Application and implications of rule-based pruning of apple trees
Labor is one of the largest operating costs associated with tree fruit production in the United States. Pruning and harvest can easily eclipse over a quarter of an orchard's operating budget. Additionally, labor sources are becoming increasingly difficult to find while worker pay keeps going up. As a result, it appears that labor costs will continue to rise for the foreseeable future. Work on developing mechanized alternatives to pruning is seen as one of the key areas to reduce an orchard's dependence on human labor. The objective of this study was to test a set of simplified pruning rules (RUL), written for possible use by an automated pruner, against pruning done by a commercial orchard crew (COM), and pruning by skilled horticulturists from Purdue University (PHP). Measurements on orchard productivity, fruit quality, light distribution, and pruning cuts were taken to determine what differences, if any, existed between the three pruning treatments. This experiment was replicated in two different commercial orchards in Indiana, U.S. using two different scion/rootstock combinations in 2013 and expanded to three locations with five different scion/rootstock combinations in 2014. In 2013, minimal differences were seen between pruning treatments. There were no differences in yield for either scion/rootstock combination. Light distribution was higher in the COM trees of 'Golden Delicious'/M.26 while soluble solid concentration (SSC) was higher in fruit from the PHP treatment of these trees. 'Pink Lady'/B.9 PHP fruit were significantly smaller than fruit from the other two treatments. In 2014, a significant difference was seen in only one of the scion/rootstock combinations in terms of light distribution. Yield was highest in the COM for 'Golden Delicious'/M.26, but individual fruit weight was also the lowest in this treatment. No other differences were seen in terms of yield or fruit quality. Pruning analysis showed that the RUL treatment made fewer cuts than the COM treatment in three of the five cultivars. Looking at the distribution of cuts, the proportion of cuts made for branches greater than 1" in diameter was highest in the RUL treatment for both 'Golden Delicious'/M.26 and 'Fuji'/M.9. The COM treatment for these cultivars had the lowest proportion of large cuts. These results show that only minimal differences existed between pruning treatments and, more importantly, show that pruning using a set of simplified pruning rules will not result in a loss of productivity or fruit quality.
Hirst, Purdue University.
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