Comparison of the most frequently used apple thinning compounds for the thinning of 'Jonagold', 'Elstar' and 'Golden Delicious' apples

Three-year apple thinning experiments were conducted on mature 'Jonagold'/M9, 'Elstar'/M9 and 'Golden Delicious'/M9 apple trees. The consistency of thinning was investigated after the application of carbaryl, 1-naphthaleneacetic acid (NAA), naphthaleneacetamide (NAAm), 6-benzyladenine (BA), ethephon and photosynthesis inhibitor terbacil. Over three years, carbaryl (800 ppm) had a good thinning effect and promoted a nice return bloom for all cultivars. Terbacil (50 ppm) and NAA (15 ppm) application had a variable influence on thinning and on return bloom in all cultivars over three years. NAAm (50 ppm) had a weak thinning effect on the average. All cultivars were thinned nicely with BA (50 ppm) spraying, except for 'Jonagold' and 'Elstar' trees in 1996. BA had no consistent effect on return bloom. In the case of spraying with ethephon (300 ppm), fruit growth did not follow the thinning effect adequately, but the promotion of return bloom was mostly enhanced.