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.
Baugher, T.A., Schupp, J., Lesser, K. & Reichard, K. 2009 Horizontal string blossom thinner reduces labor input and increases fruit size in peach trees trained to open-center systems HortTechnology 19 755 761
Bertschinger, L., Stadler, W., Stadler, P., Weibel, F. & Schumacher, R. 1998 New methods for an environmentally safe regulation of flower and fruit set and of alternate bearing of the apple crop Acta Hort. 466 65 70
Blanpied, G.D. & Silsby, K.J. 1992 Predicting harvest date windows for apples. Cornell Coop. Ext. Info. Bul. 221
Dorigoni, A., Lezzer, P., Micheli, F., Dallabetta, N. & Pasqualini, J. 2010 Diradare il melo a macchina: Cosa sapere per farlo bene L’nformatore Agrario. 22 63 67
Ferree, D.C. & Palmer, J.W. 1982 Effect of spur defoliation and ringing during bloom on fruiting, mineral level, and net photosynthesis of ‘Golden Delicious’ apple J. Amer. Soc. Hort. Sci. 107 1182 1186
Hehnen, D., Hanrahan, I., Lewis, K., McFerson, J. & Blanke, M. 2012 Mechanical flower thinning improves fruit quality of apple and promotes consistent bearing Sci. Hort. 134 241 244
Huang, C.L. & Schulte, E.E. 1985 Digestion of plant tissue for analysis by ICP emission spectroscopy Commun. Soil Sci. Plant Anal. 16 943 958
Kong, T., Damerow, L. & Blanke, M.M. 2009 Effect of mechanical thinning on ethylene efflux, yield, and fruit quality in apple Erwerbs-Obstbau 51 39 52
Larrigaudiere, C., Graell, J., Salas, J. & Vendrell, M. 1997 Cultivar differences in the influence of a short period of cold storage on ethylene biosynthesis in apples Postharvest Biol. Technol. 10 21 27
Lin, S. & Walsh, C. 2008 Studies of the tree factor and its role in the maturation and ripening of ‘Gala’ and ‘Fuji’ apples Postharvest Biol. Technol. 48 99 106
Ngugi, H.K. & Schupp, J.R. 2009 Evaluation of the risk of spreading fireblight in apple orchards with a mechanical string blossom thinner HortScience 44 862 865
Saltveit, M.E. 1982 Procedures for extracting and analyzing internal gas samples from plant tissues by gas chromatography HortScience 17 878 881
Schupp, J.R., Baugher, T.A., Miller, S.S., Harsh, R.M. & Lesser, K.M. 2008 Mechanical thinning of peach and apple trees reduces labor input and increases fruit size HortTechnology 18 660 670
Sinatsch, S., Pfeiffer, B., Toups, I., Zimmer, J. & Benduhn, B. 2010 Comparison of different thinning measures for organic grown apples. 14th Intl. Conf. on Organic Fruit Growing. 14:200–205
Stadler, W., Bertschinger, L. & Wiebel, F. 1996 Maschinelles ausdunnen—geeignet fur den umweltschonenden apfelanbau Obst-Weinbau. 132 614 616
Veal, D., Damerow, L. & Blanke, M.M. 2011 Selective mechanical thinning to regulate fruit set, improve quality and overcome alternate bearing in fruit crops Acta Hort. 903 775 782
Weibel, F.P., Chevillat, V.S., Rios, E., Tschabold, J.L. & Stadler, W. 2008 Fruit thinning in organic apple growing with optimized strategies including natural spray products and rope-device Europ. J. Hort. Sci. 73 145 154
Yuan, R. & Greene, D.W. 2000 Benzyladenine as a chemical thinner for ‘McIntosh’ apples. II. effects of benzyladenine, bourse shoot tip removal and leaf number on fruit retention J. Amer. Soc. Hort. Sci. 125 177 182