The Crush - January 2022 - 2

FEATURE STORY
Work is also in progress to breed and evaluate cultivars that will
have PD resistance and powdery mildew (PM) resistance. Six
promising sources of PM genetic resistance have been identified
from two species of native Asian Vitis cultivars, from a V. vinifera
cultivar from Asia and from Muscadinia rotundifolia - a native
southeast U.S. muscadine grape cultivar.
TRANSGENIC ROOTSTOCKS
David Gilchrist, UCD Department of Plant Pathology professor
emeritus, provided an update on field trials to evaluate crossgraft
protection of a PD susceptible scion of chardonnay against
the development of PD symptoms when grafted to transgenic
rootstocks. The two transformed rootstocks, 101-14 and 1103P,
express dual combinations of five transgenes that have shown
positive protection against PD in previous field trials. The
researchers will assess the potential cross-graft protection of
the non-transformed scions, and the effect of the transgenes to
protect the rootstocks against downward bacterial movement
into the perennial tissue from the scions. The scions were
inoculated with Xf in July 2021. The researchers will evaluate
PD symptoms, bacterial movement and fruit yield beginning in
2022. " If the rootstock is the end product and the only place in
the vine where transgenic genes are present, the hope is that this
will be more accepted by the industry, " Gilchrist said, similar to
how non-vinifera rootstocks are accepted for grafting to vinifera
varieties to produce vinifera wines.
PD CONTROL TREATMENTS
Steven Lindow, UC Berkeley Department of Plant and Microbial
Biology professor emeritus, is working with a naturally
occurring, beneficial bacterium called Paraburkholderia
phytofirmans. It grows and moves extensively within mature
grapevines and greatly reduces PD occurrence when applied
before, or even several weeks after, Xf pathogen inoculation in
vines in both greenhouse and in field studies.
Lindow's lab has tested its effectiveness through direct
inoculation by hand into the vine by a droplet puncture. More
recent studies have looked at foliar spray applications using
a penetrating surfactant that appears to be superior to direct
inoculation. Although these test foliar sprays were applied
by hand, Lindow said, " I don't see why it would not work as
well using a commercial tractor-mounted sprayer, although
it may require higher volumes of water to achieve proper leaf
penetration. "
Field tests indicate P. phytofirmans may provide best results
when used preventatively. Good results were shown in
preventing PD symptoms when it was applied three weeks
prior to inoculation with Xf. But even when applied up to five
weeks after Xf inoculation, there were great reductions in disease
severity. Findings to date indicate that a single foliar application
each year in vineyards in PD prone locations may provide
substantial prevention and reduction in PD infection. Lindow
2 JANUARY 2022
suggested a foliar spray six weeks after budbreak may be the best
application timing. Another positive aspect of P. phytofirmans is
that because it is a naturally occurring biocontrol agent, it could
have fewer regulatory requirements for commercial approval.
Leonardo De La Fuente, professor of entomology and plant
physiology at Auburn University in Alabama, discussed another
Xf chemical control option being studied using antibacterial zinc
nanoparticles. Based on trials using the product Zinkicide that
has shown effective reduction of Huanglonging (HLB) disease
symptoms (citrus greening disease) in citrus field trials, Zinkicide
was also tested in greenhouse trials against Xf infections in
tobacco and blueberry plants and it significantly reduced Xf
symptoms and pathogen populations. The researchers plan
to modify the chemical composition of Zinkicide to improve
performance against Xf at lower doses and test different zinc
nano formulations in 2022 on grapes infected with Xf. The goal
is to develop a successful formulation that is economically viable
for use in vineyards as a soil drench and/or foliar spray.
VECTOR MANAGEMENT
UC Riverside professor of entomology Peter Atkinson and his
team are researching the potential to use genetic modification of
the GWSS (Homalodisca vitripennis) vector to generate strains of
the insect that cannot transmit Xf. Explaining the need to explore
other methods of vector management, Atkinson said, " Resistance
to insecticides has been documented, " based on the repeated use
of imidacloprid in area-wide GWSS control programs in Kern
County and other locations.
CRISPR is the acronym for clustered regularly interspaced short
palindromic repeats, a system of DNA sequences found in the
genomes of organisms such as bacteria. Specific gene editing
CRISPR-based technology can be used to locate and target
specific genetic material within an organism's genome without
affecting other genetic material. The researchers have shown
it is possible to insert a gene into the GWSS genome using two
distinct CRISPR-based technologies. Their goal is to generate
strains of GWSS that cannot transmit Xf, have the ability to breed
and carry this trait across multiple generations, and to release
these new GWSS strains into
the field to replace the current
populations of wild GWSS
species that do transmit Xf.
" Our philosophy is, if you
can control the vector, you
can control the disease, "
Atkinson said.
2021 PIERCE'S DISEASE
RESEARCH PROJECTS
AT A GLANCE
December 2021
PD Research Symposium's " 2021
Research Projects at a Glance "
https://www.cdfa.ca.gov/pdcp/documents/PD_Research_Symposium_Proceedings_Exec_Summary.pdf https://www.cdfa.ca.gov/pdcp/documents/PD_Research_Symposium_Proceedings_Exec_Summary.pdf
The Crush - January 2022

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