Abstract
Thinning is one of the most important management strategies in apple orchards. Not only the fruit quality (size, colour, sugar) of the current season is influenced, but also the return bloom of the next season is influenced. Many apple cultivars are sensitive to alternate bearing, if the thinning is inadequate or performed too late in the season. To prevent or overcome alternate bearing in apple orchards chemical or mechanical flower thinning are the most effective management practices. Ammonium thiosulfate (ATS) is, beside lime sulfur the most used chemical flower thinner in South Tyrol. The objective of this work was to 1) evaluate the thinning potential of ATS in 'CIVM49' apples (Malus domestica) trees, and 2) to compare two different spraying systems. As standard treatment, the ATS sprays were conducted with a commercial air blast sprayer, this treatment was compared with the robotic system ADAM, developed by Aigritec. This system used a robotic arm with a single spray nozzle. Flower clusters were detected with computer vision and afterwards every single open flower cluster on the apple trees was treated individually. Our experiment showed that the thinning efficacy was more than doubled, when ATS was applied precisely to each flower cluster with the ADAM system, compared to the commercial air blast sprayer. INTRODUCTION Thinning is one of the most important management strategies in apple orchards. The number of fruits on a tree directly influences the yield, the fruit size, and the quality of fruits. If there are too many fruits on a tree, the fruit size is small, the colour is incisive, and the apples do not have the full flavor. So, insufficient thinning will reduce profitability of apple orchard. The return bloom of the next season can be negatively affected too. The initiation for the following year's crop is reduced and consequently the return bloom from the next year is poor. Overthinning also causes economic problems. Yield and crop value of the current year are reduced, and the fruit size is too big hence flavor, colour and flesh firmness cannot be of first quality (Robinson et al., 2021). Many apple cultivars are sensitive to alternate bearing, if thinning is inadequate or performed too late in the season. To prevent or overcome alternate bearing in apple orchards chemical or mechanical flower thinning are the most effective management practices (Solomakhin and Blanke, 2010). In chemical thinning, chemicals are sprayed when 60-80% of flowers are in bloom. The chemicals, such as lime and sulfur, ammonium thiosulphate (ATS) and others prevent flowers from pollination in different ways including preventing pollen germination and pollen tube growth, burning flower stigma and pistils, or inhibiting the plant photosynthesis causing plant carbon stress leading to flower abscission (Bhattarai et al., 2023). At present the most common thinning practice in South Tyrol is spraying ATS or lime sulfur in organic production during bloom to prevent pollen germination on the pistils of the flowers. For application farmers use a standard commercial air blast sprayer. Although chemical thinning can perform mass thinning with low costs, the performance is variable (De Ros Marchioretto et al., 2019) and unpredictable depending on