Agriculture Reference
In-Depth Information
of weed pressure in their experiments, rapeseed green manures achieved commercially
acceptable weed control in only 1 of the 2 years and under higher weed pressure may be
less effective. Thus reduction in herbicide use, rather than replacement was the target.
The yield increase was related to factors other than weed suppression as herbicide-treated
weed-free plots following rapeseed also had increased yield. Impacts of biofumigants on
various potato fungal and bacterial diseases have been variable in studies worldwide with
evidence for both GSL and non-GSL related mechanisms demonstrated (Table 9.1). In
general, biofumigation is sought as a means of rapidly reducing disease inoculum to avoid
either lengthy rotations out of potatoes or the use of synthetic fumigants. Davis
et al
.
(1996) found suppression of
Verticillium dahliae
by rapeseed was intermediate between
fallow/pea treatments and the more effective sudangrass/corn treatments and suggested
microbial antagonism as the key mechanisms but did not specifi cally investigate the role
of GSLs. Harding & Wicks (2001) found green manures of Indian mustard (
B. juncea
),
canola (
B. napus
) and radish (
R. sativus
) all reduced populations of
V. dahliae
to a greater
degree than a range of cereals but not more than a clover/ryegrass mixture. They suggested
that biofumigants may only be useful where existing soil population levels are close to the
disease threshold. Larkin & Griffi n (2007) reported a number of laboratory, glasshouse
and fi eld experiments investigating the potential for
Brassica
green manures to control
a range of soil-borne disease of potatoes in Northeastern USA. In an on-farm trial at a
site with a signifi cant powdery scab problem (
Spongospora subterranea)
, Indian mus-
tard, rapeseed, canola and ryegrass reduced the disease in the following crop by 15-40%,
and canola and rapeseed reduced black scurf (
Rhizoctonia solani
) by 70-80% compared
with a standard oat rotation. Disease control was not always associated with high GSL
crops (e.g. compare canola and rapeseed) and was observed for ryegrass indicating other
non-GSL mechanisms were involved, especially for
Rhizoctonia
. Snapp
et al
. (2007)
demonstrated that incorporation of mustard (
B. juncea
) plants was effective in reduc-
ing soil-borne fungi, principally
Rhizoctonia
and promoting healthy roots and tubers.
Compared to bare fallow, a rye cover crop increased the disease rating of tubers by 37%
while an Indian mustard cover crop reduced it by 25% (Table 9.1). The increase in healthy
white roots was similar for both cover crops (increase from 63% in fallow to ~90%)
despite a much lower amount of incorporated biomass for mustard (2.5 t ha
-1
) than rye
(4.5 t ha
-1
). Suppression of common scab (
Streptomyces scabiei)
was most effective with
a mustard green manure at one site in the USA study using various green manures (Larkin
& Griffi n, 2007), but was much greater in experiments in South Africa using dried and
ground post-harvest residues of
Brassica
vegetables (Gouws & Wehner, 2004), although
the role of GSLs was not specifi cally determined (Table 9.1). The suppression of bacterial
wilt (
Ralstonia solanacearum
) in potato crops at 5 sites in the Philippines was also shown
to be signifi cantly reduced (40-50%) by a range of
Brassica
amendments incorporated at
5 kg fresh material m
-2
(JA Kirkegaard, unpublished data) (Table 9.1). In systems such
as these in developing countries, utilization of existing residues from vegetable crops
(broccoli and radish) rather than purposeful mustard green manures were of interest to
avoid loss of income associated with a green manure. Suppression levels of around 32%
using sunfl ower (
Helianthus annus
) as a control in some experiments indicated non-GSL
suppression was also operating to suppress this disease at some sites. Related work in
northern Australia on much sandier soils demonstrated a much greater role for short-term
ITC-related suppression on sandy soils with low organic matter compared to loamy soils
