Mechanical methods of thinning fruit trees such as high-pressure spray guns, tree shakers, club thinning, rope thinners, drum shakers, and string thinners can produce a thinning response in stone fruits and some nut crops (Dennis, 2000). There has been limited adoption of mechanical thinning practices in apple as a result of two primary factors: 1) the damage and removal of spurs; and 2) the potential to spread the fireblight pathogen Erwina amylovora.
In apple, primary spur leaves emerge before anthesis and can be damaged by physical disturbance. Ferree and Palmer (1982) showed the importance of spur leaf area on young fruit development and retention. As illustrated by Ngugi and Schupp (2009), mechanical thinners can also be an efficient vector of fireblight. Precautions are advised when using the string thinner on apple such as the use of predictive models to forecast the risk of infection, avoiding use of the string thinner in blocks with a history of fireblight, and the use of an antibiotic post-treatment if conditions are conducive for fireblight infection (Ngugi and Schupp, 2009).
Recent mechanical thinning investigations on apple have used two different thinning machines and have shown good efficacy (Bertschinger et al., 1998; Damerow et al., 2007). Thinning with the Darwin (Fruit-Tec, Deggenhauserertal, Germany) string thinner reduced fruit set by 25%, enhanced return bloom when compared with the control, and did not significantly injure foliage (Weibel et al., 2008). A 50% increase in mean fruit weight, improved fruit color, and a reduction in follow-up hand-thinning time were demonstrated by Sinatsch et al. (2010). The impacts of spur leaf reduction resulting from mechanical thinning were examined by Solomakhin and Blanke (2010), but leaves were only considered damaged if one-third or more of the lamina was removed. At 320 rpm with a three-rotor string thinner (the Bonner, University of Bonn, Germany), less than 8% of leaves were injured while providing acceptable thinning efficacy. Trials in Germany with a three-rotor string thinner resulted in a 25-g increase in mean fruit weight, reduced yield, and enhanced packout by 20% when compared with an unthinned control (Veal et al., 2011).
The objectives of this study were to evaluate the efficacy of a single spindle string thinner on apple in the mid-Atlantic region, determine the influence of string number on thinning severity, and identify an optimal range of spindle speeds.
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