Influence of Mechanical Thinning Severity Treatments on Vegetative and Reproductive Tissues, Fruit Set, Yield, and Fruit Quality of ‘Buckeye Gala'

The number of chemical thinning compounds available to apple growers is expected to be reduced in the future, and there is a need to develop alternative thinning treatments. The goals of this experiment were to test the efficacy of a mechanical string thinner (Darwin PT-250, Fruit-Tec, Deggenhauserertal, Germany) on apple, and identify an optimal range of thinning severity, as influenced by spindle rotation speed. Trials were conducted in 2010 and 2011 at Penn State’s Fruit Research and Extension Center in Biglerville, PA, on five-year-old ‘Buckeye Gala’/M.9 apple trees that were trained to Tall Spindle. The following treatments were applied to the same trees for two consecutive years, at a constant ground speed of 4.8 km/h during full bloom: 1) 0 rpm (control); 2) 180 rpm; 3) 210 rpm; 4) 240 rpm; 5) 270 rpm; and 6) 300 rpm. As spindle speed increased, the removal of the number of blossom clusters per limb cross-sectional area increased. A linear reduction in the number of blossoms per spur was observed as spindle speed increased. In 2010, leaf area per spur was reduced by 9 – 45%. In 2011, the fastest spindle speed reduced leaf area per spur by 20%. Low spindle speeds improved spur quality in 2011, as 180 rpm had a greater spur leaf area than that of the control (22% increase). While increased spindle speed reduced cropload, injury to spur leaves limited any benefit of increased fruit size. The largest gain in fruit weight was 28 g (300 rpm) when compared to the control. In both years, the most severe thinning treatments reduced yield by more than 50%. Fruit quality was generally enhanced with increased spindle speed. There was no relationship between spindle speed and return bloom. The mechanical string thinner consistently removed blossoms in both years, but severe thinning treatments (240–300 rpm) resulted in high rates of spur removal and a drastic reduction in spur leaf area. Damage to spur leaves negatively influenced fruit size, fruit retention, and fruit calcium. Spindle speeds of 180–210 rpm provided the best overall thinning response and minimized injury to spur leaves.