Agriculture Reference
In-Depth Information
Morra, 1997; Smith & Kirkegaard, 2002), which provides opportunities to select
Brassica
species that produce large quantities of ITCs most toxic to the target organ-
ism. For example, screening of various pure ITCs
in vitro
demonstrated the sensitivity of
cereal fungal pathogens to the aromatic-ITC 2-phenylethyl ITC contained in canola roots
(Sarwar
et al
., 1998), while sulfur-substituted aliphatic ITCs present in radish types were
most toxic to
Fusarium
(Manici
et al
., 1997) and to
Pythium
and
Rhizoctonia
(Manici
et al
., 2000).
In vitro
screens using pure ITCs or hydrated, freeze-dried tissues must be
interpreted carefully because the apparent toxicity of individual ITCs can vary depend-
ing on whether a vapor phase or contact toxicity is established in the experimental pro-
tocol due to the variation in volatility of the different ITCs (Matthiessen & Kirkegaard,
2006). For example, the short-chain aliphatic ITCs such as 2-propenyl (found in mustard
leaves) were more toxic to cereal pathogens than the aromatic 2-phenylethyl ITC (found
in canola roots) in headspace experiments, but the reverse was true when the fungi were
grown on agar containing the ITCs. Notwithstanding the obvious simplifi cations in labo-
ratory assays compared with the complex interaction in soils (see later), matching the
most potent brassicas with target organisms shown to be sensitive to particular ITCs pro-
vides the best chance of achieving successful suppression in the fi eld.
9.5.3
Purposeful selection and development of biofumigants
The variability in the reported suppression of different pathogens in previous fi eld experi-
ments is perhaps not surprising given the failure to monitor the GSL levels of the varieties
used, and the lack of purposeful selection of cultivars high in the most appropriate GSL
precursors (Matthiessen & Kirkegaard, 2006). Recently, more systematic wide screening
of several GSL-containing plant species has led to the identifi cation of several biofumi-
gants that produce high quantities of the most potent GSLs and these have progressed to
the stage of commercialization in Europe (Lazzeri
et al
., 2004), USA (Gies, 2004) and
Australia (Kirkegaard & Matthiessen, 2004). In most cases these products are marketed
for their general soil health and soil structural benefi ts together with disease control attri-
butes (Gies, 2004; Patalano, 2004). A current example of signifi cant adoption (16 000 ha
in 2003) is in the wheat-potato rotations on light-textured soils of the Pacifi c Northwest
in USA, where mustard green manures (
Sinapis alba
and
B. juncea
) could replace con-
ventional applications of metham sodium with no penalty in disease, yield or quality of
potato tubers, demonstrable improvements in soil structure and erosion control and a
substantial economic saving (McGuire, 2004). Disease suppression is often not explicitly
separated from these other benefi ts at the stage of commercial adoption because farmers
are focussed on the overall systems benefi ts. However, the wide range in GSL profi les
and the differential toxicity of the ITCs evolved to different disease organisms provides
scope to select or breed crops with enhanced biofumigation potential for particular target
organisms.
9.6
Release effi ciency, fate and activity of hydrolysis
products in soil
The purposefully selected biofumigants discussed in the previous section contain enough
GSL to potentially produce levels of ITC equal to those applied using commercial
