- Novel approaches to crop load management of pear orchards in the Pacific Northwest (PNW) are necessary, but will need to vary according to the unique vegetative and reproductive growth habits of the cultivar. 'D'Anjou' is vigorous and non-precocious; thus, strategies to limit vigor and induce early fruiting are required. In contrast, 'Bartlett' possesses a high fruit-setting efficiency and, consequently, is prone to over-setting fruit. Thus, crop reduction is necessary to achieve marketable fruit size, but hand thinning is time-consuming and costly. Two methods were investigated to resolve these issues: Root pruning of 'd'Anjou' pear trees and, chemical thinning of 'Bartlett' pears using abscisic acid (ABA).
Root pruning was imposed on one or both sides of the tree row and compared to an untreated control plot at two sites: Moderate-density, 6th leaf 'd'Anjou'/OH x F 87; and, high-density, 4th leaf 'd'Anjou'/OH x F 87. Root pruning two-sides of the tree row consistently reduced shoot growth and the effects were partially dependent on tree age at the time of root pruning. Return bloom was positively affected by root pruning, but fruit weight was often reduced. Reduced fruit weight was not associated with mid-season water or nutrient deficits. Yield and yield efficiency the year following double-sided root pruning were improved with the greatest response occurring in the younger orchard (i.e., ~70% yield increase over control plots). Root pruning is a viable strategy to reduce vigor and improve precocity in high-density 'd'Anjou' plantings when performed in the 3rd or 4th year after planting.
Thinning efficacy of abscisic acid (ABA) applied to 'Bartlett' pear trees, between petal fall and 12mm fruit size, was inconsistent. Among four trials, ABA produced a rate-responsive, transient reduction in stomatal conductance and net photosynthesis (Pn) of ~ 80% to 95% within hours of application, the effect lasted one to two days. By day three, Pn returned to ~80% of control plots and was fully recovered by 7-10 days after application. Thinning was best achieved at 100-125 ppm ABA. Higher rates caused greater fruitlet abscission but rates exceeding 400 ppm resulted in phytotoxicity and leaf abscission. In field trials, good thinning was observed in years when low natural light (i.e., cloudy conditions) occurred during the week immediately succeeding ABA applications. Only a few days of low light appeared necessary to elicit ABA-induced fruit abscission. To test the additive effect of shade on ABA-induced thinning, an experiment was designed to expose whole canopies to one of three levels of shade (0%, 44%, or 77%) and two levels of ABA (0 or 125 ppm). Shade houses were erected within hours of ABA application (~petal fall) and were left in place for 15 days. Both ABA and shade affected fruit abscission, but 44% shade did not significantly reduce fruit set compared to control plots. Photosynthetic active radiation (PAR) was reduced relative to the intensity of shade, but Pn was not. Moderate shade (i.e., 44%) led to only minor reductions of Pn, while 77% shade reduced Pn by 50% to 75% for the duration of the treatment period. ABA-induced thinning was not significantly improved by the addition of shade; however, two days of non-forecasted, cloudy conditions (i.e., low light) within the first week of the experiment invalidated comparisons to a true control (i.e., 0% shade). Given the short-term Pn limitation induced by ABA, in combination with the high carbon reserves of pear trees, ABA application may not be consistent enough to warrant commercial application.