New Research Includes Focus on NOW and Tree Physiology in Pistachio

Sterile navel orangeworm females call to males as part of the study on Sterile Insect Technology for NOW in pistachios. (photos courtesy H. Wilson.)

The California Pistachio Research Board’s 2020 $1,946,070 research grant funding reflects the major problems currently facing the California pistachio industry.  The 30 funded projects prioritize, in this order, controlling navel orangeworm (Amyelois transitella) (NOW), improving understanding of pistachio physiology including characterizing tree carbohydrate status, dormancy, salinity and drought management, pistachio rootstock improvement, and emerging soil borne fungal diseases and other insect pests.  This article will discuss the projects focused on NOW and pistachio physiology.


Navel Orangeworm Research

Navel orangeworm (NOW) is the primary insect pest of California pistachios. Effective management of NOW involves a combination of winter sanitation, monitoring and well-timed sprays, timely harvest and, most recently, mating disruption. In the wild, NOW females emit a plume of pheromone in order to attract males, who follow these plumes to locate females and mate. Mating disruption uses various types of emitters to dispense a synthetic form of NOW pheromone throughout orchards, which effectively diminishes the males’ ability to successfully locate females. Interfering with NOW ability to mate and reproduce can lead to reduced populations and, subsequently, lower crop infest/damage.

Monitoring NOW in orchards under mating disruption remains a key challenge though, since the synthetic pheromone used to confuse males also shuts down pheromone traps commonly used to monitor NOW males. As such, Luisa Cheng (USDA Agricultural Research Service) is investigating the use of plant volatiles produced by pistachio mummies as a potential monitoring lure for orchards under mating disruption. In a related project, Dr. Tiziana Bond (Lawrence Livermore National Laboratory) is investigating the use of RAMAN spectroscopy to detect the synthetic pheromones used for monitoring NOW and mating disruption, and potentially natural pheromones emitted by female NOW themselves. If successful this highly sensitive detection method could be used to improve our understanding of how both synthetic and natural pheromones are distributed across orchards, which could lead to improved monitoring and mating disruption.

Mated female NOW deposit their eggs on pistachio nuts, and the larvae that emerge make their way into the nut in order to feed on the kernel. This not only reduces crop yield and quality, but NOW infest has been associated with increased incidence of aflatoxin.  While research to date has determined that NOW females prefer to deposit eggs on nuts with reduced hull integrity later in the season, it remains unclear exactly what physical and chemical changes in the nut actually elicit this response when hull integrity declines. Furthermore, there is little known about why changes in hull integrity occur – some years hull integrity is good and other years it is poor – new information is needed to help growers predict the timing and extent of changes in hull integrity.

Three complimentary projects will investigate how changes in hull integrity attract NOW females. In the first project, UC Davis Associate. Professor Dr. Barbara Blanco-Ulate and Extension Specialists Giulia Marino and Selina Wang will be investigating the seasonal progression of pistachio nut maturity, split and hull senescence as a function of heat accumulation. In the second project, UC Davis Professor Dr. Georgia Drakakaki will investigate shell and hull development, growth and senescence at the cellular level. In the third project, Extension Specialist Dr. Houston Wilson (UC Riverside) and Dr. Chuck Burks (USDA Agricultural Research Service) will investigate how these factors influence the timing and extent of NOW female egg deposition.

Finally, the CPRB recently initiated a major project to investigate the use of sterile insect technique (SIT) for NOW, and are currently funding research efforts in this area led by Dr. Houston Wilson and Dr. Chuck Burks. The sterile insect technique hinges on the ability to introduce large numbers of sterile NOW into orchards that will then mate with wild NOW. Mating between a sterile and wild NOW leads to inviable offspring – while a female can still deposit eggs, they will never produce a larva. Over time, introductions of sterile NOW could lead to reduced NOW populations and subsequent crop infest/damage. The CPRB-funded initiative makes use of a USDA mass-rearing and irradiation facility in Phoenix, AZ. This facility was originally developed for a pink bollworm SIT program, but with the eradication of that pest it became available for use with NOW. The facility currently produces upwards of 750,000 sterile NOW per day, but there are many challenges associated with the collection, irradiation, transport and release of these sterile moths. As mentioned, the facility was developed for pink bollworm, and changes are now needed to adapt it to NOW. Over the past two years, Wilson and Burks have carried out a series of laboratory and field experiments to improve the quality and dispersal capacity of sterile NOW. Findings to date indicate that cold storage, transport and release methods for sterile NOW are having a negative influence on moth performance, and this will be the focus of their CPRB-funded research in 2020.


Pistachio Physiology Research Projects: Carbohydrate Observatory

Understanding the whole tree physiology, to facilitate a better understanding of dormancy, bloom, set, yield potential and response to the abiotic stresses of temperature, salinity and drought is another top priority.  The core of this set of projects is support to the ongoing Carbohydrate Observatory project of Dr. Maciej Zwieniecki of Plant Sciences:

Nut producing trees rely upon process of photosynthesis, the process by which plants use sunlight to synthesize starches and sugars from carbon dioxide and water.  These sugars and starches, referred to collectively as Non-Structural-Carbohydrates, NSC, support the tree’s growth and annual bloom that produces the nut crop.

The Carbohydrate Observatory uses a citizen science approach, the citizens being the almond, pistachio and walnut growers who send monthly wood and bark samples from their orchards to be analyzed for sugars and starch. The results are made available through a website that each grower has access to. Growers can then track the carbohydrate levels of their nut trees throughout the year while pairing it with their orchard climate and management records and their records of dormancy, pollination, bud break, flowering, fruiting, harvest and leaf drop. The goal is to build a better biological understanding of the role carbohydrates and to use this data set as a tool to predict yield and understand how the environmental stresses such as lack of chilling hours, salinity and drought affect tree growth and yield.

The specific goals of the Carbohydrate Observatory are to:

  1. Understand how annual patterns of starch and soluble sugars carbohydrates concentration in orchard trees differ throughout the Central Valley as atoll for improvement of spring/fall management practices and our understanding of chilling requirements.
  2. To develop a tool that uses NSC levels as a predictor of yield for the following year and to understand variable crop yields.
  1. Create an easy interactive NSC data sharing online platform so growers can see how their orchard is performing relative the regional average.

This long-term project will result in a comprehensive understanding of how pistachios grow and produce nuts using NSC as an indicator of tree status.  Once the basic model has been completed the effects of the environmental stresses of extreme temperatures, drought, salinity can then be examined for their effects upon the trees NSC status.  A beginning example of the how the baseline date from the Carbohydrate Observatory could be used is discussed below.

UC Davis Department of Plant Sciences Professor, Dr. Maciej Zwieniecki and recent PhD graduate, Dr. Jessie Godfrey, demonstrated how this tool can be used to determine the effects of sodium on NSC reserves.  Specifically,  they found that active retrieval of sodium from xylem sap may allow for the preservation of NSC, particularly starch, pools in mature xylem tissues by limiting the demand for carbon-based osmoticum, sugars. In plants subjected to salinity the plant responds by using NSC, primarily the sugars, to increase the osmotic pressure of the xylem fluid and increase the ability to extract water from the saline soil water. They found that the young growing tissues like bark and fine roots were found to counteract salinity by degrading their carbon dense, primarily starch, tissues, into osmotically active sugars, increasing the osmotic strength of the Xylem fluid.

On a practical, production level these results demonstrate a tree’s most distal or external parts, such as bark and fine roots, are replaceable.  However, while sacrificing these young bark and root tissues to concentrate and maintain NSC in a tree’s core is a viable short salt tolerance strategy, it needs to be determined if this is a viable strategy for maintaining yields.  This also suggests that an orchard monitoring strategy that measures only total NSC may not be effective as monitoring the transient sugar and starch levels within growing tissues, which are affected before total NSC reserves. This may prove to be the best practice for utilizing carbohydrate measurements to manage pistachio orchards using saline irrigation.  This hypothesis is supported by the earlier work of Tim Spann who demonstrated that a transient drop in total NSC appears to precipitate the floral bud abscission that results in alternate bearing.

In closing, the CPRB, in supporting this major NOW project, is developing the knowledge required to address the threat of NOW.   With the Carbohydrate Observatory, a long-term project, the CPRB is developing the  knowledge to better produce pistachios in a changing climate.  Future articles in this series will discuss additional CPRB funded research projects focused on pest and disease management, young tree training, dormancy, salinity management and website development.