Walnut rootstock options were historically seedlings, either Northern California Black (Juglans nigra), or Paradox (a cross between English Walnut, Juglans regia, and Northern California Black). Those two options still exist, but as research and technology advances in walnut cloning, clonal rootstocks are becoming more available. With this new advancement, growers have questions. Hopefully, I can provide some answers.
What is the difference?
There is a large difference between clonal rootstocks and Paradox seedlings. This is due in part to the genetic variability, or genetic differences, in Paradox seeds. UC/UCCE/USDA Walnut researchers, specialists, and farm advisors studied the genetic background of Paradox seedlings and found high variability from one seed to the next. This means that each seed is different from the next one. One seed might be more vigorous, one seed might be more susceptible to phytophthora , one seed might encourage more seed production, while another encourages more leaf and branch growth. This leads to a highly variable stand of trees in an orchard. Clonal rootstocks, on the other hand, are cuttings of the same plant. Walnut varieties are a good example of this process, as every Chandler tree in California came from one single mother tree which was originally produced by a seed. Much like how every Chandler tree tends to produce the same nut (some differences do develop depending on the growing conditions), every RX1 clonal rootstock will develop similar characteristics in the tree. Therefore, a Chandler orchard on a clonal rootstock tends to be more uniform in growth than an orchard on Paradox seedlings.
What is the RIGHT choice?
I honestly cannot think of a single “right choice” in agriculture, there’s just options. Options are nice, but they can also be confusing. Here is some background information that might help the decision in the future. RX1 and VX211 are both UC selections, chosen from acres of single seedling crosses based on their potential benefits. These were developed as a part of the Paradox diversity study done by UC/UCCE/USDA researchers, specialists and farm advisors. RX1 appears to show some tolerance to Phytophtora, a root infecting fungus like organism, but if disease pressure is high, the rootstock may still succumb to Phytophthora. VX211 was selected based on its potential tolerance to some nematode populations, but again, much like RX1 and Phytophthora, if nematode pressure is high, VX211 may still succumb. Both RX1 and VX211 were field tested against a handful of other selections as well as Paradox and Vlach. Vlach was developed by a private party which originated from a Paradox seedling tree in our very own county of Stanislaus. The tree was selected based on its high level of vigor.
Are any commercially available walnut rootstocks resistant to crown gall?
Short answer: no. Long answer: Paradox seedlings, RX1, VX211, and Vlach can all be infected with the causal agent of crown gall (Agrobacterium tumefaciens) and develop galls. YET the production of clonal material AVOIDS many opportunities for infection. Paradox seedlings are collected from the field as walnut seeds. Previous UC/UCCE/USDA research (funded in large part by nurseries) demonstrated that Agrobacterium tumefaciens is picked up from the ground in seed orchards. Nurseries funded this research to find ways to make their production better and have since developed ways to reduce crown gall in new Paradox seedling rootstocks by incorporating the use of tarps or catch frames. That said, Paradox seedlings are highly susceptible to Agrobacterium tumefaciens and clonal material skips this field collection step. Please be advised that orchards on RX1, VX211, and Vlach still require proper sanitation, ie, cleaning pruners/loppers with 10% bleach solution or 70% ethanol and avoid wounding the crown, trunk, and roots during planting and other practices.
Are these our only options?
When excluding Blackline (please see my summer 2019 issue for further information http://cestanislaus.ucanr.edu/newsletters/ Walnut_News_-_Fruit_For_Thought80737.pdf), for now, yes, but not forever. The California Walnut Board in combination with the US Specialty Crop Research Initiative is currently funding ongoing research in the breeding and development of future rootstocks. We are looking at three to four selections for various reasons, one of which being resistance to crown gall. These rootstocks are being field tested now and will be made available in the future provided they prove themselves worthy, in other words capable of producing a good crop.— By Kari Arnold Ph.D. UCCE Area Orchard and Vineyard Systems Advisor, Stanislaus County