WCN December 2019

Continued from Page 8
Phytophthora root and crown rots. Kluepfel said the most
promising solution for long term tree health and major yield
limitations is genetic resistance/tolerance in rootstocks.
Playing Catch-U p
Beumel said the walnut industry is 30 years behind the
almond industry in mass rootstock propagation by cloning, a
way to ensure uniformity in traits. That is due to the biological
differences between the species. The technology to clone
walnut rootstocks is a more recent achievement. Finding
genes that are known to give certain attributes can make the
process of rootstock development move faster, he said.
The use of genomic mapping will further accelerate the
development of disease-resistant walnut rootstocks. The
USDA ARS-University of California (UC), Davis walnut rootstock
team has completed analysis of the genomic sequence
data that resulted in the complete high quality reference
genomes for J. regia and J. microcarpa, i.e. the English walnut
and its wild North American relative respectively, that are
parents to the promising disease resistant hybrid rootstocks.
The genome data will accelerate the process of identifying
genetic markers for disease and nematode resistance.
“We now have the best assembled and annotated genome
for J. microcarpa and J. regia,” Kluelpfel said. The complete
analysis of the genomic sequence data shows high quality
reference genomes for those two walnut species.
Kluepfel said the research team is looking at behavior of
the walnut breeding populations originating the crossing of
these two key species to see if the disease resistance is present.
They are in the process of fine mapping and Kluepfel said
his hope is to single out and identify a handful of the genes
responsible for resistance. In most cases, there is resistance
to one pathogen, but in a handful of cases resistance to both
crown gall and Phytophthora have been found in some of
these new and novel hybrids.
Showing enhanced performance of the new rootstock being trialed.
“We chose to cross the widely used English walnut
specifically with the wild Texas black walnut because of
its native resistance to several soil-borne diseases and root
nematodes, which are serious pests of walnut in California,”
Kluepfel said.
The assembled genome sequences of the two walnut
species also will now help researchers identify genetic
markers that breeders can use to develop new varieties with
improved pathogen and pest resistance.
Objectives for the Research
Objectives of this continuing walnut rootstock
research are:
Traditional and clonal propagation of a genetically
diverse walnut species including a fine mapping of the J.
microcarpa (Texas walnut) and J. regia (English walnut)
genomes and their disease resistant hybrids.
Identification of resistance/tolerance to the diseases and
nematode species considered most damaging to walnut
production.
Genetic, physical and functional mapping of disease
resistance genes and use of molecular marker for rapid
screening of resistant genotypes.
Outreach to growers, field trials and examining performance
of chosen rootstocks germplasm.
Moving Forward
About 1,530 unique walnut genotypes are being maintained
by the research team as micro propagated cultures.
With this material 60,000 fully rooted plants were produced
and many were entered in the disease resistance phenotyping
pipelines.
The research team reported completion of genotyping
of the germplasm collection of J. microcarpa seeds and will
soon complete the associated analysis. This will be used to
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West Coast Nut December 2019