Pacific Nut Producer Magazine

Tag: University of California Cooperative Extension

  • 2025 Results of Pre-Emergent Herbicide Demonstration Trial for Walnut

    The 2025 UCCE walnut pre-emergent herbicide demonstration trial is completed, and the results show strengths and weaknesses of 19 different herbicide treatments, applied either in February or March.

    Takeaways:

    1. All treatments were applied with a strong post-emergent mix, but hairy fleabane and white clover were not adequately controlled with post-emergent treatments alone. Control was dependent on the addition of an ALS inhibitor herbicide (products like Craze, Matrix, or Mission).

    2. Summer annual weeds were best controlled by products with long residuals (products like Alion, Chateau, Brake On! (not registered in CA), and Prowl.

    3. Yellow nutsedge was controlled by treatments with Zeus or Craze.

    4. Conclusion: Pre-emergent treatments in late winter must account for winter annual weeds that have already emerged and perennial weeds while also maintaining good residual control further into the summer.

    Introduction

    Winter pre-emergent herbicide applications are critical to get right if yearly weed management operations are going to be successful in walnut orchards. Poor selection of herbicides or missed timing could make the difference between good control and battling an overwhelming infestation of weeds. In walnut orchards I have regularly heard two comments concerning pre-emergent herbicide applications in walnut orchards:

    1. Those who apply pre-emergent herbicides in the fall, just after harvest, tend to have the cleanest orchards.

    2. February or March are the most common times to apply pre-emergent herbicides in walnut orchards.

    Maybe my perception is incorrect, but these weed control strategies seem to be at odds. When it comes to winter annual weeds, it is likely safest to apply pre-emergent herbicides before germination due to several common weed species with herbicide resistance issues (hairy fleabane, Italian ryegrass, annual bluegrass). Applications later in the winter will have to deal with emerged weeds, potentially resulting in more escapes and misses.

    However, herbicide treatments in February or March are common, and many growers have been successful with these treatments. Why is this, and what is the key to achieving good control later in the winter? Success at later timings depends on how much post-emergent activity you are getting out of your tank mix. Some pre-emergent herbicides can contribute to your post-emergent activity. Common pre-emergent herbicides like Goal, Chateau, Matrix, Pindar, and others add some extra post-emergent activity to tank mixes when used in winter months. Dr. Brad Hanson (UCCE Weed Specialist at UC Davis) discussed this topic when he addressed tank mixes of Alion and Matrix in his UC Weed Science Blog article (linked here). My recent work with rimsulfuron on established johnsongrass also explored this subject (linked here). These prior discussions focused on rimsulfuron products as tank mix partners for pre-emergent applications, but I wanted test a broader range of products applied in February and March. This year’s walnut preemergent herbicide demonstration made some progress toward that goal.

    I will discuss my findings next, and at the end of this article I have included results tables. All products except one are currently labeled for use in walnut orchards, though with some restrictions on orchard age. The one unlabeled product in this study was Brake On!, manufactured by SePRO, currently pending registration in CA. This is not an exhaustive list, and the data provided here does not constitute a recommendation of any product.

    Trial design

    In 2025, I arranged my pre-emergent demonstration trial to evaluate individual products and tank mixes for weed control efficacy when applied in February and March. This trial was installed in the Loybas Hill region of Tehama County, in a 4-year-old walnut orchard with Tehama silt loam soil. A post-emergent mix of 2 qt/A Roundup Powermax 3 and 2 qt/A Rely 280 was added to every plot in February, and pre-emergent herbicides were applied either in the mix, or one month later in March. The tables at the end of this article show the full list of treatments and when they were applied. The first table represents weeds that were present at application, hairy fleabane and white clover. The second represents weeds that grew to be prominent in May and June, so their control depended on residual activity from my herbicide treatments lasting long into the summer.

    Results overview

    White Clover and Hairy Fleabane (Table 1): Both weeds were controlled by all three group 2 herbicides tested: Craze, Revolt, and Mission. Revolt was weaker on fleabane while Craze was weaker on clover and these differences also showed up in the tank mix treatments with both products applied with Prowl. February and March treatments with these three herbicides produced very similar results. The 12 fl oz/A rate of Chateau also effectively suppressed both species.

    Summer annual weeds (Table 2): The primary weeds in the categories recorded here were grass weeds (jungle rice and large crabgrass) and broadleaf weeds (prostrate knotweed, spurge, and pigweed). Many herbicides provided good control until June (3-4 months after treatment) but only Alion, Brake on! (not currently labeled for CA), and March-applied Mission maintained “good” control of broadleaves through July. Only Alion and Prowl maintained “good” control of grasses through July.

    Yellow Nutsedge (YNS; Table 2): Zeus was the best tested product for nutsedge control, but Craze also showed some effects in July by keeping nutsedge coverage low. Rimsulfuron products like Matrix and Revolt are also labeled for YNS control, but labels suggest sequential applications, and the application timings used in this trial may not have been ideal to target YNS with this product.

    Summary and cautions:

    This is a single trial only representing one weed population and one soil type, so results may vary by location. However, hopefully this demonstrates the importance of tank mixing to control emerged weeds as well as future emergence. Group 2 herbicides like Revolt, Mission, and Craze can provide good post-emergent control in addition to their pre-emergent activity. But be cautious, this herbicide group is notorious for herbicide resistance. Do not overuse this class of herbicides or you will likely start seeing weeds escaping your treatments.

    Look through the following tables if you are interested in seeing more information on efficacy in these trials. To see weed coverage data, view the report on the UCCE Tehama website here. Thanks to the California Walnut Commission for supporting this work. For any questions or additional information, contact Ryan Hill at 530-527-3101 or rjahill@ucanr.edu. — By Ryan Hill, UCCE Agronomy and Weed Science Advisor, Tehama County

    Weed control tables:

    Control was determined by calculating % reduction in weed coverage, relative to the plot with the worst infestation. Control is summarized by four categories. These categories may not represent each grower’s threshold of tolerance for weed control, but they can still provide relative comparisons between the products tested.

    Poor control is indicated by a “p”, representing control between 0 and 50%.

    Moderate control is indicted by an “m”, representing control between 50 and 70%.

    Good control is indicated by a “g” and highlighted light green, representing control between 70 and 90%.

    Excellent control is indicated by an “e” and highlighted dark green, representing control between 90 and 100%.

    Table 1: The effect of different pre-emergent herbicides on two weeds that were present at application, hairy fleabane and white clover.

    Table 2: The effect of different pre-emergent herbicides on weeds that germinated or sprouted in summer. Summer annual broadleaves included prostrate spurge, knotweed, and pigweed and summer annual grasses were primarily crabgrass and jungle rice.

  • Almond Day Updates Growers on New Pest, Disease in California

    Late in 2023, California almond growers – responsible for a crop valued at $3.88 billion in 2023 – started to hear about a potentially devastating new pest, the carpophilus beetle (Carpophilus truncatus).

    With virtually no information about this species in California, Jhalendra Rijal and his colleagues spent 2024 investigating its extent in the almond-growing regions of the state and learning from peers in Australia, where the insect has been a persistent scourge for a decade.

    A hullsplit almond showing a large number of carpophilus beetles (Carpophilus truncatus). Photo by Jhalendra Rijal.

    “This year, we at least have the one year of experience with this pest, and we have generated information that is relevant and practical for the growers,” said Rijal, a University of California Cooperative Extension integrated pest management advisor. For example: After an insecticide trial was largely ineffective (likely because the beetle spends most of its life cycle protected within the nut), Rijal has been emphasizing cultural practices like orchard sanitation.

    Sharing those crucial updates is one reason why the annual UCCE North San Joaquin Valley Almond Day is essential for growers, industry professionals and pest control advisers. This year’s event was held on Jan. 21 at the Modesto Centre Plaza.

    During Almond Day 2025, Jhalendra Rijal, UCCE IPM advisor, discusses the differences between Carpophilus truncatus and related beetle species. Photo by Mike Hsu

    “It’s the meeting on everyone’s calendar,” said Rijal, who co-organized the event alongside his UC Agriculture and Natural Resources peers, Brent Holtz and Cameron Zuber.

    With support from Farm Credit, the 2025 Almond Day attracted about 140 people, who heard about a range of best practices on almond production and pest management from six UC experts.

    “This is a great educational tool,” said meeting attendee Ali Arshad, a Blue Diamond grower who operates several ranches in San Joaquin and Stanislaus counties. “I like to come attend their seminars as much as possible; they’re very educational and informative – and very helpful too.”

    Scientists offer advice on managing carpophilus beetle, red leaf blotch in almonds

    Fellow grower Donny Hicks, who has attended 14 Almond Day meetings, concurred about the usefulness of the event. “Everything that was covered today is applicable to what I do,” he said.

    Troubled by the presence of Carpophilus truncatus on his orchard in Hughson, Stanislaus County, Hicks said he recently partnered with Rijal to set some traps on his property, hoping to grow scientists’ understanding of the pest.

    “It takes growers to help advance that knowledge and be the ‘guinea pigs’ somewhat,” Hicks explained. “At the same time, if it helps us growers as a whole, that’s a good thing.”

    During his opening talk, Rijal reiterated the need for growers to clean up the remnant “mummy” nuts on the ground that serve as overwintering habitat and a food source for the next generation of beetles in the spring.

    “Do the sanitation so you can destroy the population from the beginning and you will have less of a problem,” said Rijal, who also shared a visual ID guide that can help growers identify the specific pest in their orchard and thus better target their control methods.

    Growers also heard practical advice on an emerging almond disease, red leaf blotch, caused by the fungus Polystigma amygdalinum. Named for the orange-red patches of discoloration on leaves, red leaf blotch in almond has led to significant crop loss across Mediterranean countries. Unknown in California prior to last year, there was an explosion of reports throughout the San Joaquin Valley during 2024.

    “It was pretty surprising, both the quickness and vastness of the spread,” said Alejandro Hernandez-Rosas, a UC Davis Ph.D. candidate in the lab of Florent Trouillas, UC Cooperative Extension specialist in the Department of Plant Pathology at UC Davis.

    During his presentation, Hernandez-Rosas said a preventative application of fungicide at petal fall – and then again at two and five weeks after petal fall – appear to work best to manage this disease. Fungicide applications made after the blotchy leaf symptoms appear will not be effective, Hernandez-Rosas said, and further research needs to be done to determine the most effective products and optimal timing for growers to make the most of their applications.

    And because leaf litter is the “primary source” of the disease inoculum, Hernandez-Rosas added that cultural practices focused on speeding cleanup or decomposition of leaf litter are critical to reduce disease severity. However, it is only effective if done over a wide area in conjunction with neighbors.

    Advanced symptoms of red leaf blotch (RLB) include large, yellow-orange blotches (roughly 1/2″) that turn reddish-brown in their center. Photo credit: Alejandro Hernandez and Florent Trouillas

    Rodents, irrigation, replanting and whole orchard recycling

    Both Arshad and Hicks mentioned that they intend to tackle the gopher problem in their orchards.

    “I plant a cover crop every year and the gophers tend to really like it – so I get overrun with those gophers,” Hicks explained. “I’m trying to find that balance of not letting them overtake my orchard but still being able to cover crop – because that’s beneficial too.”

    With many growers voicing a desire to learn about burrowing rodents, Almond Day organizers were pleased to schedule a presentation by Roger Baldwin, professor of Cooperative Extension in the Department of Wildlife, Fish and Conservation Biology at UC Davis.

    Baldwin presented the available control options for not only pocket gophers but also ground squirrels and meadow voles. He stressed the need to understand the biology and ecology of the rodents to maximize efficacy; for example, whereas ground squirrels prefer green foliage as their main food source in the spring, they switch to seeds in the summer/fall – which would thus be a better time to use pelletized baits.

    In choosing from baiting, trapping, habitat modification, burrow fumigation, biocontrols and other options, Baldwin recommended combining different strategies and approaches in an integrated way.

    “Mixing and matching tools will give you the best possible results,” he said.

    The utility of combining methods was also a key takeaway from the presentation by Moneim Mohamed, UCCE irrigation and soils advisor. Highlighting the latest tools and innovations, Mohamed outlined the soil-based, weather-based and plant-based methods to collect data that can help growers make the most of their water.

    Mohamed said using even just one approach is beneficial in optimizing irrigation scheduling – but more is even better. “A combination of the three methods is the best,” he said.

    Cameron Zuber, UCCE orchard crops farm advisor for Merced County, discussed considerations related to replanting an orchard, including field and soil conditions and the management of plant-parasitic nematodes and aggressive pathogens and pests. He also encouraged attendees to visit growingthevalleypodcast.com for more information on tree nuts and a host of other topics.

    Finally, Brent Holtz, UCCE orchard systems farm advisor for San Joaquin County and a trailblazer for whole orchard recycling, presented an overview of WOR. He then shared recent findings on its effects on fungal pathogens, and ways to mitigate drawbacks of WOR – such as the initial carbon-to-nitrogen imbalance when the recycled almond wood chips are first spread on a field.

    Hicks, who has already tried WOR in his orchard, is also the grower relations manager for RPAC, a Los Banos-based company that grows, processes and markets almonds. Hicks said he also brings what he learns during Almond Day to his fellow growers – illustrating the multiplier effect of such educational events.

    “I have growers who possibly will be doing whole orchard recycling, and I can share that information,” Hicks said.

    UC Agriculture and Natural Resources brings UC information and practices to all 58 California counties. Through research and Cooperative Extension in agriculture, natural resources, nutrition, economic and youth development, our mission is to improve the lives of all Californians. Learn more at ucanr.edu.