Allelochemicals Biologica... - Name

ALLEOPATHIC ORGANISMS AND
MOLECULES
incorporation (DAI) of red clover and compost, with significant differences (α= 0.05)
at two out of four sampling dates in 1997, and four out of four sampling dates in
1998. Incidence of Pythium infection was 10-70% greater in the amended soil treatment
during that period. Asymptomatic wild mustard seedlings grown in amended soil
were also on average 2.5 cm shorter (α= 0.05) at 5 DAI than those grown in unamended
soil in one year out of two. Concentration of phenolic compounds in soil solution was
correlated with decreased shoot and root growth (r = 0.50, 0.28, respectively) and
increased incidence of disease (r = 0.48) in wild mustard seedlings in one year out of
two. Dissolved organic carbon concentration was correlated with increased disease in
wild mustard seedlings in both years (r = 0.51, 0.33, respectively). Growth of corn
seedlings did not differ between the two soil treatments, suggesting that red clover
green manure and compost may selectively reduce density and competitive ability of
wild mustard in the field. Bioassay results corresponded well with emergence and
shoot weight results from a related field study, indicating that this technique may be
useful for screening potential soil treatments prior to field studies.
Anaya et al. (1987) applied leaves of Alnus firmifolia, Berula erecta and Juncus
sp., as green manures in corn fields with corn, bean, and squash grown using traditional
techniques. The growth of weeds during the crop period was decreased by the presence
of the green manures. At the same time, stimulation of bean root nodulation by
Rhizobium was obtained with these particular green manure species, nodulation was
also increased in plots with abundant weed growth. These results suggest that the
presence of different secondary metabolites liberated by these green manures and by
some living weeds in the field plots increase the ability of Rhizobium sp. to infect
bean roots.
Weed control by rye, crimson clover, subterranean clover, and hairy vetch cover
crops was evaluated in no-tillage corn during 1992 and 1993 at two North Carolina
locations (Yenish, et al., 1996). Weed biomass reduction was similar with rye, crimson
clover, and subterranean clover treatments, ranging between 19 and 95% less biomass
than a conventional tillage treatment without cover. Weed biomass reduction using
hairy vetch or no cover in a notillage system was similar averaging between 0 and
49%, but less than other covers approximately 45 and 90 d after planting. Weed
biomass was eliminated or nearly eliminated in all cover systems with pre- plus postherbicide
treatments. Weed species present varied greatly between years and locations,
but were predominantly common lambsquarters, smooth pigweed, redroot pigweed,
and broadleaf signalgrass. Corn grain yield was greatest using pre-herbicides or preplus
post-herbicides, averaging between 16 to 100% greater than the nontreated control
across all cover treatments depending on the year and location.
Studies were conducted by Burgos and Talbert (1996) at the Main Agricultural
Experiment Station in Fayetteville and the Vegetable Substation in Kibler, Arkansas,
in 1992 and 1993 on the same plots to evaluate weed suppression by winter cover
crops alone or in combination with reduced herbicide rates in no-till sweet corn and
to evaluate cover crop effects on growth and yield of sweet corn. Plots seeded to rye
plus hairy vetch, rye, or wheat had at least 50% fewer early season weeds than hairy
vetch alone or no cover crop. None of the cover crops reduced population of yellow
51