Promoting fruit set of 'Ingeborg' pears in a northern climate

The triploid pear cultivar 'Ingeborg' is currently the main commercial pear cultivar grown in Norway. However, fruit set and subsequent yields of this cultivar have proven to be variable and overall rather low. In order to promote the fruit set, different bioregulators were applied during and after bloom and compared with an untreated control. Investigations were done during the period 2017-2018, at NIBIO Ullensvang, western Norway. Different dosages of both gibberellins (GA₃, trade name GIBB 3, 10% active ingredient (a.i.) and GA_4/7, trade name Novagib^®, 1% a.i.) were applied at full bloom and at petal fall. Additional applications of the growth retardant prohexadione-Ca (trade name Regalis^®, 10% a.i.) were applied twice, when bourse shoots had 3-5 leaves and after one month later. Ethephon (Cerone (480 g a.i. L^‑1)) was applied three times starting about 7 days after petal fall with ca. 7- to 10-day intervals. All gibberellin applications significantly increased fruit set compared to the untreated control. One single application with GA₃ (3 g ha^‑1) almost tripled the fruit number per 100 flower clusters when compared with the control (136 and 46, respectively). The yield response was similar (16.8 to 9.6 kg tree^‑1, respectively). Similar results occurred with one application of GA_4/7 (12 g ha^‑1) with the same crop load level, and the fruit weights were similar to the control (130 g). Prohexadione-Ca treatments significantly reduced shoot growth of the pear trees. Two treatments with 125 g ha^‑1 or one treatment of 250 g ha^‑1 reduced the growth by ~35% but had no significant effect on fruit set and yield. The multiple ethephon applications (275 mL ha^‑1 in total) had no effect on both set and shoot growth, and return bloom compared to the untreated or gibberellin treated trees.