S16 - Session P3 - Preliminary results on the effect of AVG (ReTain®) and 1-MCP (HarvistaTM) applied at bloom on fruit set, yield, and apple size in WA 38 cultivar

Authors: Sara Serra *

WA 38 (Enterprise x Honeycrisp) is a new apple variety developed by Washington State University and released to WA growers in 2017. WA 38 is attractive, crisp, juicy, and highly appreciated by US consumers. It is considered a self-thinning cultivar, setting one or two fruit per flower cluster without the need for chemical thinning. Fruitlet shedding typically lasts 7-8 weeks and fruitlet retention is around 17% of total flowers. However, historical yield data shows fluctuations in number of fruit from year to year. This study focused on evaluating two chemical methods to increase the fruit set in WA 38. Considering the role of ethylene in post-bloom fruitlet drop, the present approach tested the efficacy of two ethylene inhibitors, AVG (ReTain ® ) and 1-MCP (Harvista TM ), in extending flower longevity by disrupting ethylene signaling. The study took place in a WA 38/M9-Nic29 experimental block located in Quincy (WA, USA) and planted in 2018 (3,870 trees/ha). At bloom 2021, Retain ® and Harvista TM were applied at label rate at two phenological stages: 31% open king flowers and 7 days later at beginning of petal fall. Monthly fruit retention (%), flower/fruitlet ethylene production, and number of apples and yield per tree were recorded and compared to an untreated control ("CTRL"). The entire harvest was graded for packout and size distribution data. Apple retention did not differ significantly in response to ethylene inhibitors sprayed at different bloom stages. Production per tree and crop load at harvest also did not significantly differ among the 5 treatments. However, average fruit weight and size distribution was significantly affected, with "Retain 7 days" showing the highest proportion of graded apples in the small size category (≤ 215 g), while "Retain 31% king bloom" and "CTRL" presented significantly higher proportions of apples in medium + large size classes (216-339 g).