Root Pruning Reduces Vigor in High-density d'Anjou Pear Orchards

It is crucial that expenses associated with orchard establishment are recovered early.  This is especially important with high-density plantings; however, for pear, non-precocious and vigorous young trees limit the commercial adoption of high-density designs.  Root pruning has been successfully used to control shoot and canopy growth in high-density, European pear orchards, while simultaneously improving fruit set and yield efficiency.  A thorough evaluation of root pruning in US pear orchards, however, has not been performed.  Therefore, we used the inherently high-vigor cultivar ‘d’Anjou’ on OH×F 87 rootstock to investigate root pruning at two separate sites: 6^th-leaf, moderate-density (769 trees/ha); and, 4^th-leaf, high-density (2,243 trees/ha) orchards.  Initially, we hypothesized that root pruning would result in an inverse relationship between tree vigor (the season of root pruning) and return bloom, fruit set and yield (the year following application).  We compared single- and double-sided root pruning treatments (applied at ~10% of full bloom, 45 cm from tree trunks to a depth of 45 cm) with and without application of AVG (aminoethoxyvinylglycine hydrochloride, ReTain®, Valent Biosciences; 823 g per ha rate, applied 10 days after full bloom), to untreated controls.  Vegetative growth was reduced at both sites relative to the severity of root pruning (i.e., single- vs. double-sided), though the magnitude of the response differed between sites as a function of tree age.  At the conclusion of the second season (root pruning was not performed in year 2), carry-over effects of root pruning on vegetative growth were roughly half those observed after year-one.  In the 6^th-leaf orchard, root pruning resulted in reduced fruit set, yield and harvested fruit size the year of the application, likely attributed to higher stress associated with a greater percentage of root volume removed compared to the 4^th-leaf, younger trees.  In year 2, only the double-sided root pruning treatment produced greater return bloom, fruit set and yield relative to controls, but fruit size remained smaller.  This yield recovery, however, was not high enough to compensate for year-1 deficits.  At the 4^th-leaf site, we did not observe any negative effects on fruit set, fruit growth rate, yield or fruit size the year of root pruning.  Application of AVG led to a twofold increase in fruit set and yield, irrespective of root pruning.  AVG-treated trees had slightly smaller fruit size, though this was likely an indirect effect of higher crop loads.  New insights for developing root pruning programs for pear will be discussed.