A precision pruning strategy for improving efficiency of vibratory mechanical harvesting of apples

The state of Washington is the biggest fresh market apple (Malus domestica Borkh.) producer in the U.S., and the state's annual apple production has exceeded 60% of the national production. Due to the extensive labor requirements for harvesting fresh market apples, there is burgeoning demand for mechanical harvest solutions. Our transdisciplinary studies on mechanical harvest systems for apples have shown that fruit removal efficiency (FRE) with a vibratory system can be improved with precision canopy management. In this study, we evaluated the effect of precision pruning strategies on FRE in two groups (106 and 107, respectively) of randomly selected horizontal branches of 'Jazz/M.9' apple trees in a commercial orchard. Fruiting lateral branches were pruned to either shorter than 15 cm (guideline 1, G1) or 23 cm (guideline 2, G2). Harvest tests were conducted using a shake-and-catch harvester prototype developed by Washington State University with a fixed vibrating frequency of 20 Hz and shaking duration of 5 s. FRE for branches treated with G1 was significantly higher (91%) than FRE for branches treated with G2 (81%). We recorded a negative relationship between FRE and lateral shoot length. FRE was up to 98% when shoots were shorter than 5 cm, and FRE was only 56% for shoots of 25 cm or longer. We developed a shoot diameter-to-length index (S-index) to better understand the effect of shoot size on FRE. FRE was as high as 98% when the S-index was greater than 0.15. In addition, mechanically harvested fruit quality was assessed by categorizing the fruit into Extra Fancy, Fancy, and Downgrade fresh market classes based on USDA standards; however, no significant difference was found between the two treated groups. These results suggest that pruning lateral fruiting branches to less than 15 cm or to an S-index greater than 0.03 is required to achieve FRE of 85% with no negative impacts on fruit quality.