Thinning treatments used with considerable success in North America for many years comprised a spray of Elgetol (DNOC) as a flower thinner, followed by sprays of carbaryl or NAAm.
- Recently, researchers at East Malling have investigated the use of other, more environmentally sensitive combinations of spray treatments to achieve effective apple thinning.
Royal Gala
- In a trial conducted in 1998, a spray of ATS (0.5 or 1.0%) at full bloom, followed by a spray of BA (Perlan 100 ppm), all at high volume (1000 litres/ha), showed some promise for thinning the variety Royal Gala.
Effects of combinations of ATS and BA on the fruit set, yield and fruit size of Royal Gala in 1998
| Treatment | Final set/100 floral buds | Total yield/tree (kg) | Mean fruit wt. (g) | Yield/tree Class 1 (%) | |
|---|---|---|---|---|---|
| >70 mm | >65 mm | ||||
| Unthinned | 183 | 27.7 | 101 | 7 | 25 |
| ATS (0.5%) + BA (100 ppm) | 131 | 23.7 | 122 | 13 | 52 |
| ATS (1.0%) + BA (100 ppm) | 105 | 21.2 | 130 | 13 | 62 |
| Hand thinned | 66 | 14.2 | 138 | 11 | 78 |
Jonagold
- Combined treatments of 0.5% ATS, applied at full bloom, followed by 100 ppm BA, at the 10-12 mm fruitlet diameter stage, gave promising thinning results on Jonagored trees in 1998.
- Although the degree of thinning was much less than that achieved by hand, reductions in total yield were less.
Effects of ATS and BA spray treatments on the thinning of Jonagold in 1998
(all sprays applied at 1000 litres/ha)
| Treatment | Final set/100 floral buds | Total yield/tree (kg) | Mean fruit wt. (g) | % Class 1 >90 mm |
|---|---|---|---|---|
| Unthinned | 57 | 44 | 179 | 0.5 |
| ATS (0.5%) + BA (100 ppm) | 47 | 42 | 208 | 13.4 |
| ATS (1.0%) + BA (100 ppm) | 36 | 34 | 219 | 15.4 |
| BA (100 ppm) | 50 | 42 | 190 | 4.9 |
| Hand thinned | 26 | 29 | 260 | 32.4 |
- 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.
Combined chemical thinning strategies tested abroad
Trials conducted in mainland Europe have on occasions shown the benefits of using more than one chemical for thinning apples.
- Trials in the Netherlands have recently shown that a combination of the flower thinner lime sulphur and the fruitlet thinner BA can prove effective on the variety Elstar.
- In Spanish trials, mixtures of Ethrel (applied at 20% open flower) and BA (applied at the 10 mm stage) have thinned Fuji effectively, as have combinations of NAA and carbaryl.
- Similarly, Gala and Golden Delicious were very effectively thinned with mixtures of BA and NAA in Slovenia.
- The same combination of chemicals has proved effective in thinning the variety Elstar in Dutch trials.
- In Northern Italy sprays of NAAm or Ethrel, followed by later sprays of BA have proved very effective in thinning Royal Gala.
- However, in French trials this combination treatment overthinned the variety Pink Lady.