Selecting the Best Walnut Rootstock for Your Situation

Janine Hasey, UCCE Tree Crops Advisor Emerita, Sutter/ Yuba/ Colusa Counties
Luke Milliron UCCE Orchards Advisor Butte/ Glenn/ Tehama Counties
Clarissa Reyes UCCE Orchards Staff Research Associate Butte/ Glenn/ Tehama Counties

Once a site is selected, the three most important decisions in establishing a new walnut orchard are variety selection, rootstock selection, and tree/row spacing. The spacing should be determined after the variety and rootstock are chosen since that combination will determine the ultimate size of the tree, along with the soil type and orchard management. Rootstocks are the foundation of a walnut orchard. Therefore, selecting one with attributes that will tolerate or resist site problems, while providing the vigor for optimum tree performance, is key to orchard health and longevity. There has been extensive team research including nurseries, growers, UC, UCCE, and USDA-ARS with funding from the CA Walnut Board and the USDA’s National Institute of Food and Agriculture (NIFA), that have contributed to the great strides in clonal walnut rootstock development over the last 22 plus years. This article provides updates from the latest rootstock research statewide.

Commercially available Paradox clones include Vlach, VX211, RX1, and the most recent clone Grizzly. Vlach has been available since 1999 and came from a vigorous Paradox tree in Stanislaus County. Originally identified as superior seedlings from the statewide Paradox diversity trial in the 1990’s and then cloned VX211 and RX1 were released by UC and USDA in 2007, available to growers in 2008, and patented in 2010 after years of evaluation for vigor, resistance to nematodes, crown gall, and Phytophthora. Clonal Paradox rootstocks are propagated in a lab and sold either as potted rootstock that is fall budded or spring grafted in the field, or as a June budded or nursery grafted bare root tree. For information on training different nursery products, see sacvalleyorchards.com/walnuts/horticulture-walnuts/walnut-tree-training-different-nursery-products.

Clonal Paradox walnut rootstock attributes

Vlach: One of the first clonal Paradox, and the first to be widely planted. Parentage is Northern California black (Juglans hindsii) x English walnut (J. regia). It continues to have high growth rates and yields well in ongoing trials. It typically has much less crown gall than seedling Paradox but more than the other clones. It’s been a good consistent rootstock for many different sites.

VX211: Parentage of VX211 is Northern California black x English walnut and it is very vigorous. It continues to have high growth rates and yields well in ongoing trials. A main attribute is lesion and root knot nematode tolerance. The nematodes can still reproduce but this rootstock is able to tolerate higher levels of nematodes than the other rootstocks. Recommended for where there have been nematode problems or for replant trees in existing orchards. Replants are very problematic in more mature orchards because of competition for nutrients, light, and different irrigation needs. With the nematode tolerance advantage paired with the vigor, VX211 is a great candidate for replants. Being a clonal Paradox, VX211 has notably less crown gall than seedling Paradox and consistently less crown gall than Vlach in field trials. You can find more replant considerations at: sacvalleyorchards.com/walnuts/cost-and-expense-considerations/replanting

RX1: The genetics for RX1 is different than for Vlach or VX211. RX1 parentage is Texas black (J. microcarpa) x English walnut. The J. microcarpa seems to instill resistance to Phytophthora and maybe to crown gall disease and has been used extensively to breed the new rootstock genotypes we’re testing in statewide field trials. As a rootstock, it has more moderate vigor, but grafted trees on RX1 sometimes have equal or more vigor compared to trees on VX211 depending on the field location. In our ongoing statewide rootstock trials planted in 2016, RX1 along with Vlach and VX211 have had the highest scion growth rates across counties. It also has yielded well at most sites.

Disease resistance – RX1 has either moderate to high resistance to Phytophthora depending on the species. In a 5-year field trial concluded in late 2020, RX1 inoculated with Phytophthora developed no symptoms, meaning the resistance is still holding. RX1 is the preferred rootstock for any site with Phytophthora. It also can have what we call moderate resistance to crown gall. Consistently in our greenhouse screening and statewide field trials, RX1 has the least crown gall of the three standard clones.

Water relations – RX1 appears to handle drier conditions in terms of irrigation scheduling and still be vigorous and productive. Note that it’s a very subtle difference and we’re still in the early stages of researching it.

Salinity tolerance – In early stages of research, RX1 appears to show more salinity intolerance (leaf scorch) than the other clonal rootstocks. It may not be the best choice in situations with soil or water salinity concerns.

Vlach, VX211, RX1 summary (see Table 1): Regardless of whether they are a potted vs. a field grown tree, none of these three clonal rootstocks is resistant to Agrobacterium tumefaciens that causes crown gall. Consistently in our long-term field trials however, these clonal Paradox rootstocks have a substantially lower percentage of crown gall compared to the highly susceptible seedling Paradox hybrid rootstock. A well-known nurseryman once said, “It takes 20 years to prove a rootstock”.  These three clonal Paradox rootstocks have been subjected to 20+ years of screening, field research trials, and observations in growers’ orchards for us to have confidence in the attributes listed above. That is why we refer to them as the “standard clones”. These three standards are now the controls for comparison against new genotypes in UCCE/USDA rootstock trials.

Grizzly:   The most recent commercially available clonal Paradox rootstock, Grizzly is patented by a grower and a nurseryman. Grizzly parentage is Northern California black x English walnut. It came from a mother tree in a replant situation with very sandy soil and lesion nematodes. From observations over 20 years, this tree had twice the size and production of the other trees that were starting to decline much earlier. We lack long-term data on Grizzly but have it in rootstock trials planted in 2016 – at one replicated trial in Lake County, and nine observational trees in Sutter County. So far it has high vigor and no crown gall at either site. The yield on the observational trees is the same as the standard clones, so it looks promising. It would be one to consider where there are tougher, coarse soils or for replant situations. Tolerance or resistance to nematodes has not yet been established in UCCE trials. It can be difficult to propagate so availability can be an issue.

Rootstock disease ratings

Table 1. Disease rating of the standard clonal Paradox rootstocks for problem situations. Based on data from ongoing UC and USDA-ARS trials.

For more information on disease resistance ratings and mechanisms of the Paradox clones, see the bulletin on walnuts in the nursery trade, how they are propagated and understanding the terminology.

Seedling walnut rootstock attributes

Northern California black walnut: This was the rootstock of choice in the early 1900’s. Fast forward to the present, its main attribute is having the most tolerance to salinity compared to Paradox hybrid seedling or clonal Paradox rootstocks. Vigor is only moderate and in UC trials, yields were always lower compared to trees on Paradox rootstocks. Traditionally they were planted and grew well in deep loamy alluvial soils near rivers. They often have stunted growth when planted on more marginal soils. Northern California black has a lower susceptibility to crown gall vs. seedling Paradox but are very susceptible to Phytophthora and to lesion nematode. Clonal Paradox rootstocks are recommended where any of these problems exist (Table 1). Black walnut rootstocks should be considered where there are salt problems like chloride and only where soils are loamy and well-draining.

Paradox hybrid seedling: This rootstock gained popularity in the 1950s because of its vigor and more tolerance to Phytophthora and lesion nematode than black walnut rootstock. However, Paradox seedling rootstocks are very susceptible to crown gall disease. There are seed sources that have lower crown gall incidence and certain nursery practices that prevent infection by the bacterium during the seed collection phase which can alleviate crown gall from developing. However, clonal Paradox rootstocks typically have much lower crown gall infection than Paradox seedling rootstocks across numerous trials statewide and have the other specific advantages as shown in Table 1. Seedling rootstocks also have genetic variability, so tree size differences are more common than in clonal rootstock orchards. With clonally propagated rootstocks, we have the advantages of Paradox, plus some added advantages (see Table 1), all without the disadvantages of being a seedling with genetic variability across the field.

New clonal walnut rootstock genotype trials: With the walnut rootstock breeding program’s long-term goal of developing rootstocks with genetic resistance to all three of the major soil-borne pathogens shown in Table 1, several new genotypes have been bred. Four new genotypes are in statewide field trials planted in 2015 and 2016 and being compared to the standard clonal Paradox rootstocks. The 2016 trials are ongoing, and we are planning to have field meetings at each location in early summer so stay tuned for dates. Several newer walnut rootstock genotypes have been planted in 2022 in various locations statewide making the prospect of future new superior rootstocks an attainable possibility.

For a discussion on walnut rootstocks, visit: growingthevalleypodcast.com/podcastfeed/walnutrootstock

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