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BIOLOGICAL CONTROL OF ANNUAL
GRASS WEEDS
Ann C. Kennedy
ABSTRACT
to occur at a critical point in the growth of the target organism. The microorganism needs to survive in the soil and
be tolerant of low soil moisture. Finally, the biological control procedure needs to be feasible and be combined with
other methods of weed control. Biological control will not
necessarily be used alone, but rather in concert with cultural practices and herbicides, with the end result being
the suppression of the pest.
Biological control of weeds is based on the premise that
biotic factors have a significant influence on the distribution,
abundance, and competitive abilities ofplant species. Soil
bacteria that selectively suppress grass weed species may
alter competition among range plants. The bacteria can
function as a direct delivery system for the natural plantsuppressive compounds they produce. Biological control
offers alternative means of suppressing annual grass weed
growth and establishment in range systems.
INSECTS
BIOLOGICAL CONTROL
Biological control is the use of natural enemies to control
pests and has been utilized ever since humans altered the
environment for food production. Biological control can be
used to control insects, pathogens, and weeds and is defined
as "the action of parasites, predators, or pathogens in maintaining another organism's population density at a lower
average than would occur in their absence" (DeBach 1964)
or alternatively as "the use of natural or modified organisms, genes, or gene products to reduce the effects of undesirable organisms (pests), and to favor desirable organisms
such as crops, trees, animals, and beneficial insects and
microorganisms" (Cook 1987).
The three main strategies for biological control are:
1. Classical, which is the importation of exotics or natural enemies for release, dissemination, and self perpetuation on target pests,
2. Augmentation, which involves the introduction of a
virulent strain to suppress pests, also called the bioherbicide approach, and
3. Integrated pest management, which uses management
techniques to conserve or enhance indigenous enemies of
various pests.
An effective biological control agent needs to have certain
characteristics in order to be effective. First and most important, biological control agents need to have a narrow host
specificity or selectively suppress the target organism and
not have any adverse effect on the nontarget population.
The suppression of the pest does not necessarily have to be
100 percent lethal to be effective. Second, the agent must
survive and act in the environment. It is very easy to find
antagonistic relationships in the lab, but often these relationships do not exist in the field under varying environmental conditions. Third, the suppressive activity needs
Paper presented at the Symposium on Ecology, Management, and Restoration of Intermountain Annual Rangelands, Boise, ID, May 18-22, 1992.
Ann C. Kennedy is a Soil Scientist, Agricultural Research Service, U.S.
Department of Agriculture, Pullman, WA 99164-6421.
188
Weeds are a problem in range establishment and have
the potential to be controlled by biological means, either
by the use of insects or microorganisms. The greatest emphasis in biological weed control research in the past has
been on the use of insects. The first insect release on record
for weed control was the release of a moth (Dactylopius
ceylonicus) in the mid-1800's to control the pricldypear cactus (Opuntia vulgaris) in India. The list of insects used to
successfully control many different weed pests continues
to expand (Julien 1987). Such biological control agents
include a beetle to combat St. Johnswort (Jlypericum perforatum), a gall-forming fly and a gall-forming mite which
attack rush skeleton weed (Chondrillajuncea), and beetles
to control lantana (Lantana camara).
MICROBES
Microorganisms can have a profound effect on plant
growth. Phytotoxic effects of microorganisms are often
plant species and cultivar specific. These plant-microbe
interactions can greatly influence distribution, abundance,
and competition among plant species. Plant pathogens potentially may be used to regulate the growth of unwanted
plant species growing simultaneously with more desirable
plants. This would be especially true if competitive weed
growth coincided with environmental factors conducive to
bacterial growth and weed-suppressive activity.
Fungal pathogens have also been used to control weeds.
Most notable is the use of rusts (Puccinia jaceae Otth.) for
the control of diffuse knapweed (Centuria diffusa Lam.)
(Mortensen 1986; Watson and Clement 1986) and skeleton
weed (Chondrilla juncea) control with the use of Puccinia
chondrillina (Cullen and others 1973). Mycoherbicides,
such as the fungal pathogens of weeds sold under the trade
names of DevineTM and CollegoTM, are commercially available. Devine is being used to control stranglervine (Morrenia odorata) in citrus (Ridings 1986) and Collego is used
for the control of northern joint vetch (Aeschynomene virginica, Templeton and others 1986) in rice and soybean.
The success of these two commercial mycoherbicides indicates the potential of microorganisms for weed control
Plant-suppressive bacteria potentially may be used to
regulate the growth of unwanted plant species growing
simutaneously with more desirable plants. Downy brome
is an excellent weed species for this type of investigation.
Bacteria can exert a subtle, yet profound, effect on plant
growth. Rhizobacteria that suppress plant growth can impair seed germination and delay plant development by the
production of phytotoxic substances (Alstrom 1987; Schippers
and others 1987; Suslow and Schroth 1982). Many plantsuppressive bacteria are effective colonizers of roots and residues (Fredrickson and Elliott 1985; Stroo and others 1988;
Suslow and Schroth 1982) and subsequent retardation of
plant growth is demonstrated in the laboratory.
Our biological weed control research developed from
investigations of the early spring growth of winter wheat
(Fredrickson and Elliott 1985). Stunted wheat plants were
heavily colonized by bacteria that produced plant inhibitory
compounds. These compounds specifically inhibited winter
wheat and did not injure other small grains or legumes
(Bolton and Elliott 1989). Recognizing this specificity, our
initial hypothesis for the use of similar bacteria for biological weed control was that other soil bacteria could be found
that inhibited only the grass weed downy brome and not
the crop or other plant species.
To date, we have found a number of bacteria that specifically inhibit various grass weeds, but do not affect the crop
<Kennedy and others 1991). These naturally occurring soil
bacteria inhibit plant growth by the production of plantsuppressive compounds. These bacteria are excellent biological control agents because they are aggressive colonizers of the roots and residue. The bacteria can function as
a direct delivery system for the natural plant-suppressive
compounds they produce. They tend to be fairly tolerant of
low soil moisture, although they do not survive well under
hot, dry conditions. They survive well at low temperatures
with an optimum temperature often below 15 oc. The greatest amount of toxin is produced at these low temperatures.
They are most prevalent in the soil in late fall and early
spring. Application of these bacteria during seedbed preparation and the resultant suppression of downy brome root
growth may allow other plant species to out-compete weeds,
thus leading to the establishment of more desirable range
species. If this occurs during the seedling stage, the crop
may gain a competitive growth advantage. This increase
in crop competitive ability may further suppress weed
growth at later growth stages.
Downy brome (Bromus tectorum L.), commonly called
cheatgrass, was chosen as a biological control target because it is an especially troublesome weed. It has more
vigorous root growth in the winter months, and completes
its life cycle early in the spring. It is an invader species
from Eurasia that germinates in fall or spring, over wide
ranges of temperature and moisture. Downy brome was
introduced into North America about 100 years ago (Mack
1981) and continues to be unmanageable in winter annual
cropland and rangeland (Morrow and Stahlman 1984). It
often is considered an important forage species providing
early spring grazing; however, its short growth period, fluctuating forage production, and high fire hazard make it less
desirable than other species. Downy brome is an effective
competitor for space, water, and nutrients because its roots
grow at low temperatures. Perennial grass seedings often
187
fail because downy brome is so competitive. Downy brome
is an excellent plant to be used in biological weed control
programs. Application of the downy brome-suppressive bacteria and the resultant suppression of root growth may allow another plant species to out-compete the weed.
In a survey of 3,500 naturally occurring soil bacteria,
half were inhibitory to downy brome seedling growth in the
in vitro screening. Two hundred forty isolates inhibited
downy brome and not winter wheat in the in vitro assay. In
greenhouse studies, 14 isolates continued to inhibit downy
brome more than 35 percent with no adverse effect on winter wheat (Kennedy and others 1989). Several of these isolates from the initial screenings have proved to be biologically active in field tests.
Most bacterial isolates investigated thus far inhibit root
growth, but some, such as Pseudomonas fluorescens strain
D7 (P.f. D7), also reduce weed seed germination. The toxin
apparently slows down the root mass accumulation by
downy brome and reduces its competitive ability thereby
giving wheat the competitive edge. An additional characteristic of the toxin is that it may affect germination and
seedling vigor of the weeds, which results in a reduction
in stand or population. The toxin will also reduce tiller
number and the total number of seeds, with the end result
being less plants with fewer tillers and fewer seeds. Thus,
in theory we have a good match for a biological control
pair.
Field studies were conducted in eastern Washington to
evaluate the effects of the inhibitory bacteria on the growth
of downy brome (Kennedy and others 1991). In field trials,
in which downy brome was planted in rows, bacterial isolates reduced downy brome populations up to 30 percent
and shoot dry weight up to 42 percent (fig. 1). In other studies, bacteria were applied to wheat fields infested with natural populations of downy brome. Downy brome and winter
wheat growth and development were measured throughout
the growing season. Reduction in downy brome growth
varied and was dependent on the specific bacterial strain.
One strain of inhibitory bacteria, P.f. D7, reduced plant
populations and aboveground growth of downy brome 31
and 53 percent, respectively (fig. 2). In the same experiment, seed production of downy brome was reduced 64 percent. Winter wheat yields were increased by 35 percent
with the application of the bacteria and subsequent suppression of downy brome growth (fig. 3). This increase in
yield is similar to the yield increase expected from the elimination of a moderate infestation of downy brome. Another
bacterium, Pseudomonas syringae strain 2V19, suppressed
downy brome growth by 25 percent in the same field study
with a 27 percent increase in winter wheat yield. In winter wheat field studies conducted in 1988, 1989, and 1990,
suppression of downy brome by P.f. D7 at Lind, La Crosse,
Ralston, and Pullman, WA, was 34 percent when the bacterium survived (Ogg and others, unpublished). These studies illustrate the potential for inhibitory bacteria to suppress
the growth of weeds, resulting in significant increases in
winter wheat yields. Even though the bacteria did not control the downy brome 100 percent, it suppressed downy
brome growth so that winter wheat was able to out-compete the weed. Root colonization and plant suppressive
ability by these bacteria may be sensitive to many soil factors, including soil water content (Fredrickson and Elliott
120
120
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Growth Parameters
Shoot weight
Seed production
Growth Parameters
Figure 1-Downy brome growth from seeding field
studies inoculated with rhizobacteria directly or spread
with straw inoculated with the bacteria. Asterisks indicate significant differences at p s 0.05 {Kennedy and
others 1991).
Figure 2-Downy brome growth and seed production
from fields inoculated with rhizobacteria and planted to
winter wheat at three locations in eastern Washington.
Asterisks indicate significant differences at p s 0.05
{Kennedy and others 1991 ).
5,000
1985; Howie and others 1987; Liddell and Parke 1989;
Schippers and others 1987) and soil temperature (Kenerley
and Jeger 1990; Loper and others 1985). Root exudate quality or quantity (Van Vuurde and Schippers 1980) or microfloral competition also play a part in bacterial survival
(Suslow and Schroth 1982). The production of plant-suppressive compounds and subsequent weed suppression will
change with colonization of the root by the bacteria (Bolton
and Elliott 1989). In field studies with P.f. D7, time of application was critical to suppression of downy brome. Greatest inhibition of downy brome occurred when the bacterium
was applied just prior to a significant rain event, indicating
the importance of bacterial survival on biological control efficacy (Kennedy, unpublished).
These rhizobacteria can be compatible with several herbicides for grass control. Compatibility would be advantageous because it would allow for the bacteria to be used to
supplement herbicides with poor activity on downy brome.
Thus, a combination of the biological control agent and reduced rates of herbicides may reduce significantly the infestations of downy brome and may act synergistically with
herbicides to increase activity, selectivity, or spectrum of
control.
Microbial weed control, while reducing weed pressures,
should significantly reduce costs, the need for tillage, and
chemical pesticide usage, thus reduce erosion and water
pollution. Microbial amendments, while reducing chemical
herbicide rates, could substantially decrease the potential
for groundwater contamination from herbicide runoff or
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Figure 3-Winter wheat yield from fields inoculated
with rhizobacteria and planted to winter wheat at three
locations in eastern Washington. Asterisks indicate
significant differences at p s 0.05 {Kennedy and others
1991).
188
seepage into the water table. This new weed control technology has potential for use in rangeland systems.
Kennedy, A. C.; Elliott, L. F.; Young, F. L.; Douglas, C. L.
1991. Rhizobacteria suppressive to the weed downy
brome. Soil Science Society of America Journal. 55:
722-727.
Liddell, C. M.; Parke, J. L. 1989. Enhanced colonization
of pea taproots by a fluorescent pseudomonad biocontrol
agent by water infiltration into soil. Phytopathology.
79: 1327-1332.
Loper, J. E.; Haack, A.; Schroth, M. N. 1985. Population
dynamics of soil pseudomonads in the rhizosphere of
potato (Solanum tuberosum L.). Applied and Environmental Microbiology. 49: 416-422.
Mack, R. N. 1981. Invasion of Bromus tectorum L. into
western North America: an ecological chronicle. Agroecosystems. 7: 145-165.
Morrow, L.A.; Stahlman, P. W. 1984. The history and distribution of downy brome (Bromus tectorum) in North
America. Weed Science. 32(Suppl. 1): 2-6.
Mortensen, K. 1986. Biological control of weeds with plant
pathogens. Canadian Journal of Plant Pathology. 8:
229-231.
Ridings, W. H. 1986. Biological control of stranglervine
(Morrenia odorata Lindl.) in citrus-a researcher's view.
Weed Science Supplement. 34: Supp. 1.
Schippers, B.; Bakker, A W.; Bakker, P. A 1987. Interaction of deleterious and beneficial rhizosphere microorganisms and the effect of cropping practices. Annual Review
Phytopathology. 25:339-358.
Stroo, H. F.; Elliott, L. F.; Papendick, R.I. 1988. Growth,
survival and toxin production of root-inhibitory pseudomonads on crop residues. Soil Biology Biochemistry. 20:
201-207.
Suslow, T.V.; Schroth, M. N. 1982. Role of deleterious
rhizobacteria as minor pathogens in reducing crop growth.
Phytopathology. 72: 111-115.
Templeton, G. E.; Smith, R. J., Jr.; Tebeest, D. 0. 1986.
Progress and potential of weed control with mycoherbicides. Reviews of Weed Science. 2: 1-14.
Van Vuurde, J. W. L.; Schippers, B. 1980. Bacterial colonization of seminal wheat roots. Soil Biology Biochemistry.
12: 559-565.
Watson A. K.; Clement, M. 1986. Evaluation of rust fungi
as biological control agents of weedy Centaurea in North
America. Weed Science. 34:7-10.
ACKNOWLEDGMENTS
Support from the 0. A Vogel Research Fund is gratefully acknowledged.
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189