Mechanical Pruning Trial in ‘French’ Prune – Through Year Three

Becky Wheeler-Dykes, Orchard Researcher, CSU Chico
Dr. Rich Rosecrance, Professor, College of Agriculture, CSU Chico
Luke Milliron, UCCE Farm Advisor Butte, Tehama and Glenn Counties
Franz Niederholzer, UCCE Farm Advisor, Colusa and Sutter/Yuba Counties

Work continues at the mechanical pruning trial site in Red Bluff. The trial, funded by the California Prune Board and led by Dr. Rich Rosecrance at Chico State, was initiated in 2019 and aims to identify lower cost pruning alternatives for growers without compromising yield or fruit quality. The study site is an orchard planted in 2011 at a spacing of 15’ x 18’ on Myrobalan seedling rootstock and irrigated with buried drip. This is a vigorous, well managed orchard with a history of producing 4-5 dry ton/acre. The study treatments are:

  1. Fall: Grower standard — ladders and loppers pruning, no topping (i.e. ‘control’).
  2. Spring: Topping and hedging both ways — cutting 5 sides of the canopy, with the tree row and across the tree row, plus topping (i.e. ‘boxed’).
  3. Fall: Topping and hedging both ways — cutting 5 sides of the canopy, with the tree row and across the tree row, plus topping (i.e. ‘boxed’).
  4. Spring: Hedging both sides of the tree row, no cross hedging — cutting 2 sides of the canopy, no topping (i.e. ‘hedged’).
  5. Fall: Hedging both sides of the tree row, no cross hedging — cutting 2 sides of the canopy, no topping (i.e. ‘hedged’).

Several measurements are collected annually to assess the effects of the pruning treatments. Highlights of 2021 results and planned 2022 measurements include:

  • Canopy volume was measured in May 2021. Trees boxed in spring were significantly smaller than the hand-pruned control. Trees boxed in fall or hedged in fall or spring did not differ significantly from the control.
  • A bark canker pathogen survey (Cytospora, Botryosphaeria, etc.) was conducted to establish a baseline of pathogens present in the field. Several canker pathogens were found, and the control treatment had the shortest canker length. A more thorough evaluation of disease presence and severity will be performed in 2022.
  • In 2021, a difference in severity of sunburn damage after extreme heat events was observed. The boxed treatments both had significantly less blue prune drop due to sunburn damage than the control or hedged treatments. This was surprising – fruit is typically exposed to higher light environments in the boxed trees. Though there is no definitive cause at the moment, it may be possible that bowing branches in the hedged treatments or reduced water stress in boxed trees due to smaller canopy size may have contributed. We will continue to assess these differences.

2021 treatment yields ranged between 3.0 and 4.3 dry ton marketable (A+B screens) yield per acre and no significant differences were found among the treatments in 2021 nor in cumulative yields (Table 1). Large fruit (A + B screens) comprised between 92% and 100% of the four-pound sample from all the treatments (Table 1). The spring hedged treatment did have significantly lower percentage of marketable (A+B screen) fruit than the control and fall boxed treatments.

Table 1. Percent large fruit, canopy circumference trunk and canopy diameter, and pruning weights in 2019, 2020, and 2021.

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