The Effect of Mechanical String Thinner Spindle Rotation Speed on Apple Spur Bud Removal

Reducing apple crop load potential by spur pruning (artificial spur extinction; ASE) has been the focus of research since the mid-1990s. A hand-held tool was developed by researchers at Institut National de la Recherche Agronomique (INRA, Montpelier, France) to aid in ASE treatments [Equilifruit; 6 spurs per cm² limb cross-sectional area (LCSA)]. The Equilifruit was developed for the Centrifugal training system, which utilizes ASE as the primary means of reducing bearing surface. The Equilifruit disk was tested in 2009 and 2010 as a hand-thinning gauge on tall spindle-trained apple trees in Pennsylvania, and resulted in appropriate crop loads and increased fruit size. ASE was tested on tall spindle apple trees in Australia, where researchers confirmed the validity of the concept. As a result, ASE was recommended as a crop load management strategy. It is purportedly easy to train pruning crews to do ASE, but the practice is labor intensive. Studies were conducted on tall spindle ‘GoldRush’ / 'M.9', and vertical axis ‘Golden Delicious’ / 'Bud. 9' apple trees to determine an appropriate spindle speed for mechanized ASE. Mechanical ASE was performed with a tractor-mounted mechanical string thinner (Darwin 300; Fruit-Tec, Deggenhauserertal, Germany) operated at 4.4 km ∙ h^-1 forward speed. To determine an appropriate spindle rotation speed, a string density of 270 strings was rotated at 210, 240, 270 and 300 rpm on ‘GoldRush’ apple trees, and at 270, 300, 330, 360 and 390 rpm on ‘Golden Delicious’. Treatments were applied at green tip. Limb diameter was measured, and spurs were counted on two to three limbs per tree. Spur counts occurred before and after mechanical ASE. Spur removal (%) and spur density per cm² LCSA were calculated and analyzed using regression. ‘GoldRush’ spur removal increased linearly with increasing spindle speed. Spur density was linearly reduced from 12.9 to 8.9 buds per cm² LCSA. The number of remaining buds was supraoptimal, even at 300 rpm. ‘Golden Delicious’ spur removal increased linearly with increasing spindle speed. Spur density was linearly reduced from 13.6 to 2.9 buds per cm² LCSA. The regression equation showed that a spindle speed of 345 rpm would result in the target spur density of 6 buds per cm² LCSA. This technology has potential for mechanizing ASE.