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Manipulating crop load by chemical thinning is critical to optimizing orchard performance.
A well-managed chemical thinning programme will markedly reduce hand thinning labour costs. For instance, with good chemical thinning, the hand thinning cost for Royal Gala is usually in the range of $1,000 to $2,000 per hectare. In the absence of a chemical thinning programme, it is not unusual to spend as much as $8,000/ha on hand thinning.
Secondly, chemical thinning enables the excess crop load to be removed very early in the fruitlet development period, when it is not practical to hand thin. The benefits from early thinning are larger and firmer fruit, and less risk of biennial bearing.
Chemical thinners fall into two categories:
- Blossom thinners - ANA (naphthalene acetic acid)
- Post-blossom thinners - Cylex, Carbaryl and Thiram.
These are the only chemical thinners that at present have full label claims for use on apples in New Zealand.
Overseas, additional blossom thinners are in use, including ATS (ammonium thiosulphate), ethephon, and NAD (naphthalene acetamide). These thinners are under development here at present.
Chemical thinning is as much an art as a science, so its success is highly dependant on fully understanding the factors that may influence the result.
These factors can be broken down into three groups:
- Orchard factors.
- Weather conditions prior to, during and particularly after application.
- Application techniques.
Orchard Factors
Variety - Among our present varieties, Granny Smith and Braeburn are relatively easily thinned, with other varieties much harder to thin. Fuji is generally considered the most difficult to thin.
Other orchard factors are listed below.
Factors favoring easy thinning:
- Young trees and trees with high vigour.
- Trees on vigorous rootstocks.
- Heavy shading or dense tree canopies.
- upright branch structures
- trees in "off" year of biennial cycle
- Trees under water stress or suffering poor drainage
- Trees deficient in nitrogen
- Inadequate cross-pollination
- Trees sprayed with Hicane
Factors making trees hard to thin
- Mature trees with low vigour
- Trees with good light exposure
- Most branches horizontal or pendant.
- Trees on dwarf rootstocks.
- Trees with adequate nitrogen levels.
- Trees in the "on" crop year of the biennial cycle.
- Good cross-pollination.
With more established orchard blocks, historical behaviour towards chemical thinning is often a good guide to the chemical thinning strategies that are likely to be successful.
Weather Conditions
Judging the weather conditions, particularly for the next three or four days following application, is absolutely critical.
Most of our chemical thinners, particularly Carbaryl and Cylex require warm weather following application for a good result - eg, minimum of three days with maximum temperatures in excess of 180C. The importance of getting the right weather conditions is more important than the precise development stage of the crop at time of application.
ANA uptake is affected by weather conditions prior to application. Dull, warm, cloudy weather preceding application softens the growth and uptake is enhanced. Conversely, cool sunny dry weather hardens the growth and strengthens cuticle development, reducing ANA uptake. Addition of Regulaid is vital as it helps overcome the barrier to uptake so that ANA results are more predictable.
Warm, dull, cloudy weather 14 to 28 days after full bloom reduces fruitset markedly and will increase the activity of the post blossom thinner, carbaryl.
High winds sufficient to cause tissue injury in the immediate post flowering period also appear to cause poor fruit set.
Application Techniques
It is generally agreed that morning application gives a better response than afternoon application. Adequate spray coverage, particularly to the upper tree is absolutely critical to a good chemical thinning job.
Research has shown that very high water rates (4,000 l/ha) using low dilution rates of chemical thinner or low volume concentrate spraying are the best application techniques for chemical thinning.
With high-volume low-concentrations, it is possible to get adequate wetting of the upper tree without danger of the run off over-thinning the lower tree.
Medium water rates (eg 2,000 l/ha) with higher concentrations generally under-thin tree tops and over-thin the lower tree.
Using low volumes, eg 200 to 700 l/ha, which do not produce drip points and run off of spray to fall through the lower tree, it is possible to specifically target the upper tree and avoid over-spraying the lower/middle tree area.
Where concentrate spraying is used, it is necessary to think in terms of rates per hectare rather than rates per 100 litres or ppm.
For ANA the following are approximate per hectare rates based on ppm.
5.0 ppm 70 to 100 ml ANA 10 % product/ha
7.5 ppm 150 ml ANA 10 % product/ha
10.0 ppm 200 ml ANA 10 % product/ha
Special Fuji rate 250 ml ANA 10 % product/ha
Do not increase the Regulaid concentration ie use Regulaid at the same rate per 100 litres of water as for dilute applications.
In humid climates where dew usually occurs at night, concentrate application of carbaryl also appears to give satisfactory results.
With hard to thin varieties such as Royal Gala, two applications of ANA with the second application targeted at the upper tree is common practice.
Synergic reactions occur when ANA, Cylex or Thiram is added to carbaryl, giving the combination much greater thinning activity than when these products are used on their own.
When using chemical thinners a few unsprayed trees should be left so that the effect of chemical thinners can be accurately assessed.
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