These technologies also lessen the competition from a portion of the excess crop early and rapidly--thereby improving fruit size, quality and return bloom. Being non-chemical, the obstacle of registration for a new compound is avoided. New mechanical thinning strategies coupled with narrow tree architectures have potential to favorably impact grower profitability both by reducing labor requirement and by improving fruit size and quality.
In a Specialty Crop Research Initiative project led by Penn State, trials with a mechanical blossom string thinner were performed in 30 Pennsylvania, 9 South Carolina, 22 Washington and 9 California commercial and research orchards. A USDA drum shaker was tested in PA orchards during the bloom and green fruit stages. Conventional hand thinning at the green fruit stage was the control treatment. Varying operational speeds, blossom stages and pruning modifications to improve access by the thinner were assessed. Data were uniformly collected across all regions to determine blossom removal rates, fruit set, labor required for follow-up hand thinning, fruit size distribution at harvest, yield and economic impact.
String thinner trials to assess optimum operational parameters for varying growing regions and tree forms showed reduced labor costs compared to hand thinned controls and increased crop value due to a larger distribution of fruit in higher market value sizes. Blossom removal ranged from 20% to 55%, hand thinning requirement was reduced by 25% to 65%, and fruit size distribution improved in all but one trial. Net economic impact at optimum tractor and spindle speeds was $462 to $1490 and $230 to $847 per acre for processing and fresh market peaches, respectively. Trials with the string thinner at varying bloom stages showed the thinning window is from pink to petal fall. Trials on modifications in tree training to improve access by the string thinner indicated detailed pruning for targeted crop loads was superior to standard pruning. Studies with a new drum shaker prototype adapted from a blackberry harvester demonstrated increased thinning consistency compared to previous research with a citrus drum shaker. Joystick control of the Darwin thinning unit was developed and successfully tested to improve the positioning performance of the spindle. This allows the tractor operator to more easily maneuver the spindle for proper alignment with the tree without having to use the vehicle steering for that purpose, reducing operator fatigue and improving thinning performance. Sensors to automate the positioning of the spindle were also tested.
While our research resulted in the Darwin string thinner being successfully deployed to reduce hand thinning labor in peach, results from apple have been a mixed bag. Apple flowers form from a mixed bud, and the spur leaves under the flowers are crucial to fruit growth during the cell division phase of growth. Although thinning has been achieved in our apple studies, fruit size has been increased in only one of several apple trials conducted in Pennsylvania. It is thought that the spur leaf damage that accompanies the thinning explains the lack of fruit growth promotion.
Research on the string thinning units has led to improvements in the commercially-available products. A new development that will soon be on the market is a joystick-controlled thinner with a side-shifting spindle. The concept is based on Penn State research to improve the operator control of the thinning equipment. Development of a selective thinning unit is on-going. Prototype designs for the components of the unit, including image analysis, controls, robotic arm selection and blossom removal mechanism, are being investigated and tested.
Prepared by Jim Schupp, Tara Baugher, Paul Heinemann, Edwin Winzeler, Jude Liu, David Lyons