Managing weeds with insects and pathogens - Ecological Management of Agricultural Weeds - page 397

Deleterious rhizobacteria
Deleterious rhizobacteria (DRB) are a second group of microorgan-
isms that have received attention as potential weed biocontrol agents. DRB
colonize seeds, root mucigel, epidermal and cortical tissues, and intercellular
spaces, and can reduce seed viability, seedling emergence, and plant growth
through the release of toxic compounds (Kremer, 1993; Kennedy, 1997). Data
from a number of researchers indicate that the effects of different DRB taxa on
plants can be host- and rhizobacterial-isolate-specific (Boyetchko, 1996,
1997). Certain DRB can harm crop species, such as potato, sugar beet, wheat,
citrus, and bean, but others have benign or neutral effects on crops and detri-
mental effects on weeds, such as Abutilon theophrasti , Aegilops cylindrica ,
Amaranthus spp., Bromus japonicum , B.tectorum , Chenopodium spp., Datura stramo-
nium , Ipomoea spp., Polygonum spp., and Xanthium canadense (Kremer, 1993;
Boyetchko,1996; Kennedy & Kremer,1996).Genera of DRB that may be useful
for weed suppression include Achromobacter , Alcaligenes , Arthrobacter , Citrobacter ,
Enterobacter , Erwinia , Flavobacterium , Klebsiella , and Pseudomonas (Boyetchko,
1996; Kremer & Kennedy, 1996).
Most of the research work focused on DRB has been conducted in laborato-
ries and glasshouses. In one of the few experiments examining the potential
of DRB as selective agents for weed suppression under field conditions,
Kennedy et al .(1991) tested whether an isolate of Pseudomonas fluorescens (strain
D7) could inhibit the grass weed Bromus tectorum without harming winter
wheat. Bromus tectorum is poorly controlled by conventional herbicides avail-
able for winter wheat production. It is often the dominant weed species in
cereal fields of the western USA and Canada, where it causes losses estimated
at $300 million per year (Skipper, Ogg & Kennedy, 1996; Kennedy, 1997). In
laboratory experiments, strain D7 has been shown to produce toxins that
inhibit B. tectorum but not wheat (Tranel, Gealy & Kennedy, 1993;
Gurusiddaiah et al ., 1994; Gealy et al ., 1996). In the field, application of strain
D7 to soil of plots sown with wheat and infested with B. tectorum reduced the
weed's density up to 35%, reduced its late-season biomass up to 54%, and
reduced its seed production up to 64%. At two of three field sites, application
of strain D7 increased winter wheat yields 18% to 35%,an effect attributed to a
reduction in B. tectorum 's ability to compete against the wheat crop.
In subsequent field experiments with strain D7, B. tectorum was suppressed
in only one of 20 trials, and the bacterium consistently failed to increase
winter wheat yield compared to uninoculated controls (Skipper, Ogg &
Kennedy, 1996). At the present time, it appears that DRB have potential
but lack consistency as agents for biological control of weeds under field