisation that specialises in research, development, and extension on temperate fruit crops. She attends meetings of the EUFRIN fruit thinning working group. Theron shared some of her insights on chemical thinners with Hortgro Science’s Wiehann Steyn.

Q. What makes a good chemical thinner?

A.  A good chemical thinner needs to be reliable, consistent, and predictable in the response obtained, while not inducing phytotoxicity on leaves or fruit. This is however not so easy, as many factors influence the efficacy of a chemical thinning application, among others tree condition, and weather before, during and after application.

A good thinner will also have a positive linear dose response — the higher the dose, the stronger the thinning effect. The thinning response should preferably happen quickly and be uniform. This means that only one or two fruitlets remain in a cluster, that clusters are evenly distributed in the tree canopy, and that the larger fruitlets remain while smaller ones are abscised.

Unfortunately, this is not always achieved in South Africa as our bloom is often protracted. In addition, bloom in a tree varies, with the lower canopy reaching full bloom earlier than the upper canopy. Large variation can also be found between trees in an orchard.

Q. Is development of new chemical thinners ongoing? Are there any exciting compounds on the horizon?

A.  A limited number of new products are currently being evaluated. The one that’s the furthest in development is S-abscisic acid [S-ABA] which we evaluated on Forelle pears. We were looking for a chemical thinner for Forelle because 6-benzyladenine [6-BA] is usually not strong enough. The advantage is that S-ABA is already registered for use as a plant growth regulator in South Africa, which should make it easier to extend the registration to another crop and application.

In the case of apples, we’re evaluating 1-aminocyclopropane-1-carboxylic acid [ACC]. This will be a longer process as ACC is not registered as a plant growth regulator anywhere in the world and, according to Valent BioSciences, it’ll take another year or so to complete all the toxicological screening and other requirements before the registration processes can be started.

I’m excited about both these products as they are naturally occurring in plants and therefore environmentally softer than the synthetic auxins and carbaryl. Also it appears as if ACC might work as a “rescue” thinner in a late window when fruitlet diameter is 15–18 mm.

Q. What chemical thinners are available to South African producers targeting the organic market?

A.   What’s allowed in organic production depends on the specific organic certification, as far as I know: I recall that lime sulphur was allowed in certain cases while in others not.

Depending on formulation, it’s possible that S-ABA and/or ACC might be regarded as suitable for use in organic production. S-ABA has Organic Materials Review Institute [OMRI] classification in the U.S. OMRI is an international non-profit organisation that determines which input products are allowed for use in organic production and processing. OMRI Listed products are allowed for use in certified organic operations under the USDA [U.S. Department of Agriculture] National Organic Program.

Q. It’s been noted that producers try to increase fruit set in pears but then end up having to thin. Does that make any sense?

A.  Some pear cultivars set fruit with very few seeds and fruit set needs to be stimulated by applying gibberellins as the yield will otherwise be too low. However, the set following these gibberellin applications is often too high and therefore needs to be reduced again chemically or by hand. In an ideal world, one would be able to increase set only to the extent that the correct crop load is achieved, but increasing set with plant growth regulators is as difficult as chemical thinning.

Q. How should a producer evaluate new chemical thinning compounds or recipes?

A.   I believe that it’s important for a grower to evaluate new products or programmes relative to the one they know. I suggest they apply their old recipe to one part of the orchard and then apply the new option to at least a few rows. It’s important that these rows are representative of the orchard and that the trial does not include a weaker part of the orchard. These trees can then be harvested separately, and yield determined as well graded for size distribution. It’s also very important to keep accurate information on aspects like tree condition, bloom density and distribution, previous crop load, and weather conditions before, during and after application. Such trials should be repeated over a few seasons.

Q. Tell us more about your involvement with EUFRIN.

A.  I was informally asked to join the group and attended my first meeting in 201A I’ve been privileged to attend annually since then. The big advantage of being at these meetings is that one is at the forefront of what’s being researched internationally, and data is discussed that’s still confidential. Therefore, I’m aware of what is happening long before any scientific papers are published. Also, the bigger companies that are involved in chemical thinner development, for example Valent BioSciences, Fine Agrochemicals, and Adama Agricultural, are present and as researchers we are also aware of what is happening on the commercial side of things.

Image supplied by Elise-Marie Steenkamp | Hortgro.