Agriculture Reference
In-Depth Information
9.3.1
Rotation or intercrops - a role for root GSLs
Biofumigation by rotation crops or intercrops, where above-ground material is harvested
or left to mature above-ground, relies on root exudates of growing plants throughout the
season, leaf washings or root and stubble residues. In these circumstances it is important
to distinguish between active disease suppression through biofumigation and the well
established non-host or break-crop effect (Kirkegaard
et al
., 2000). Several studies have
detected both GSLs and ITCs in the rhizospheres of intact plants, and these have been
implicated in the suppression of pests and pathogens in both natural and managed ecosys-
tems (van Dam
et al
., 2008). An extensively studied instance in broad-acre agricultural
systems involves the possible role of biofumigation in the widely observed rotational
benefi ts of the
Brassica
break crops canola (
Brassica napus
) and Indian mustard (
Brassica
juncea
) to subsequent cereals in the rotation (Angus
et al
., 1994; Kirkegaard
et al
., 2000).
This was thought to be associated with the release of 2-phenylethyl ITC from the roots
of
Brassica
rotation crops, a compound shown to be highly toxic to cereal pathogens
in
vitro
(Sarwar
et al
., 1998; Smith & Kirkegaard, 2002). The roots of canola varieties with
high levels of the precursor 2-phenylethyl GSL reduced the inoculum levels of the take-
all fungus (
Gaeumannomyces graminins
var
. tritici
) in the soil (Kirkegaard
et al
., 2000),
and also reduced the hosting of
Pratylenchus neglectus
nematodes on canola, which could
otherwise multiply to infect subsequent wheat crops (Potter
et al
., 1999). Rumberger &
Marschner (2003, 2004) demonstrated ITC-induced changes in rhizosphere bacteria of
canola, although the low levels of ITC detected (1-2 nmol g
-1
soil) were two orders of
magnitude lower than those detected under commercial fumigation with methyl ITC, and
cast some doubt as to a direct suppression of disease related to ITC release. Indeed, Smith
et al
. (2004) failed to demonstrate yield improvements in wheat associated with reduced
disease in the fi eld using biofumigation and considered the limitation in ITC release from
intact, growing roots would reduce disease suppression. Recent studies demonstrating the
localization of 2-phenylethyl GSL in the outer tissues of secondary thickened canola roots
suggest an evolutionary role in protection of the larger root system from soil-dwelling her-
bivores and a mechanism to ensure a continuous localized source of biotoxic hydrosolates
into the rhizosphere (McCully
et al
., 2008). Such mechanisms may account for the pest
control properties of mustard intercrops grown in mixtures with crops such as potatoes
(Akhtar & Alam, 1991). Generally, there is considerably more scope to utilize biofumiga-
tion strategies where purposely selected green manures from a range of species can be
selected, grown and the whole plant incorporated to improve the timing and amount of
ITC release for pest control. As root GSLs can constitute 24% (range 2-81%) of total
plant GSL content during the vegetative stages (Kirkegaard & Sarwar, 1998), they should
not be overlooked as a source of biofumigation when whole plants are incorporated as
green manure.
9.3.2
Seed meals and other processed plant products
The seed meal or 'oilcake' byproducts which remain after pressing rapeseed or mustard
seed for oil constitute a convenient, high GSL material suitable for soil amendment for
high-value horticultural crops. These products contain suffi cient intact myrosinase to
