Fruits and Nuts
Walnut ‘RX1’ – Phytophthora-Resistant Rootstock
BREEDING AND COMMERCIAL RELEASE OF PHYTOPHTHORA-RESISTANT WALNUT ROOTSTOCK ‘RX1’
Charles A. Leslie
Department of Plant Sciences, UC Davis, One Shields Avenue, Davis, California 95616.
Corresponding author: caleslie@ucdavis.edu
OUTLINE
1. SUMMARY
Phytophthora crown and root rots can cause extensive tree losses that degrade and occasionally decimate walnut orchards. Seedlings generated from various sources were tested for host-plant resistance to these diseases in greenhouse and field trials. A single seedling derived from the USDA-ARS National Plant Germplasm System (NPGS) accession DJUG 29.11, maintained in Davis, CA, showed excellent resistance in trials, was clonally propagated by tissue culture, and has now become a major commercial rootstock.
The goal was to develop and release a commercially viable walnut rootstock with host-plant resistance to Phytophthora crown and root rot.
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2. PROBLEMS ADDRESSED
Species of Phytophthora are widespread in orchard soils, can move across locations through surface water, are difficult to control, and can be deadly to trees. Young trees are particularly susceptible and can collapse and die rapidly, or trees may go years before succumbing, while experiencing reduced growth and yield. Chemical control options are limited. Proper water management is helpful but not always successful in prevention, particularly on poorly drained or periodically flooded soils. Resistant rootstocks provide an additional disease management tool.
3. SOLUTIONS DEVELOPED
A collaboration between UC Davis and the NPGS’s National Clonal Germplasm Repository (NCGR) in Davis, California sought to develop novel walnut rootstocks. They produced ‘RX1’, an open pollinated hybrid seedling from the female parent DJUG 29.11, an accession of little walnut (Juglans microcarpa Berland.) maintained at NCGR-Davis, and an English walnut (Juglans regia L.) male parent. ‘RX1’ performed well in a screening of commercial nursery seed sources for Phytophthora resistance during the Paradox Diversity Study—a project designed to evaluate the genetic diversity of walnut rootstocks—and was subsequently clonally propagated for further testing. Following superior performance in rootstock field trials in orchards, particularly those with previous losses to Phytophthora, ‘RX1’ was patented and released commercially in 2010 (McGranahan et al., 2010).
‘RX1’ also appears somewhat less susceptible to crown gall disease, another major walnut rootstock problem. Grower interest in the added value of ‘RX1’ and its relative ease of commercial clonal propagation also have helped drive the industry movement towards clonal walnut rootstocks. Subsequent research supported by funding from the USDA-NIFA Specialty Crop Research Initiative confirmed that progeny from DJUG 29.11 and other Juglans microcarpa trees in the NCGR-Davis collection segregate for resistance to crown gall and lesion nematode as well as to Phytophthora (Browne et al., 2015; Ramasamy, et al., 2021; Westphal, et al., 2022).
Collaborators involved in developing solution:
- Gale McGranahan, Charles Leslie, Wesley Hackett, James McKenna, UC Davis, Davis, California
- Gregory Browne, USDA-Agricultural Research Service, Davis, California
- Joseph Grant, UC ANR, Davis, California
4. IMPACT
Sales very considerably year-to-year, but on average 166,000 trees are sold per year over the past 10 years. As of January 2025, total sales of ‘RX1’ have exceeded 1.7 million trees. Ungrafted rootstocks typically sell at $10-$12 per tree, so at the sales-to-grower level, ‘RX1’ value is $1.66 to 1.99 million per year, or about $17 to 20.4 million total.
5. GERMPLASM
DJUG 29.11 is the female parent used to breed ‘RX1’. It was collected by Loy Shreve from a wild-growing tree in Edwards County, Texas and accessioned to the National Plant Germplasm System in 1983.
‘RX1’ was patented by McGranahan et al. on January 19, 2010 (US PP20,649 P3).
6. REFERENCES
Browne GT, Leslie CA, Grant JA, Bhat RG, Schmidt LS, Hackett WP, Kluepfel DA. Robinson R, McGranahan GH. 2015. Resistance to Species of Phytophthora Identified among Clones of Juglans microcarpa × J. regia. HortScience 50:1136-1142. https://doi.org/10.21273/HORTSCI.50.8.1136
McGranahan G, Browne G, Leslie C, Hackett W, McKenna, J. 2010. Walnut Rootstock ‘RX1’. (U.S. Patent No. PP20,649 P3). U.S. Patent and Trademark Office. https://patents.google.com/patent/USPP20649P3/
Ramasamy RK, Luo M-C, Leslie CA, Velasco D, Ott N, McClean A, Dandekar AM, Aradhya M, Brown PJ, Browne GT, Kluepfel DA, Westphal A, Dvorak J. 2021. Co-located quantitative trait loci mediate resistance to Agrobacterium tumefaciens, Phytophthora cinnamomi, and P. pini in Juglans microcarpa × J. regia hybrids. Horticulture Research 8:111. https://doi.org/10.1038/s41438-021-00546-7
Westphal A, Buzo TR, Maung ZTZ, McKenry M, Leslie CA, Brown PJ, Kluepfel DA. 2022. Identifying walnut (Juglans spp.) rootstocks with resistance and tolerance to plant-parasitic nematodes. Acta Horticulturae 1346:647-654. https://doi.org/10.17660/ActaHortic.2022.1346.81
7. CHAPTER INFORMATION
Citation: Leslie CA. 2025. Walnut ‘RX1’ – Phytophthora-Resistant Rootstock. In: Volk GM, Chen K, Byrne P (Eds.) Plant Genetic Resources: Success Stories. Fort Collins, Colorado: Colorado State University. Date accessed. Available from https://colostate.pressbooks.pub/pgrsuccessstories/chapter/walnut-rx1-phytophthora-resistant-rootstock/
Content originally submitted: March 25, 2024
Date of publication: February 24, 2025
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