Pilot comparative trials with organic thinners in two apple cultivars (Malus domestica L. Borkh.)

Considering the scarce availability of effective thinning methods for organic apple cultivation, and the importance of fruit load management to obtain both high-quality apples at harvest and an adequate return bloom in the following season, a pilot study was conducted on two apple cultivars, ‘Renè Civren’ and ‘Gaia’, which were subjected to different thinning treatments. These two cultivars are characterized by different behaviors in terms of physiological fruit drop, with the former showing an almost self-thinning attitude. In addition to the techniques already used in organic farming, the research also examined the potential of alternative methods, such as the use of vinasse and soft soap (potassium salt of unsaturated fatty acids), as more environmentally conscious alternatives. The results demonstrated that treatments with calcium polysulfide and mineral oil did not point out a notable reduction in total final fruit drop when applied to ‘Renè Civren’, when compared to the untreated control. In contrast, ‘Gaia’ exhibited a higher fruit drop. The alternative use of vinasse demonstrated a considerable potential as a thinning agent, with results in some cases better than those obtained with the previous treatments. On the other hand, soft soap, although effective, was applied at very high rates, which might render the treatment unsustainable in production contexts due to its high costs. The present trials pointed out the need to tailor organic thinning treatments to specific cultivars and local conditions, as well as to explore new substances that can enhance the competitiveness and sustainability of organic apple farming.