Most chemicals that have been tested and found effective in preventing apple flowers from setting fruits work by desiccating the flower organs and preventing pollination and/or fertilisation (fruit set). There are a number of chemicals that work in this way but the only one currently available to UK growers is the nutrient ammonium thiosulphate (ATS).
Other blossom thinners that have shown promise in trials both in the UK and abroad are:
- Elgetol (DNOC)
- Endothallic acid
- Lime sulphur
- Pelargonic acid
- Wilthin
- Armothin
- Ethephon
- Urea
- Zinc
- Vinegar
- Common salt (sodium chloride)
Other chemicals applied at the time of flowering for flower thinning may be effective through a different action to those listed above.
- Ethephon releases the gas ethylene and stimulates the drop of flowers and young fruitlets.
- Part of the thinning reaction induced by high rate urea sprays is undoubtedly attributable, indirectly, to the damage the sprays cause to the primary leaves surrounding the flowering spurs.
- The mode of action of zinc in causing thinning is not understood.
- The auxin naphthylaceticacid (NAA– Planofix) has also been applied close to full bloom as an apple thinner.
- This is thought to trigger the early drop of flowers and fruitlets by causing a temporary check to the production and movement of foodstuffs (carbohydrates) to the flowers.