Orchard Notes: May 2022

A Message from the Executive Director

A few weeks ago, I attended the International Nut & Dried Fruit Congress (INC) in Dubai which brought together more than 1100 industry professionals from 64 countries.  Issues ranging from the supply, market development, and shipping challenges to nutrition and sustainability were covered. Our posture as an active member of INC and sponsor of the Congress gives us a platform to reach the trade and showcase our premium difference as the global prune category leader. In addition to heavy foot traffic at our booth, Esther participated in a marketing panel and presented on behalf of the UK’s Dried Fruit Alliance to help educate key opinion leaders. I presented during the Dried Fruit Roundtable and shared global prune statistics as well as current and future market trends.

The panel discussion around global prune industry statistics was a stark reminder of how volatile prune production by country and supply have been in recent years. The significant oversupply from just a few years ago has morphed into a very tight market, particularly for California Prunes. With France headed for yet another crop disaster (est.~15,000 MT in 2022 after zero carry-in), larger prunes will be the primary domain of California. On the other hand, Chile has estimated its recent harvest at 65,000 MT, with average sizes being smaller than normal, and Argentina at 18,000 MT, setting up for a significant amount of supply in the smaller prune sizes.  California Prune growers will be well-served to deliver the optimum sizes they’re known for in a space largely unoccupied in this upcoming season.

With the timeframe for thinning closing rapidly, it is worth reviewing one more time the information provided by Franz Niederholzer, UC Advisor. View Article

– Donn Zea
Executive Director

Orchard Considerations


  • Irrigation: Continue monitoring pressure chamber, soil moisture, and/or weekly ET to manage irrigation and maintain adequate orchard moisture. May and June are the most critical months for end-cracking. Also, watch the weather forecast for sudden jumps in temperature that can occur in late May or early June as the weather switches the mode from spring to summer. Sudden 15-20 degree increases in temperature can result in fruit sunburn damage; making sure orchards are not behind on irrigation when the heat hits may reduce damage.
  • Rust: Monitoring commences with the start of the month, surveying 40 trees weekly until July 15. Check lower parts of the canopy for leaf symptoms (spots) and pay close attention to non-bearing replants, exceptionally vigorous trees, and previous hot spots. Consider treating when the first leaf with rust is found. For more on rust see: ucanr.edu/PMG/r606100611.html
  • Peach twig borer (PTB) and Oblique-banded leaf roller (OBLR): Inspect fruit 400 degree days after the first PTB biofix. Look for larval entry points on the fruit (ideally 15 fruit from 80 trees), especially at fruit to fruit or fruit to leaf contact points. Treat if 2% or more (24+ of 1,200) of the fruit has damage. For OBLR, begin fruit inspections at 930 degree days after biofix for that pest, following the same sampling protocol and treatment threshold. More on PTB at: ucanr.edu/PMG/r606300211.html and on OBLR at: ipm.ucanr.edu/PMG/r611300511.html
  • Aphids: Leaf curl plum aphids move to summer hosts in May, but mealy plum aphids stay in orchards until mid-July. Heavy infestation of mealy plum aphid can limit flower bud development this year, which can mean less crop next year.
  • Fertility: Continue with the nitrogen and potassium fertilization program if a good crop is set. More than 50% of the annual N budget should be applied before June 1st.


  • Pest and disease management: Continue monitoring for aphids and rust.
  • Spider mites: Begin scouting by checking two different sections of the orchard each week. Spend about five minutes in each section, checking 2-3 leaves (some inside and outside of the canopy) on 10 trees. Look for spider mites as well as predators (predaceous mites and sixspotted thrips). Treatment decisions should be based on the population levels of both mites and predators. If more than 20% of leaves have mites, but less than 50% of the leaves have predators, treat for mites. If more than 60% of leaves have mites, treat them even if most leaves have predators. For more, see ucanr.edu/PMG/r606400411.html.
  • Irrigation: Mild to moderate tree water stress (-12 to -16 bars, measured by a pressure chamber) can help avoid excessive vegetative growth and associated pruning costs next winter without slowing fruit sizing this season. Maintain this water stress until the fruit has reached physiological maturity (when fruit averages 4lbs internal pressure), typically in early to late August.

Learn more at sacvalleyorchards.com/prunes/irrigation-prunes/pre-and-post-harvest.

Note: In late June, consider the weather forecasts when deciding on irrigation through early July. Traditionally, many growers reduce irrigation going into the July 4 Holiday to reduce orchard humidity and chances of fruit side cracking caused by dew events if the weather suddenly cools. Reduced irrigation to reduce side cracking may increase the risk of fruit sunburn if the weather stays hot. The weather is hard to get right but adding sunburn risk to the conversation along with side cracking risk is suggested.  Side cracking is more of an economic risk in years with light crops and larger fruit while sunburn may be a higher economic risk in heavier crop years with less extra-large fruit. See more information on sunburn (“blue prune”) at sacvalleyorchards.com/prunes/blue-prune-drop/.

Summer Resources

Irrigation Strategies in Drought Conditions

As we head into another summer, here is an article from UC Cooperative Extension on managing drought and best practices for regulated deficit irrigation. View 2015 Article

Heat Illness Prevention

Cal/OSHA reminds all employers with outdoor workers to be prepared and take the necessary precautions to prevent heat illness, as high temperatures are expected throughout the state this week. Employers in California must take steps to protect outdoor workers from heat illness by providing water, rest, shade, and training.

Read More

Mechanical Pruning Meeting

In early May, the Prune Board hosted UCCE researchers, and representatives from the Olive, Cherry, and Pistachio industries to discuss details of mechanical pruning and how it is impacting the production of other commodities as well as what is working for them and what isn’t.

Key takeaways from Gary Obenauf:

  1. Mechanical pruning provides reduced labor for pruning.
  2. Cost savings are significant at least in the short run.
  3. New pruning methods work best if you start with a first leaf to set up the tree correctly.

The research that is currently being conducted in relation to mechanical pruning will help the prune industry determine if mechanical pruning will work for the industry. More research is needed to determine the long-term effects of cost, disease development, the life of orchards, fruit quality, and size among other impacts. There are several industry members already doing some mechanical pruning on their own. Still, more scientific research in paired comparisons of different pruning methods is needed before this practice is adopted industry-wide.

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 aimed 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 a buried drip. This is a vigorous, well-managed orchard with a history of producing 4-5 dry tons/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 the 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 a 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.

Treatment Percent large fruit (screen A+B) 2019 Percent large fruit (screen A+B) 2020 Percent large fruit (screen A+B) 2021 Large fruit yields (A+B screens, dry t/a) 2019 Large fruit yields (A+B screens, dry t/a) 2020 Large fruit yields (A+B screens, dry t/a) 2021 Cumulative Yields (A+B screens, dry t/a)
Control 94 95 100 5.3 3.2 3.5 12
Box in Spring 86 91 98 3.9 2.8 3 9.7
Box in Fall 89 86 99 4.4 2.9 3.2 10.5
Hedge both sides in Spring 89 85 92 4 3 3.7 10.7
Hedge both sides in Fall 89 84 99 4.4 2.9 4.3 11.6
P value ns ns 0.02 ns ns ns ns


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