Allelochemicals Biologica... - Name

ALLELOPATHIC BACTERIA IN WEED MANAGEMENT 151
biocontrol agents, however, field performance to date has been inconsistent (Sturz
and Christie, 2003).
Cover crops and mulches as components of sustainable management systems
may be used for integrating bioherbicides by delivering the agents on seeds and
promoting their establishment in soils for attack of weeds and seedlings prior to
planting. Recent research demonstrated that several cover crop species inoculated
with selected AB at planting established and maintained the selected bacterial
populations on their roots and in adjacent soil. When giant foxtail (Setaria faberi)
emerged later in the season, the selected bacteria colonized seedling roots after the
cover crop was terminated (Kremer, 2000). The selected AB and allelopathic cover
crop residues acted synergistically to suppress the growth of the weeds.
5.4. Suppressive Soils and Conservation Biological Control
Soils under sustainable management may develop antagonistic microbial populations
in rhizospheres of selected weeds that suppress their growth. This occurrence is similar
to natural disease-suppressive soils in which indigenous soil microorganisms effectively
protect crop plants from soilborne plant pathogens (Weller et al., 2002). Diseasesuppressive
soils may be defined as soils in which a pathogen does not establish or
persist, establishes but causes little or no damage, or establishes and causes disease
for a short time but thereafter the disease is less important even though the pathogen
may persist in the soil (Baker and Cook, 1974). Suppression is due primarily to
antagonistic microorganisms, however, soil physical and chemical factors also may
be involved (Weller et al., 2002). Similarly, weed-suppressive soils may be defined as
soils in which certain weeds do not establish or persist, or establish and grow with the
crop but cause little interference due to suppressed growth and vigor caused by native
AB. Native and desirable plants may also in stimulate high populations of AB in their
rhizospheres that reduce growth of invasive weed species, suggesting that plant-soil
interactions are also involved in development of weed-suppressive soils (Kulmatiski
et al., 2004).
Evidence of apparent weed-suppressive soils has been reported for a variety of
sustainable cropping systems. A study of crop management practices on claypan soils
(Epiaqualfs) that involved reduced tillage, maintenance of high soil organic matter,
and limited inputs of agrichemicals found increased levels of AB associated with
weed seedlings that likely contributed to natural weed suppression (Li and Kremer,
2000). It was reported that agronomic practices that resulted in relatively high organic
matter, such as uncultivated prairie, organic farming and integrated cropping systems,
supported higher proportions of weed AB. Compost-amended soils planted to winter
wheat showed 29 and 78% reductions in broadleaf and grassy weed densities,
respectively, compared to soils amended with inorganic fertilizers only (Carpenter-
Boggs et al., 2000). Organic amendments (composts and cover crops) increased soil
microbial biomass and decreased the seedbank density and emergence of shepherd’s