Crop load has been managed over the last 25–30 years by pruning and chemical thinning, followed up with hand-thinning.
Tools for managing crop load (cont from last issue)
We are fairly lucky in Australia with a choice of chemicals available. While our knowledge of chemical thinning and tree response has come a long way over the past 25 years, juggling application of chemical thinning agents with unstable spring weather conditions is a fine balancing act; using chemicals to control crop load is always likely to be unpredictable.
ASE
A new tool introduced into Australia recently for use on apples is artificial spur extinction (ASE).
This method of crop load management stems from observations that regular bearing cultivars have high natural spur extinction and enables crop load to be precisely managed to consistently achieve target yields, unlike the ‘hit and miss’ often experienced with chemical thinning.
Pruning
Whether using a chemical thinning program for crop load management or implementing ASE technology, pruning during the dormant period is the first stage of any program.
Good sized, high quality fruit is associated with open, healthy trees.
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 producing good quality fruit and initiating 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 in a centre leader system 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 to a pendant position will also help to calm trees.
In vase and bush systems, scaffold limbs should be limited to around 10 and the centre kept open for good light penetration.
Chemical thinning
Responses to chemical thinning can be unpredictable, making optimal crop load management a difficult task.
There are many interacting factors influencing the thinning response of chemical thinning agents, including cultivar, climate, pollination and tree history.
A structured program combining both blossom 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.
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 cyanamide have been applied.
In this situation, careful fruitlet counts should be undertaken after primary thinners have been applied to determine whether further thinning is necessary. If using ATS, only one application might be required to reduce the crop load to a suitable level.
Blossom thinners
Ammonium thiosulfate (ATS): ATS works by desiccating or 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 is critical to achieve a satisfactory level of thinning. The chemical must be applied when sufficient flowers have already been fertilised to give a good crop load—in apples this can be as early as 20% bloom.
Multiple applications are recommended—the aim is to remove the later-opening flowers. In cultivars with an extended flowering period, such as Gala, three applications might be necessary.
Armothin: Desiccant. Useful for cultivars that have an extended flowering period.
(cont next issue)
Download the Orchard plant protection guide
See this article in Tree Fruit Aug 2020
