Native soil bacteria as biocontrol

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Cal-IPC News
Vol. 24, No. 2 Summer 2016
Newsletter of the California Invasive Plant Council
Sarah Minnick, Stewardship Coordinator of
Ring Mountain Preserve for Marin County
Parks, with a young volunteer.
Testing oxalis control 4
Celebrating park stewards 8
Soil bacteria vs. invasive grasses 9
New threat to forests 10
WMAs: Who needs’em? 11
Celebrating
park stewardship
Protecting California’s Environment and Economy from Invasive Plants
Testing oxalis control 4
Celebrating park stewards 8
Soil bacteria vs. invasive grasses 9
New threat to forests 10
WMAs: Who needs’em? 11

Cal-IPC News Summer 2016 9
Biological control of invasive plants
can have many advantages over more
conventional management methods,
namely the potential for regional scale
control of target species over the long
term if the control agent can be effectively
established. It also avoids the often-un-
popular use of herbicides and the habitat
disturbances involved with mechanical
and physical control. However, because
biological control often relies on moving
the control agent from one region to an-
other, it can require extensive research to
ensure there is neglible risk of unwanted
impacts on non-target native or desirable
organisms. In the best case, the control
agent would be native to the region, just
not present in sufficient numbers to cause
the desired effect on the target invader.
This is an actual scenario that
may yield a novel tool for managing
invasive Eurasian winter annual grasses
in the western United States, thanks to
USDA-ARS researchers in Oregon and
Washington. They have recently publi-
cized the results of a long-term field trial
investigating the utility of native soil bac-
teria for selective control of the cheatgrass
(Bromus tectorum, a.k.a. downy brome),
jointed goatgrass (Aegilops cylindrica),
and medusahead (Taeniatherum caput-
medusae), while not adversely impacting
native plant species.
Research into this promising man-
agement tool started in the 1980s in
eastern Washington when Pseudomonas (a
common genus of soil bacteria native to
much of the western U.S. and elsewhere)
was found on the roots of stunted winter
wheat and associated with a reduction
in tiller number in affected plants.
Recognizing that many of the Eurasian
winter annual grasses which are ecosystem
transformers in the western U.S. are close
relatives of wheat, researchers wondered
if the bacteria might also negatively affect
these invaders and offer a potential man-
agement tool to mitigate their negative
effects on biodiversity and productivity
of range and croplands. To investigate
this possibility, researchers have screened
over 20,000 potential bacterial candidates
through greenhouse experiments and field
trials over the last 20 years located around
the inland Pacific Northwest for selective
control of cheatgrass, medusahead, and
jointed goatgrass.
Recently, they made a breakthrough
with Pseudomonas fluorescens strain
ACK55. By using ACK55 similarly to a
pre-emergent herbicide, researchers found
that at a rate of one pint of active culture
(1 x 108 colony forming units mL-1) per
acre, sprayed in the fall prior to emergence
of the target invasive plant species, P.
fluorescens ACK55 is extremely effective at
controlling cheatgrass, jointed goatgrass,
and medusahead by inhibiting root cell
elongation and tiller initiation. In fact, a
single application appears to be adequate
for almost eliminating these annual weeds
from the seed bank in the long-term (4-5
years). This is about the normal lifespan
of P. fluorescens in the soil.
Other positive results that would
increase its utility in an Integrated Pest
Management (IPM) program for Eurasian
winter annual grasses include tests of over
200 non-target plants demonstrating that
the bacteria do not affect crop or native
species, meaning that P. fluorescens can
provide selective control of the target
invaders. In the presence of P. fluorescens,
crops and natives are able to competi-
tively dominate the suppressed weeds and
increase in cover. Research has also found
that P. fluorescens does not inhibit fish,
birds, bees, and other insects. Moreover,
since the strain has no known anti-
fungal or anti-bacterial activity, it is not
expected to disturb the native microbial
communities.
As is the case with pre-emergent herbi-
cides, which this biological control agent
most resembles in terms of application
protocols, researchers have stressed that
several other factors, such as soil proper-
ties, temperatures, and precipitation, can
Native soil bacteria as biocontrol
By Travis Bean, UC Riverside and Elise Gornish, UC Davis, bean@ucr.edu
...continued page 14
Cheatgrass invades pinyon/juniper/sagebrush country in the intermountain west,
changing the vegetation community by altering the wildfire regime. BLM photo from
the Salt Lake Tribune.

14 Cal-IPC News Summer 2016
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mediate treatment effectiveness, spe-
cifically noting that the bacteria may not
perform as well under hot and arid condi-
tions experienced in summer months. As
with all invasive plant management and
restoration methods, timing is critical;
along with cooler temperatures, precipita-
tion is required within two weeks of
application for the bacteria to effectively
colonize the soil. This means that success-
ful application relies heavily on seasonal
precipitation and a lack of unseasonably
warm fall weather, which may be more
limiting in future years under some
climate change scenarios.
P. fluorescens ACK55 is currently
undergoing the EPA registration process,
which can take about 17 months. If
accepted by EPA, the inoculate could be
available for widespread use. In California,
pathogens intended for controlling weeds
require authorization from the California
Department of Pesticide Regulation,
and other states may have similar
requirements.
Other than state-specific restrictions
on using pathogens for invasive plant
control, there are also the hurdles of
propagation: what is the cost of growing
enough of the bacteria to supply it to
farmers, ranchers and conservationists
interested in using it to control invasive
winter annual grasses in the western U.S.?
Because it’s likely to be cost-prohibitive to
apply the bacteria to all of the millions of
acres currently infested by these problem
species, the researchers suggest that a
more targeted approach be considered.
One approach would be direct applica-
tion of the bacteria to leading edges of
existing infestations to quarantine existing
populations and prevent spread into new
areas. Other priority targets are firebreaks
to control wildfire spread and recently
burned areas where the existing popula-
tion of invasive grasses is restricted to the
seed bank, allowing the bacteria to have
the largest impact.
Additionally, site-specific factors may
limit effectiveness of the bacteria for
invasive grass control. As we often hear
restoration ecologists and practitioners
emphasize, control tools are most effective
when used as part of an IPM program,
rather than as a stand-alone treatment. In
fact, as the researchers themselves suggest,
the bacteria is unlikely to be successful if
simply applied to invasive grass monocul-
tures, as the grasses will simply regenerate
given sufficient time. Instead, researchers
suggest that post-emergent herbicides be
applied to reduce the standing crop of
invaders, while also applying the bacteria
to attack germinating seedlings and
provide seed bank control over the longer
term. Seeding or planting natives or desir-
able forage species can help reestablish a
diverse and resilient plant community that
can resist or prevent recolonization by
invasive grasses.
For more information:
Ibekwe, A.M. et al. 2010. An assessment of
environmental conditions for control of downy
brome by Pseudomonas fluorescens D7. www.ars.
usda.gov/SP2UserFiles/Place/20360500/pdf_pubs/
P2098.pdf
Kennedy, A. et al. Microbial control of
cheatgrass, jointed goatgrass and medusahead
(presentation). www.fwaa.org/accounts/fwaa/
data_documents/60/files/10b-dl-2011-12-
13_130p_kennedy,ann.pdf
Kennedy, A. et al. Cheatgrass suppressive
bacteria research (factsheet). http://sfc.smallfarm-
central.com/dynamic_content/uploadfiles/152/
biocontrols%202.pdf
Dickie, G. 2015. Researchers find an answer
to invasive cheatgrass. High Country News, www.
hcn.org/articles/researchers-find-formidable-foe-for-
invasive-cheatgrass
Solomon, C. 2015. Researcher finds way
to fight cheatgrass, a western scourge New York
Times, www.nytimes.com/2015/10/06/science/
researcher-finds-way-to-fight-cheatgrass-a-western-
scourge.html
...Soil bacteria from page 9