California produces 99% of the Persian (English) walnuts produced in North America and nearly all of this production occurs on trees growing on rootstocks genetically distinct from the scion. Dependent on the soil type, soil conditions and presence of known pathogens, growers must choose from a limited selection of commercially available rootstock genotypes.
To date, these available rootstocks have served the industry well. This is evidenced by the fact that walnut production has grown from around 70,000 tons in 1960, to over 600,000 tons in 2015. In addition, the average production per acre over this same period has risen from 1,200 lbs/acre to nearly 4,000 lbs/acre. However, even with these significant increases in yield over the years, California growers and nursery operators do experience yield and tree losses due to soil borne pests and diseases. One estimate puts annual walnut orchard/nursery losses from key soil borne pathogens such as lesion nematode, Phytophthora spp, Armillaria, and Agrobacterium tumefaciens (crown gall) at roughly 18%, which translates into an annual loss of greater than $150 million.
Currently, a grower has three general choices when it comes to rootstock selection: 1) Paradox seedlings, 2) one of three commercially available clonal rootstocks, and 3) own-rooted trees (or English rootstocks). These rootstock choices represent a fairly narrow genetic base upon which you will be depending for up to 30 years. Let’s look at these three classes of available rootstocks a bit more closely. Paradox seedlings, which represent the majority of rootstocks currently in the ground, originate from open pollinated seeds resulting from a cross between Northern California Black walnut (J. hindsii) and nearby English walnut (J. regia) tees. Two of the three commercially available Paradox clonal rootstocks, VX211 and Vlach are clones of a given Paradox tree of seedling origin while RX1 is a hybrid between J. microcarpa (Texas Black Walnut) and English walnut. Finally, some growers in areas where cherry leaf roll virus is prevalent, choose to grow trees on their own roots or English seedling rootstocks to avoid blackline disease. (More detailed information on the characteristics of available rootstock genotypes can be found in the publication; http://ucanr.edu/datastoreFiles/391-536.pdf )
Even though this limited array of rootstock genotypes has performed well for the walnut industry, collectively they offer little protection against crown gall, Armillaria and, with one or two exceptions, lesion nematodes and Phytophthora crown and root rot. Since there are limited post-plant options to control these three soil borne pathogens, it is generally agreed that the best way to manage these diseases is to develop rootstocks which are tolerant or resistant to them. This has been the long term goal of our rootstock development research team: the development of disease resistant rootstocks and identification of the walnut genes which control this resistance.
As a first step, our team exploited a tremendous resource available in our own back yard, i.e. the walnut germplasm collection maintained by the USDA ARS National Clonal Germplasm Repository in Davis, CA. This collection contains about 800 walnut genotypes representing about 14 Juglans species gathered from around the world. We started by collecting open pollinated (OP) seeds from mother trees in this collection and screened the resulting seedlings for resistance to crown gall (Fig. 2), and on a more limited initial scale, resistance to Phytophthora and lesion nematodes. The result of this work was the identification of several mother trees that produced a high incidence of OP seeds resistant to one or more of the key soil borne pathogens mentioned above. Recently we narrowed our focus to J. microcarpa mother trees since this species appears to throw a higher incidence of disease resistant OP progeny than other Juglans species. In addition, given their natural range across the arid southwestern U.S., we anticipate this species may contribute useful drought tolerant traits into our rootstock breeding efforts. Several of these putative disease resistant OP J. microcarpa progeny have already been cloned and are now in field trials to assess their commercial potential.
As mention above, our goal is to not only identify useful disease resistant rootstocks but also to locate and identify the walnut genes which control disease resistance. Towards that end we crossed the most promising J. microcarpa mother trees with pollen from J. regia to generate a family of hybrid offspring (a breeding population) which we are using to map and identify the genes for disease resistance. (Fig.1) To accomplish this goal, we also sequenced the entire genome of both parents of this cross (i.e. J. microcarpa and J. regia). Now that we have the complete genomic DNA sequence of both parents, and understand their basic chromosomal organization, we are ready to interpret how the offspring of these parents respond to the pathogens of interest. For the first time ever, we are in a position to identify the genes in one or both parents that were passed along to their offspring and contributed to disease resistance in a given rootstock. This level of genetic knowledge and sophistication has never before been possible in the walnut-pathogen system.
Why are these advances important and how are they being utilized? First, one of the direct and immediately useful aspects of this approach is the identification of disease resistant individuals in the hybrid breeding populations which may be useful to the industry. In a sense, we are generating novel “Paradox-type” hybrids for the industry which will broaden the genetic base of the currently available rootstocks. Second, by understanding which genes mediate disease resistance we will be able to dramatically enhance our future breeding efforts to produce new disease resistant individuals. We will be able to simply look for the presence of “resistance” genes which will allow us to label a given seedling as resistant. This avoids the need to grow out trees to various levels of maturity before conducting disease resistance screens. In short, the new knowledge and techniques we are developing will take years off the breeding and selection process involved in discovering new rootstocks. In addition, once we identify the genes which mediate resistance, it may be possible to directly alter these genes to perhaps enhance or extend their ability to impart disease resistance to a given rootstock.
Finally, after each year’s disease resistant screening efforts, one of the by-products of our gene-discovery effort is the identification of “elite” individuals exhibiting useful levels of disease resistance to one or more of the pathogens mentioned above. These “elite” selections are then clonally propagated and prepared for field trials. Currently, we have 5 “elite” selections grafted to the cultivar Chandler and growing in five large scale rootstock plots around the central valley of California where they are being examined for both horticultural and disease resistance traits. In cooperation with our nursery collaborators, the next sets of “elite” selections are being propagated now and prepared for additional field trials.
In conclusion, we have:
- Identified individual hybrid selections which exhibit resistance to one or more of the key soil-borne pathogens impacting walnut production in California
- Sequenced and characterize the organization of the genomes of two walnut species which have been used as parents to generate novel “Paradox-like” hybrids.
- Made progress towards identification of the genes in walnut which mediate disease resistance in hybrid selections
- Established large scale field trials to evaluate “elite” rootstock selections for their potential commercial viability
Acknowledgements:
The work presented here is possible do to the support received from the California Walnut Board, the USDA-ARS, University of California, and grants from the USDA-NIFA-SCRI program. This project is truly a team effort which is only possible due to the contributions made by all the key principle investigators involved. For additional information on any aspect of this project, please contact Daniel Kluepfel at daniel.kluepfel@ars.usda.gov.
A) Resistant genotype
B) Limited resistance
C) & D) Examples of susceptible genotypes which are discarded from the screen.
If the plant shown in panel A continues to exhibit limited gall development, this plant would be retested and considered for entry in field trials.
