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“The most important decision you will ever make in establishing an orchard is where,” UCCE plant science specialist Louise Ferguson said.
In her presentation at the UC Statewide Pistachio Day, Ferguson noted the location of the bulk of California’s pistachio orchards and how climate and microclimate in specific areas affects production and tree health.
Climate is the limiting factor in tree crop production, and climate is temperature, Ferguson said. All plants have genetically determined heat accumulation requirements for growth and health. Additionally, all plants have optimal temperature ranges effective at specific growth stages: dormancy, bloom and nut set, nut growth and maturation, and entering dormancy.
Warm winter temperatures, spring frosts or weeks of blazing hot summer temperatures can impact orchard health and production, and there are few management practices that can lessen the impact of unpredictable climate.
Ferguson said that within the expected temperature ranges, normal growth will occur. With unexpected higher or lower temperatures, growth and reproductive development of the plant are impacted.
“Temperatures are uncontrollable and we have very few tools to mitigate their effect,” she noted.
Ferguson explained how temperatures influence the growth stages of pistachios. Using dynamic model chill portion at 45 degrees F as a baseline, pistachios require 54-65 chill portions during dormancy. A research report from Australia found late February chill correlated best with decreased blanks. The same study found highest correlation with splits was steady even heat accumulation through the summer.
Bloom and Set Time
“Key temperature influences in the spring is one area where we are learning lessons the hard way,” Ferguson said. This is bloom and fruit set time. Post bloom is when there is shell and hull enlargement and formation. The bloom date and heat accumulation in the spring affect yield and nut quality. One example is spring 2004 where later bloom, after April 20, and temperatures greater than 70 degrees F resulted in decreased yields, increased blanks, low shell split, poor hull integrity and low harvestability.
Temperature influences nut growth and maturation during the summer. Ferguson pointed out difference in fruit development between cultivars. A 2021 study at eight pistachio-growing locations showed that Kerman required the most growing degree days and Golden Hills required the least.
Entering dormancy, more erratic temperatures, including sudden freezes, can damage or kill young actively growing non-bearing scions on UCB1 rootstock. This occurred in 2009 where during a warm fall, temperatures dropped below freezing. Trees that have not been treated to enter dormancy or were still being irrigated were affected.
Ferguson also cited studies on growing degree hours, or GDH, needed for 60 percent bloom at different chill portions and effect of sunlight on bud development. At 50 chill portions, Golden Hills and Randy require 5,500 GDH while Kerman and Peters require 7,000 GDH accumulated in April. At 60 chill portions, Golden Hills and Randy require 3,000 GDH while Kerman and Peters require 5,500 GDH. She noted that trademark product Dormex can be used if chill portions are low and the site has low spring temperatures.
Growers have no control over temperatures, Ferguson noted, but by monitoring them, growers may be able to change timing of their management practices. For example, in dormancy, monitoring can help with decisions on a Dormex application. Monitoring can help with determining irrigation timing for Stage 3 and to predict pest management timing and harvest date.
Another study showed sunlight on winter buds can make a difference in chill portion accumulation. Ferguson said basing CP on air temperatures will underestimate chill portion accumulation.
UC Davis researchers Barbara Blanco-Ulate and Guilia Marino, Ferguson said, are working on showing production of split nuts as a function of growing degree days, or GDD. The greatest predictor of split nuts, she added, is heat accumulation in the summer. Blanco Ulate is also working on using GDD to determine navel orangeworm generations and hull susceptibility as it increases in Stage 3.
Ferguson also noted UC researcher Georgia Drakakaki is building on her work with the ethylene-releasing compound Accede, trademark, in pistachio production. The aim with this use of this material, Ferguson said, is not to advance harvest, but to take off more nuts with a single shake, lowering harvest costs.
In her Pistachio Day presentation, Marino set the stage for the climate discussion, noting how tree carbon dynamics affect pistachio orchard management.
She explained that carbon is captured from the atmosphere by the leaves and transformed with photosynthesis into sugars. Sugars are used to grow, becoming structural carbohydrates as they become fixed in fruits, roots and trees. Sugars are lost through respiration, which is necessary for energy. Sugars are stored as sugars and starch, NSC, non-structural carbohydrates, to support tree function when photosynthesis is lower than use.
Crop load should be integrated in management practices as yield is a primary driver of carbon depletion. High temperatures, Marino said, increase tree respiration and reduce photosynthesis, impairing the carbon budget.
Best management practices in the orchard, Marino noted, target maximizing carbon assimilation. The non-structural carbohydrate dynamic in the winter is a promising indicator of dormancy and could be used to support management.
In addition to her work with harvest efficiency, Drakakaki’s laboratory at UC Davis is investigating pistachio nut development from ‘roots to fruits,’ noting climate effects.
In the fruit development studies, she noted that studies that showed the shape of suture and shape and size of cell matters for shell split. Hull split findings include fruit water status was important. Different pistachio varieties have different hull split rates.
Use of the abscission compound marketed as Accede was tested at a field trial, proving it can improve efficiency of mechanical harvest. Drakakaki said the using Accede at 250 ppm and 100 GPA rate leading to a 3,060-pound increase of in-hull nuts per acre in the first shake.
Cost of the treatment was about $167 per acre.
The Accede treatments, she noted, significantly decreases the fruit removal rate. No phytotoxicity was observed and there was no significant increase in blank percentage in the treated trees.
Cecilia Parsons | Associate Editor
Cecilia Parsons has lived in the Central Valley community of Ducor since 1976, covering agriculture for numerous agricultural publications over the years. She has found and nurtured many wonderful and helpful contacts in the ag community, including the UCCE advisors, allowing for news coverage that focuses on the basics of food production.
She is always on the search for new ag topics that can help growers and processors in the San Joaquin Valley improve their bottom line.
In her free time, Cecilia rides her horse, Holly in ranch versatility shows and raises registered Shetland sheep which she exhibits at county and state fairs during the summer.
