Hand thinning is the best way to achieve the correct crop load and ensures that the largest and best shaped fruits are retained. It also allows the best fruitlet distribution on the tree to be established by allowing fruits in poor positions on the branches or spurs to be removed.
- Even when hand chemical thinning has been carried out, growers should double check the fruit number and hand thin down to the correct crop load per tree.
Procedures for hand thinning:
Check the fruit numbers on a small sample of trees and compare with target numbers.
Calculate how many fruits per cluster or per foot of branch length need to remain.
Decide on a simple set of rules for the fruit thinners, e.g:
- Remove all fruits under branches.
- Remove all fruits on one-year-old wood.
- Leave one (or occasionally 2) fruit per cluster.
- Leave fruits to be spaced 10 cm (4 inches) apart.
- On short-stalked varieties (Cox, Discovery, Egremont Russet and Bramley) remove the ‘king’ fruit as this may be misshapen. Leave the next largest lateral fruit in the cluster.
- On long-stalked varieties (Gala and Jonagored) the king fruit is retained as it is the largest and is not misshapen.
- Check the crop load by counting or by using special binoculars provided by advisory specialists. This aid to assessing crop load has proved quick and very reliable.
- Begin thinning as soon as possible as this maximises the benefits to fruit size and texture.
Timing of hand thinning
In a research trial conducted at East Malling in 1997, semi-mature trees of Royal Gala on M.9 rootstock were thinned to single fruits per cluster at different timings and the effects on yields and grade outs recorded. The table below shows some of the results:
Effects of different timings of hand thinning Royal Gala trees in 1997
| Treatment timing of hand thinning | No. fruits/tree | Weight of fruit harvested/tree (kg) | |||
|---|---|---|---|---|---|
| Total | >65 mm diameter | >70 mm diameter | % of total 65 mm diameter | ||
| Non (control) | 235 | 18.6 | 1.4 | 0.1 | 8 |
| Full bloom | 52 | 7.8 | 6.1 | 2.6 | 78 |
| Late initial set | 80 | 10.9 | 8.0 | 4.4 | 73 |
| 12 mm diameter | 100 | 12.6 | 5.9 | 2.7 | 43 |
| 18 mm diameter | 84 | 9.5 | 2.8 | 0.2 | 29 |
| 24 mm diameter | 83 | 9.1 | 3.3 | 0.8 | 36 |
- Trial results show that thinning at or before the 12 mm fruitlet diameter stage is essential.
- Thinning Gala at the time of initial set is particularly beneficial.
- Very early thinning of Cox may, however result in some problems in storage.
Chemical thinning of fruitlets
- Currently there are no approved products for thinning of fruitlets in the UK.
- Trials with benzyladenine (BA) appear promising and it is hoped that a registration for this may be approved in 2010 or 2011.
- Other chemicals trialled for thinning fruitlets include NAA/NAD and ethephon.
Combined flower and fruitlet thinning treatments
Trials and grower experience have shown the benefits of combining flower and fruitlet thinning treatments for maximum thinning effect.
- Consider supplementing any blossom thinning achieved using ATS with supplementary hand thinning at the 12 mm fruitlet diameter stage.
- Recently promising results have been achieved using combinations of ATS for blossom thinning and BA for fruitlet thinning.
- However, BA is not currently approved for use as a fruitlet thinner in the UK.