Pistachio harvest ended earlier than usual this year, but growers should not stop irrigating their orchards just yet. The early finish creates a longer post-harvest period before trees enter dormancy, making proper irrigation especially important.
Research by Daniele Zaccaria, Mae Culumber and Anderson Safre of the University of California Cooperative Extension indicates that pistachio trees continue using water after the nuts have been removed. Although harvest is complete, evapotranspiration—the loss of water from the soil and through the trees’ leaves—continues until the trees become dormant.
Because the post-harvest season will be longer this year, growers should continue monitoring their orchards’ water needs. Stopping irrigation too soon could place unnecessary stress on trees as they prepare for next season.
However, the researchers suggest growers may be able to conserve water through carefully timed deficit irrigation. This practice involves applying less water than the orchard would normally use during selected periods. When managed properly, deficit irrigation may reduce water use by about 20% with little to no effect on the following year’s harvest.
The key is timing. Growers should consider orchard conditions, soil moisture and the trees’ water needs before reducing irrigation. An early harvest does not mean water use has ended, and post-harvest management can play an important role in supporting the next crop.
Although almond harvest may be wrapping up, the work needed to prepare orchards for next season has already begun. Post-harvest priorities include monitoring for disease, irrigating trees, pruning and managing pests.
Brent Holtz with the University of California Cooperative Extension advises growers to watch for red leaf blotch and rust. Bright orange spots on leaves may indicate red leaf blotch, while orange and black spores can be signs of rust. This year’s warm March appears to have increased the severity of rust, making it important to avoid irrigation practices that keep orchards humid for extended periods.
However, growers should not stop irrigating altogether. Water should be applied as soon as the harvested nuts are removed from the orchard floor. Dr. David Goldhamer, emeritus irrigation specialist with UC Davis, says almond trees can use up to 11 acre-inches of water after harvest if their leaves remain active. Proper post-harvest irrigation can also help move salt below the trees’ root zone.
Growers can also begin pruning mature orchards immediately after harvest instead of waiting until dormancy. Pruning early gives wounds time to heal before rainfall, reducing the risk of fungal diseases. While studies indicate pruning does not significantly improve almond yields, it remains necessary to provide enough space for orchard equipment to pass between trees.
Fall is also a good time to manage gophers, ground squirrels and mice. Baiting or fumigation can help control ground squirrels before they enter hibernation. Growers should also look for pieces of almond shells left in tree crotches, which may indicate mouse activity.
For more information about post-harvest almond orchard management, read the full article in the September issue of Pacific Nut Producer Magazine.
Recent studies conducted by scientists at UCCE and CSU Bakersfield demonstrate that a common disease prevalent in vineyards and orchards may impact long-term orchard longevity of almonds in the southern San Joaquin Valley.Almond orchards with a high incidence of crown gall, a disease caused by a ubiquitous soilborne pathogen, exhibit high rates of tree failure caused by a newly identified wood rotting disease.
Crown gall is common in walnut and almond orchards and can be readily identified by the tumors it induces on the roots and crowns of trees. Unless severe, crown gall generally does not cause tree mortality, but may limit tree size, thus affecting overall yield.Growers with crown gall-affected orchards often replant young trees that exhibit girdling by the tumors but leave mature trees untreated. Within the past decade, however, growers have noticed heightened prevalence of tree mortality due to butt rot in crown gall-affected almond orchards. The butt rot disease is caused by an introduced fungal pathogen, Ganoderma adspersum, that was first reported in California in 2016.
From 2024 through 2026, UCCE researchers have surveyed over 23,000 almond trees in Kern and Tulare Counties, documenting prevalence of both butt rot and crown gall.Molecular identification of the butt rot pathogen in each surveyed orchard was conducted by scientists at CSU Bakersfield. The study found that Ganoderma adspersum was the only species of Ganoderma isolated in all surveyed orchards. Additionally, the results indicated that orchards with crown gall were six times more likely to have butt rot disease. Trees with historic or active signs of butt rot also exhibited 1.5 times the mortality rate of healthy trees over a 26-month survey period.
The results of this work demonstrate that the introduction of the new butt rot pathogen may elevate the importance of crown gall as a predictor of orchard longevity. The implication of this finding is that the planting of crown gall-free rootstocks during orchard establishment may be key to future reduction of risk from G. adspersum. Similarly, Mohammad Yaghmour and Elizabeth Fichtner, Farm Advisors in Kern and Tulare Counties, respectively, found that crown gall was also associated with increased incidence of thousand cankers disease on walnut.Fichtner has also reported that crown gall is related to heightened root predation by ten-lined June beetle on walnut.
The current study was conducted with support from the Almond Board of California and as a collaborative effort between UCCE scientists including Elizabeth Fichtner, Santosh Bhandari, Mohammad Yaghmour, and Raymond Mireles, and CSU Bakersfield Professor Isolde Francis and Chance Kophamer, graduate student. — Story by Elizabeth J. Fichtner, UC Ag and Natural Resources
How can almond, pistachio, and walnut growers control navel orangeworm and reduce aflatoxin risk? UC Cooperative Extension entomologist Houston Wilson joins pistachio and almond grower and pest control adviser (PCA) Joe Coelho and Arielle O’Connor of the Arizona Cotton Research & Protection Council to discuss practical strategies for preventing infestations, limiting crop damage, and reducing the effects of aflatoxin.
Presented by title sponsor Liphatech and premier sponsor the Arizona Cotton Research & Protection Council.
As growers navigate SGMA, different avenues are being explored for access to the needed water for crops, including groundwater markets. This has been experimented with in the Mojave Basin, resulting in stabilized implementationand aggregate gains, but also a consolidation of water rights. At the World Ag Expo, UC Cooperative Extension Associate Professor Ellen Bruno discussed the findings on groundwater markets with Matthew Malcolm at California Ag Network. Watch this quick video and read more in California Fruit & Vegetable Magazine.
Please thank this video’s sponsor Simplot for their industry support.
This work was partially funded by the Stockton East Water District — Crown rot and root rot caused by Phytophthora species are diseases of great concern for almond growers. Many people believe that irrigating with surface water puts your trees at risk of Phytophthora, and that irrigating with groundwater is the best way to prevent these diseases. However, a recent study of Phytophthora in irrigation water and orchard soils tells a different story.
The Bottom Line:
Irrigation management is Phytophthora management. This study showed that Phytophthora is common in orchard soils. Regardless of whether they are irrigated with surface water or groundwater, over 30% of tested orchard soils were positive for Phytophthora. Since water (either standing water or saturated soil) allows Phytophthora to cause disease, good irrigation management is crucial to prevention, no matter what your water source.
— Reduce ponding: irrigation application rate should not exceed soil infiltration rate.
—Reduce length of soil saturation: run shorter sets (24 hrs max) more frequently.
— Avoid water on the trunk of the tree and avoid soil saturation at the trunk: plant on berms, use stream splitters, choose microsprinkler wetting patterns that avoid the trunk, move drip emitters away from the trunk.
Using a resistant rootstock is an excellent tool to help limit Phytophthora disease if you are planting a new orchard (see this chart for rootstock info) or putting in replants. Even with a resistant rootstock, good irrigation management is crucial because no rootstock is immune.
The Details:
Phytophthora is a genus of fungus-like organisms which contains over 200 different species. Many of these species are important pathogens of orchards, causing root rot or crown rot (for example Phytophthora cinnamomi, Phytophthora cactorum, and Phytophthora mediterranea) or pruning wound cankers (mainly Phytophthora syringae). Many previous studies have shown that Phytophthora species are common in surface sources of irrigation water, such as rivers, canals, and sloughs. Phytophthorahas not been found in groundwater from wells, unless that well has been contaminated with surface water. From this, people have assumed that irrigating with surface water puts an orchard at risk of Phytophthora root or crown rot, and that irrigating with groundwater is ”safe.” However, biology is rarely that simple: in my experience, orchards irrigated with groundwater can struggle with Phytophthora, and many orchards irrigated with surface water do not show symptoms of Phytophthora disease.
In 2021 and 2022, collaborators and I conducted a study to look at the effects of Phytophthora in irrigation water from a broader perspective. This study took place in the Stockton East Water District (SEWD), a local water agency that manages both groundwater and surface water use by agricultural producers east of Stockton, CA. The study had three objectives:
— Test SEWD surface water for Phytophthora during the irrigation season.
— Test for live Phytophthora coming through irrigation emitters.
— Test orchard soils for Phytophthora, comparing orchards irrigated with surface water vs groundwater.
Objective 1. Test SEWD surface water for Phytophthora during the irrigation season
During the 2021 irrigation season, we sampled water from surface water irrigation sources throughout the SEWD. Some locations (core sites) were sampled monthly from June through October, and some locations (additional sites) were sampled twice during the season, in July and October. Samples were taken back to the lab and tested for Phytophthora using DNA sequencing.
Overview of the Stockton East Water District (SEWD) with water sampling sites marked.
We found that Phytophthora species were common in SEWD waterways. Over the course of sampling, we found 39 Phytophthora species, 10 of which are known pathogens of orchards crops grown in the SEWD. Many of these species were found throughout the irrigation season. We did find that different waterways had different Phytophthora species, and some waterways had more orchard pathogens than others. However, every waterway, and nearly every site, had at least one Phytophthora species of concern to orchards. These results are consistent with many previous studies, which have determined that Phytophthora species are common in surface sources of irrigation water.
Phytophthora species detected in Stockton East Water District waterways throughout the irrigation season.
The Phytophthora species listed in this table are reported to cause disease on walnut (W), cherry (C), almond (A), or pistachio (P). If the crop is bold, the Phytophthora species is particularly aggressive on that crop. A plus (+) indicates that the Phytophthora species was detected at least once in that waterway. A period (.) indicates that the species was not detected in that waterway. Nineteen other Phytophthora species (not listed) were detected, but these are not known to cause disease on orchard crops.
Objective 2. Test for live Phytophthora coming through irrigation emitters
In objective 1 we determined that Phytophthora is common in the SEWD waterways. However, very few studies have looked at whether Phytophthora in a waterway can get into an orchard through the irrigation system. This is especially true of drip irrigation systems, which require substantial filtration of surface water to keep emitters from plugging.
During the 2021 and 2022 irrigation seasons, we collected water directly from irrigation emitters during a normal irrigation and tested this water for the presence of live Phytophthora. This was done in three surface-water-irrigated orchards, two with drip emitters and one with sprinklers, as well as two groundwater-irrigated orchards, one with drip emitters and one with sprinklers.
Orchard sampling stations used to detect live Phytophthora in water from A, sprinklers and B,
drip emitters.
We found that Phytophthora commonly survives the journey from surface water sources into the orchard, and that the irrigation system type did not seem to matter. Also, the presence of a sand media filter did not seem to affect how regularly we detected live Phytophthora coming through irrigation emitters.
Summary of Phytophthora detections in water collected from irrigation emitters
Objective 3. Test orchard soils for Phytophthora, compare orchards irrigated with surface water vs groundwater
In objectives 1 and 2 we determined that Phytophthora is common in SEWD waterways and that it can survive the trip through an irrigation system and into the orchard. However, in most cases Phytophthora must survive in the soil to infect the orchard when conditions are right. Even though we knew that irrigation with surface water is bringing Phytophthora into orchards, we didn’t know if this affects the incidence of Phytophthora in orchard soils.
In 2021, we collected soil from 20 SEWD orchards exclusively irrigated with groundwater for at least 60 years and from 20 SEWD orchards mainly or exclusively irrigated with surface water over the same timeframe. This soil was tested for the presence of Phytophthora species using both the DNA sequencing from objective 1 and the live detection methods from objective 2.
We found that Phytophthora is common in orchard soils, with 32.5% of sampled orchards (13 out of 40) testing positive. We also found that the source of irrigation water did not affect the chances of finding Phytophthora in the soil: groundwater irrigated orchards were as likely to have Phytophthora as orchards irrigated with surface water. This indicates that irrigation with surface water was not the main factor determining whether Phytophthora was present in orchard soils and that irrigation with surface water may not increase risk of Phytophthora disease in orchards.
Hytophthora species in bold are particularly aggressive on orchard crops
The results from this study confirm that Phytophthora is common in surface water but show that irrigating with surface water was not the main factor determining whether Phytophthora was present in an orchard. Where is the Phytophthora coming from? It is hard to know, but historical flooding may play a role. We also know that Phytophthora can be moved into an orchard on planting material and in soil on equipment. Where does this leave us in terms of management? This study shows that Phytophthora is very common in orchard soils, regardless of the source of irrigation water. Since over 30% of tested orchards were positive for Phytophthora, it would be prudent to assume you have Phytophthora in your orchard. Saturated soil allows Phytophthora to infect and cause disease, so good irrigation management is crucial to prevention. By “good irrigation management”, I mean that you want to reduce the length of time that orchard soils are fully saturated and avoid having standing water. This can be done by using irrigation emitters with output volumes suited for your soil infiltration rate and by irrigating more frequently for a shorter duration (24 hrs maximum). In addition, you want to apply irrigation water in the root zone but away from the trunk to reduce the opportunity for infection of the trunk or major roots. This can be done using stream splitters with sprinklers to protect the trunk, choosing a microsprinker wetting pattern that avoids the tree trunk, or moving drip emitters away from the trunk of the tree. At planting, consider using a resistant rootstock, and plant on berms. Note that even if you are using a resistant rootstock, good irrigation management is crucial because no rootstock is immune.
I want to leave you with this thought: irrigating well is much more important for preventing Phytophthora than the source of your irrigation water. I have been to many surface-water-irrigated orchards with no symptoms of Phytophthora. Some of the worst orchards I have seen, in terms of Phytophthora disease, were irrigated with groundwater with the driplines right against the trunk on 3rdleaf trees. Irrigation management is Phytophthora management.
Thank you to my collaborators on this project, Greg Browne (USDA-ARS) and Mohamed Nouri (UCCE San Joaquin County). Special thanks to Justin Hopkins with the SEWD for his help planning and executing this project. Thank you also to the SEWD growers who welcomed me into their orchards for sampling. This work was partially funded by the Stockton East Water District. — By Jaime Ott, UC Extension Tehama, Shasta, Glenn, and Butte Counties
Irrigation systems today are amazing at getting just the right amount of water where it needs to be to meet a crop’s needs; however, if these systems are not well-maintained, things can go south pretty fast, resulting in distribution uniformity-related losses and expensive repairs. During his presentation at Malcolm Media’s recent Tree & Vine Expo, UC Cooperative Extension Irrigation & Soils Advisor Moneim Mohamed noted that regular and proper irrigation system maintenance can prevent expensive repairs. Watch his video interview with Matthew Malcolm on California Ag Network for five key tasks for preventative maintenance.
Leaf Monitor, a new mobile tool backed by artificial intelligence and predictive modeling, could revolutionize how farmers monitor crops and make decisions by providing real-time nutrition and leaf trait information in the field.
“Having this information is very valuable for the farmers,” said Alireza Pourreza, associate professor of Cooperative Extension and director of the Digital Agriculture Laboratory in the Department of Biological and Agricultural Engineering at the University of California, Davis. “In five seconds, they can have a sense of how much nutrition they have in a leaf.”
Development of the AI model was funded by the U.S. Department of Agriculture’s National Institute of Food and Agriculture’s HiRes Vineyard Nutrition multistate project and its Animal and Plant Health Inspection Service, as well as the California Table Grape Commission.
Maha Afifi, director of viticulture research at the California Table Grape Commission, said the tool could be a game changer for the table grape industry if it leads to faster decision-making about fertilizer use. The right amount typically leads to healthier vines that produce more grapes with optimal size, weight and color.
“The evaluation of vine nutrient status is one of our top priorities,” Afifi said. “At the same time, exploring new technology tools like this project is a high priority for us because they will be important to the future of the table grape industry.”
Field testing
The Leaf Monitor tool uses a handheld spectrometer to measure leaf reflectance beyond the range of light visible to the human eye.
Once a leaf is scanned, its spectral data is uploaded to a cloud-based machine learning system designed to predict leaf traits and nutrient content. This algorithm was developed and trained by the Digital Agriculture Laboratory over five years using a dataset of thousands of leaf samples collected from California’s specialty crops, primarily grapevines and almonds. The samples were chemically analyzed to determine nutrient levels and structural leaf traits, providing the data needed to build an accurate prediction model.
“Nutrient deficiencies in plants often go unnoticed until late in the season, by which point the damage is already irreversible,” said graduate student Parastoo Farajpoor, who is running the project. “This is why early detection is essential. Spectrometry provides a rapid and reliable way to identify these deficiencies before visible symptoms appear.”
After a recent demonstration, Bulleseye Farms Irrigation Manager Geoff Klein said the tool could help save money and improve yields. Bullseye grows walnuts, pistachios, tomatoes, corn, wheat, rice and sunflowers in Yolo and Solano counties.
Tailored crop management
Currently, farmers typically take leaf samples, dry them, grind them up and send the samples off to a lab for testing, which can take up to two weeks to return results. Bullseye samples leaf tissues about three times a year.
“Right now, it doesn’t really make sense to go out and take tissues in every single corner just because it’s expensive,” Klein said. “It’d be really cool if I could just walk out there and test a couple of different places.”
The Leaf Monitor tool helps farmers tailor management decisions to specific areas rather than an entire field. Calibrating fertilizer use to real-time data can prevent overuse and nitrogen runoff, a financial and environmental challenge that many growers face.
“I feel like there’s a lot of times we do need to put less [fertilizer] on, where we end up putting more, because that’s what the nitrogen removal formula says,” Klein said. “But with this app we can use less because we know the actual conditions at the time. I think it opens a lot of doors in terms of getting data back in real time and also utilizing the level of control we have with the data.”
The app can also aggregate the scans and map out spatial patterns over a large area.
“What we know is every field has variability that is not necessarily visible to the farmer’s eye,” Pourreza said.
The prototype Leaf Monitor tool is free and included in a set of tools that can be downloaded on the Digital Agriculture Laboratory website. A web-based version of the tool will follow while the team continues to feed new data into the algorithm to refine the predictions. On average, it achieves about 65% accuracy across all traits, with predictions for certain nutrients, such as nitrogen and phosphorus, performing better than the overall average. Users will need to pair it with a spectrometer.
“We need to produce more food while using less resources so we need to have some kind of monitoring system to give us precise and accurate feedback on our management practice,” Pourreza said. “This technology is growing very fast.” — By Emily Dooley, UC Davis
Rats have taken an unusual interest in the specialty crop capital of the nation. Many attribute the cause to the pest multiplying unchecked in orchards and vineyards in the Central Valley that have been abandoned due to water restrictions and economic challenges. This becomes a real problem when the rats move into neighboring farms in active production, tearing up driplines, feeding on crops and damaging trees. Growers are in a state of emergency and have limited tools for managing this pest.Malcolm Media Editor-in-Chief Matthew Malcolm met with UCCE Wildlife Specialist Roger Baldwin at the California Association of Pest Control Adviser’s annual conference to gain answers on how to knock down populations effectively and keep the pest at bay.Watch this brief interview and learn more by attending Baldwin’s presentation on rat control at the Grape, Nut & Tree Fruit Expo at the Fresno Fairgrounds on Nov. 14 (register to attend at agexpo.biz).
American Pistachio Growers (APG), in collaboration with Washington State University, University of California – Riverside, UC Cooperative Extension, the USDA Agricultural Research Service and regional nut industry partners, has been awarded a $1 million grant through the California Department of Food and Agriculture’s Biologically Integrated Farming Systems (BIFS) Program to develop a Regional Integrated Pest Management (IPM) network for Navel Orangeworm (NOW) — the most damaging pest in California’s tree nut industry.
The project, led by Dr. Houston Wilson (UC Riverside), Dr. David Crowder (WSU), Dr. Jhalendra Rijal (UC IPM), and Dr. Charles Burks (USDA ARS) will pilot a groundbreaking Decision Aid System (DAS) that integrates real-time trap data, weather models, and crop phenology into a single communication platform. The goal is to improve the timing and precision of pest control decisions while fostering regional cooperation among almond, pistachio, and walnut growers.
“This project moves us beyond the farm gate,” said Joe Coelho, APG’s Director of Sustainability and Member Outreach, who serves as Technical Agronomist and PCA on the project. “For the first time, growers across commodities will have access to shared regional data and communication tools that allow them to anticipate pest pressure before it hits their fields and ultimately make precise, timely treatment decisions. The outcome is fewer sprays, lower costs, and higher quality nuts.”
Through field-level data acquisition, the system’s meta-analytics will identify regional flight trends coupled with crop-specific phenological development and enhance forecasting of NOW flights — critical steps in breaking the pest’s lifecycle across neighboring farms. Ultimately, the program is expected to help reduce pesticide use, improve nut quality, and lower aflatoxin risk associated with pest damage.
APG will serve as the grower administration partner, coordinating grower participation and outreach. Carlee Branco, APG Grant Programs Administrator, will conduct on-farm grower coordination, engagement and data collection. “This is a major milestone for sustainable pest management,” said Coelho, “and it demonstrates APG’s leadership in advancing research that directly benefits growers.”
The Regional IPM for Navel Orangeworm Project represents a pivotal step toward the state’s Sustainable Pest Management (SPM) Roadmap by providing a scalable, data-driven framework that can be expanded statewide.
“This is exactly the kind of innovation California agriculture needs,” said Dr. Wilson. “Regional coordination is essential to long-term pest reduction, especially for highly mobile insects like the navel orangeworm, and this project will now put those ideas into practice at scale.”
The program launches in early 2026, with pilot regions in West Fresno County and Modesto, serving as the foundation for a future statewide expansion. Growers within these territories who are interested should contact Carlee Branco for more information at cbranco@americanpistachios.org.
American Pistachio Growers (APG) is a non-profit trade association representing more than 800 growers and processors across California, Arizona, and New Mexico. APG’s mission is to enhance grower profitability through global marketing, industry research, and sustainability initiatives that promote economically viable and environmentally responsible pistachio production.
