Agriculture Reference
In-Depth Information
where longer-term suppression related to non-GSL mechanisms associated with organic
inputs were operating (Matthiessen & Kirkegaard, 2006). The most advanced commer-
cial adoption of biofumigants in potato production is that of mustard green manures
(
B. juncea
and
S. alba
) in the wheat/potato rotations of the pacifi c Northwest in USA.
A system involving incorporated mustard green manures to replace conventional metham
sodium treatment has provided adequate control of diseases (
Verticillium, Sclerotinia,
Helminthosporium, Meloidogyne
and weeds), an improvement in soil structure and soil
protection, maintenance of yield and quality, and a cost saving of around USD$169 ha
-1
(Gies, 2004; McGuire, 2004). Central to the wide adoption of this on over 16 000 ha (in
2003) has been the identifi cation, commercialization and availability of effective biofumi-
gant varieties and the involvement of growers at all stages of the research to develop and
adapt the concept in a practical way to the farming system (Gies, 2004). These various
studies worldwide in potato production systems demonstrate several important issues in
relation to biofumigation and disease control: (a) the variability in the success of dis-
ease control, even for the same pathogens (b) the clear involvement of non-GSL-related
mechanisms in disease suppression at several sites (c) the need to combine biofumigation
with other strategies to achieve acceptable commercial disease control and (d) the wider
farming system benefi ts which may be associated with biofumigant green manures. Few
of the studies utilized all of the best-bet strategies summarized above to maximize ITC-
based suppression so some improvements in disease suppression may be possible. The
clear involvement of non-ITC mechanisms indicates this is not necessarily inevitable.
9.8.2.2
Control of apple replant disease using Brassica seed meal
A series of recent studies on the mechanisms underlying the suppression of the disease
complex responsible for apple replant disease using
Brassica napus
seed meals exempli-
fi es the complexity of the soil processes which can be involved. The studies set out to
identify suitable alternatives to chemical soil fumigants to control apple replant disease,
a complex involving species in the genera
Cylindrocarpon
,
Phytophthora, Pythium
and
Rhizoctonia,
as well as nematodes (Mazzola, 1998). The goal was to integrate methods that
selectively suppress these organisms with procedures that enhance the activity of resident
microbial antagonists. A specifi c aim was to combine applications of rapeseed meal to
suppress disease organisms with pre-crops of a wheat variety shown to induce microbial
antagonists of the diseases in soil (Mazzola
et al
., 2001). In studies using high and low
GSL rapeseed meal,
Rhizoctonia solani
and
Pratylenchus penetrans
were suppressed by
both high and low GSL meal while
Pythium
spp. increased in response to the low GSL
meal. The suppression of
Rhizoctonia
by low GSL meal suggested non-GSL mecha-
nisms were operating and dramatic changes were measured in various components of
the microbial community. These included a 100-fold increase in
Streptomyces
spp. and a
predominance of those which produced nitric oxide (NO) rather than those demonstrating
direct antagonism to
R. solani
(Cohen
et al
., 2005). NO can induce plant systemic resis-
tance and is predominately generated in amended soils by the activity of nitrifying bac-
teria that oxidize the ammonium released from the incorporated seed meal. A subsequent
study provided evidence that both direct antagonism by
Streptomyces
and indirect effects
of NO were involved in the suppression of
R. solani
by low GSL rapeseed meal (Cohen &
Mazzola, 2006) although the exact mechanism remained uncertain. Preventing
Pythium
