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The California walnut industry goes through difficult times. In addition to market challenges, several production issues task the sustainability of walnut production. Quickly changing regulatory requirements modify the way walnuts can be grown. The Sustainable Groundwater Management Act (SGMA) makes production in southern parts of the valley problematic, and many orchards have been or are being removed. Another critical change in policy is the increasing stringency of regulations for application of 1,3-dichoropropene (1,3-D). This material has been widely used, often in co-applications with chloropicrin when walnut followed walnut, to reduce soil-borne diseases.
Regulatory Pressures and Soil Fumigation Challenges
Walnuts are produced on rootstocks more adopted to soil conditions than the scion cultivars would be. The long-term go-to rootstock was ‘Paradox,’ a cross of Northern California Black Walnut and English walnut. It is susceptible, among others, to root lesion nematodes, Phytophthora root and crown rot, crown gall, and the so-called replant disease. Preplant soil fumigation with 1,3-D has reduced the damaging effects, especially of root lesion nematodes, that can reduce plant growth at 1 nematode per 250 cc of soil at planting. The replant disease was mitigated by soil treatments with concomitant application of chloropicrin.
With the new regulations in effect since January 1, 2024, and January 1, 2026, soil moisture conditions have to be much higher than previously and are not conducive for sufficient 1,3-D gas movement through the soil to reach plant-parasitic nematodes. Requirements of deeper injection and the possibility of covering the soil by totally impermeable film (TIF) add expense to the process. The restriction to smaller field sizes that may be treated in a single day add to logistical challenges. In essence, soil fumigation with 1,3-D becomes much more expensive, less effective and more cumbersome to use.
Rootstocks and Biological Considerations
Soil fumigation practice is viewed as troublesome because large amounts of chemicals are used per acre. The Sustainable Pest Management Roadmap that aims at a reduction of synthetic pesticide use by 90% and for 90% of the farms to practice sustainably by 2050 further exacerbates the challenge to establish productive walnut orchards for coming generations. In essence, the production system needs a thorough overhaul. Among the soil-borne plant pathogens, the root lesion nematode is probably the most notorious. In surveys, it was found in 85% of the orchards tested. In addition to its low threshold level, it occurs at least 5 ft deep in soil and is difficult to reach by management tools. Using rootstocks that don’t get damaged by this nematode has high sustainable appeal because these hypothetically “solve the problem for good.” Root lesion nematode-tolerant rootstocks are commercially available.
‘VX211’ and ‘Grizzly’ are promoted for their medium to high levels of nematode tolerance. It is important to remember that these rootstocks are susceptible to nematode parasites but don’t seem to suffer damage as much as others. While an enticing practical approach, biologically there are limitations of using nematode-tolerant rootstocks. It is not known how the high population densities that can develop under these rootstocks will interact with the trees in decades to come. There is also a lack of information how expansive this tolerance is.
‘Using tolerant rootstocks alone may not be enough, as high nematode populations can still pose long-term risks to orchard health.’
For example, it appears that VX211 often can tolerate root lesion nematode but does get damaged when root-knot nematodes are feeding on its roots. This latter species is less in the growers’ mind since pure English rootstocks (that are very susceptible and sensitive to this nematode) are seldom used anymore. In root lesion nematode, large population densities over time could create the possibility of selection of new “types” of this species. Perhaps, such nematodes could “learn” how to damage a tolerant rootstock. Because the current tolerant rootstocks allow for copious nematode reproduction, the risk for such shift in the nematode population appears real. After all, selection is a numbers game.
The more individuals are exposed to certain environmental queues, the higher the risk to encounter a new or different type of nematodes. Limiting nematode reproduction is liable to reduce these risks. While resistance has its own biological problems, broad resistance to root lesion nematode combined with tolerance is an important goal in rootstock development. These matters probably sound theoretical, but alertness of possible nematode population changes must not go ignored.
In a large cooperative effort of (molecular) breeders, engineers, plant scientists and plant pathologists (different UC campuses and USDA-ARS), farm advisors (UCANR), nurseries, and grower collaborators, a rootstock development program is in place. Graciously supported by the California Walnut Board and Commission, by CDFA, and the federal grantor NIFA, numerous rootstock trials are in place on commercial farms and on research stations. Out of these efforts, one rootstock, tentatively called ‘K3,’ expressed reduced susceptibility AND tolerance to root lesion and root-knot nematodes.
K3 performed well in regional rootstock trials in the Sacramento Valley. It also showed tolerance to the walnut replant disease when planted to non-treated root lesion nematode-infested walnut replant sites. This rootstock is nearing release for commercial use. In the said breeding and development program more rootstocks are expected to become available in the next few years. These rootstocks combine resistances to root lesion nematode, root-knot nematode, crown gall, and Phytophthora crown and root rot. These rootstocks are currently undergoing on-farm testing. Readers are invited to participate in testing of ‘K3’ and other rootstocks to get a first-hand look at the new technology as it nears release.
Alternatives to Fumigation and Future Directions
Developing rootstocks is a tedious process. Despite the high appeal of using such superior rootstocks, growers can’t wait if they plan to replace walnut orchards. Comprehensive studies to identify possible alternatives to soil fumigation with 1,3-D have been ongoing for decades. In recent years, new chemistries have become available that were perceived to have potential as pre-plant soil treatments. These are so-called “non-fumigant nematicides.”
In contrast to 1,3-D, these chemicals cannot redistribute in the soil environment after application. They need to be delivered to the target sites in the soil profile. Such delivery can be done with copious amounts of drench water that wets the soil profile 5 ft deep. Approximately six inches of water are necessary to accomplish this delivery. Such treatment requires complex irrigation systems and capabilities to inject the chemical into the irrigation stream.
Fascinatingly, several of the non-fumigant chemicals had efficacy in reducing nematode numbers in the soil profile at just a few pounds of active ingredient per acre compared to the several hundred pounds of 1,3-D necessary. But in addition to the application challenges, the level of efficacy of these materials was insufficient for a stand-alone preplant treatment. Only the (bio-) fumigant Dominus (AITC-containing material) was effective in reducing nematode numbers throughout the soil profile when delivered with irrigation water to the target sites. Simplified application methods had been developed. But high amounts per acre (40-80 GPA) of this material were necessary for efficacy, rendering it a poor fit for SPM. In addition, volatile organic compound regulation may encumber its registration.
The biorational method of anaerobic soil disinfestation (ASD) showed more promise for acceptability concerning air quality regulations and other human health concerns. In this method, easily decomposable organic matter is spread on the soil surface, incorporated, drip irrigation lines and totally impermeable film (TIF) installed, and the soil kept moisture-saturated for about one month. With the proper amount of substrate, preferably rice bran, and the correct irrigation schedule, this method is highly effective in reducing plant-parasitic nematodes and the replant disease from soil.
Key challenge is its expense and the excessive amount of plastic necessary for this procedure. This method requires copious amounts of drench water and is well-suited for flat level ground but probably would be difficult to use in hilly fields. Research work is ongoing to reduce application expenses by methods that eliminate the need for plastic cover and drip irrigation lines. Also, use of alternative substrates is investigated. Root lesion nematode-resistant cover crops are cropped during the fallow period between orchard cultivation.
In late summer, biomass produced by such crops is then macerated and incorporated in the soil as substrate for ASD. This may further reduce the expense of this method and make it better locally adoptable. Here, another offer to growers who plan to plant a new walnut orchard in the near future. The author of this article is on standby to test these new ASD tools on commercial farms. Growers interested in seeing these procedures first-hand and learning how to implement them on their farms are encouraged to reach out to the author.
Post-Plant Management and Orchard Recovery
Efforts invested in testing post-plant remedies for existing walnut orchards confirmed that Movento applications following the original protocol developed by Dr. Mike McKenry offered yield protection. Of other post-plant tools, Salibro was the most effective in reducing population densities of root lesion nematodes. Currently, this material is registered for non-bearing orchards only. In essence, it is only useful to establish a new orchard, but for bearing years it cannot be used until the registration is encompassing that use as well.
At the current time of reduced planting, growers may wonder if extended fallow periods or “crop rotation” rid their soils of the soil-borne plant pathogens that can create so much trouble. While there is some alleviation of the soil-borne pest and disease problems, extended fallow is typically not sufficient to forego any type of corrective action before planting a new orchard. For example, an over-the-thumb rule is that the replant disease in a fallow field after walnut orchard removal declines by 50% per year, meaning from 50%, 25%, 12.5% and so forth. So, some alleviation will come from patience.
Root lesion nematodes will also decline, but their very low threshold level coupled with their high reproductive capacity can make them resurge quickly. “Rotating” to other crops has limited value. Almond, stone fruit, and pistachio all host root lesion nematode and will be damaged by it.
So in summary, sustainable production of walnut remains challenging under the new conditions. There is hope on having new and improved rootstocks available that will reduce the negative impacts of soil-borne pathogens on walnut within the next few years. Until then, soil treatment options are being developed that carry the potential to partially replace chemical soil preplant fumigation. These methods become more affordable and are set to compete with the ever more cumbersome and costly chemical treatments.
Research never stops, so additional tools are being tested and developed. It is the nature of perennial crops that developing additional alternatives will take time. Progressive and curious growers are encouraged to participate in on-farm studies to support the quick adoption and implementation of technologies available now. The researchers “can’t do it without you.”
