Fruit Thinning in Organic Apple Growing with optimized Strategies including natural Spray Products and Rope-Devices

Summary

The aim of our study was to find new thinning methods for organic apple production able to fulfil the standards of Bio Suisse, the main label organisation of the organic food sector in Switzerland.
The 17 trials reported were carried out during flowering period in the years 2003, 2004 and 2005 on the cultivars ‘Pinova’, ‘Gala’, ‘Maigold’, ‘Elstar’, ‘Breaburn’, ‘Golden Delicious’ and ‘Otava’ at different sites. The results confirm the good efficiency of mechanical thinning with the rope thinner and the thinning potential of vinasse, a by product of molasses. Different vinasse products were tested (K-Vinasse, light-vinasse, N Vinasse; NK-Vinasse) and it was observed that those containing less nitrogen had a minor thinning effect. To avoid phytotoxicity on leaves, the concentration of N-Vinasse should not exceed 10%. It seems that for ‘Gala’ higher concentrations are needed, 7.5-10%, whereas with ‘Maigold’ the efficacy of N-Vinasse was the same with the 2.5% concentration as with 5% and 10%. On the other hand, partly good results were obtained when N-Vinasse was over-concentrated to 12 or 15%, applied at warm weather and rinsed with clear water 4-6 hours later to avoid the phytotoxic effect (“burn & rinse” method). Corn Oil (5%), a commercial product from New Zealand, gave encouraging results. Lime sulphur (2%) did not effectuate a significantly better thinning effect. The best method tested with a fruit set reduction by 62% and 3,05 times higher flower bud set next year was achieved with a combination of rope thinner and N-Vinasse: rope thinner to thin the peripheral branches and to exert a physiological stress; and N-Vinasse to thin the inner parts of the canopy that the ropes can’t reach. Other tested products did not show satisfying effects. Trials also confirmed the importance of applying these thinning agents at least two times, at the appropriate phenological stage of flowering period mainly at F2 (full bloom) and G (= F2 of the one-year old shoots) and in case at F (king flower open). Hitting also the flowers of the one-year old shoots is important to break bi-annual bearing. Pollen tube growth was assessed by fluorescence microscopy. Almost complete inhibition of pollen tube germination was observed with Lime Sulphur and over concentrated N-Vinasse (12%%) whereas with N-Vinasse at 5%% inhibition was 50%% less compared to untreated styles.. Overlooking all trials, combining N-Vinasse and rope thinner was the most efficient strategy for flower thinning. However, after our trials, its efficacy seems to depend strongly on climatic conditions and phenological stages: treatments with N-Vinasse should be carried out at warm and sunny days above 16, better 18-20 deg. C. to maximize desiccation on the styles; whereas the rope thinner should be applied during periods of cold and clouded weather to maximize the thinning effect due to physiological stress and shortage of assimilates to the fruitlets.