In South Tyrol 2022 was a difficult year for chemical thinning. Especially in the case of Gala, thinning was often too strong. According to Gerhard Baab (Agroselection), various factors influence the thinning result, such as variety, growth and active ingredient. But especially the carbohydrate balance, the auxin flow and the gibberellin content affect natural fruit drop.
A small number of seeds and a small supply of photosynthetic products promote fruit drop. Carbohydrates play a decisive role, however. During flowering they are the most important nutrient and at the same time the source of energy for the plant. The consumption of carbohydrates depends on fruit load and growth. A well-balanced carbohydrate balance results in a smaller June drop. Intensive (short) pruning of fruit-bearing wood boosts fruit drop and provides larger leaves and therefore more photosynthesis.
The various active ingredients differ in their efficacy. ATS disturbs fertilisation, 6-BA boosts auxin hierarchy, NAA reduces auxin export from the fruit and metamitron reduces photosynthesis. The wetter and warmer the air, the more active ingredients are absorbed by the leaves. Ultimately, the carbohydrate balance is the decisive factor, however.
Causes of too strong thinning
The primary cause for the (too) strong thinning in 2022, especially in the case of Gala (especially the dark strains) and Golden Delicious, is the heightened natural fruit drop due to the very high night temperatures and the resulting stress situation (Christian Andergassen, Research station Laimburg, and Jos de Wit, Fruitconsult). Uniform fruit size, strong growth, heightened efficacy of metamitron on fruits bigger than 20 mm and the sometimes poor leaf quality strengthen fruit drop.
When determining the thinning strategy for 2023, not all old experiences should be thrown overboard. Almost certainly, 2023 will be a completely different year. ATS (+ Ethrel) remains the basis for thinning. It is necessary to pay more attention to the differences between the shady side and the sunny side and to be more careful with metamitron lower down in the tree.
(Source: Presentations Gerhard Baab, Christian Andergassen and Jos de Wit at the fruit growing seminar that is annually organised by the graduate association of the agricultural and horticultural colleges (A.L.S) in South Tyrol.)