Biofumigation for plant disease control – from the fundamentals to the farming system - Disease Control in Crops - page 176

ensure effective hydrolysis of the GSLs upon wetting. These have been investigated as a
biopesticide (Brown & Morra, 1997) and biofertilizer (Balesh et al ., 2005) for some time,
and interest has grown recently as a high-value end use for the seed meal arising from high-
GSL biodiesel oilseeds is sought. Brassicaceous seed meals have demonstrated signifi cant
suppressive activity to a range of insect (Borek et al ., 1997), nematode (Mazzola et al .,
2001; Rahman & Somers, 2005), fungi (Smolinska et al ., 1997) and weeds (Brown &
Morra, 1995), although at the high rates of amendment sometimes used (~8 t ha -1 ), non-
GSL related impacts on plant diseases have also been demonstrated (Mazzola et al ., 2001;
Cohen et al ., 2005) (see Section 9.5). In addition to products derived from seed meals,
other commercial products comprising the dry pellets of dehydrated high-GSL plants have
also been developed for use as biofumigants (Lazzeri et al. , 2004). In both cases, these
products provide an opportunity to maximize the amount plant tissues with high GSL con-
centration applied to the soil, and a great deal of fl exibility in the timing and incorporation
management of the biofumigant material. However, the high cost of transport is likely to
limit the use to high-value horticultural industries such as fruit, vegetables and cut fl owers
or in potting media for glasshouse production. In general, the disease control principles
associated with biofumigation using these products align with those of incorporated green
manures, but without the sacrifi ce of time and space required to grow the material on-site.

9.3.3 Green manuring and biofumigation

Incorporated biofumigant green manures or plow-downs can potentially combine the
benefi cial elements of rotation crops (Section 9.3.1) with a more concentrated release
of biocidal GSL-hydrolysis products at the time of incorporation. Green manures also
provide benefi ts to subsequent crops and farming systems through maintenance of soil
cover, soil sanitization, reduced erosion, increased soil organic matter and soil structural
improvements (Bailey & Lazarovts, 2003; Thorup-Kristensen et al ., 2003). Brassicaceous
green manures are no exception, and improvements in soil structure (Chan & Heenan,
1996), erosion control (McGuire, 2004) and nutrient cycling (Thorup-Kristensen et al .,
2003) have been reported for various Brassicaceous green manures in different parts of
the world. Kirkegaard & Matthiessen (2004) recognized the diffi culty in isolating the
ITC-related disease control effects of green manures from these other effects in many
biofumigation fi eld studies and summarized this diagrammatically (Figure 9.2). This
diagram provides a framework to reconsider the widely varying impacts of biofumigant
green manures previously reported, and highlights the need for caution in ascribing these
to ITC-related biofumigation alone.

9.4 Separating GSL-related suppression from other

effects of biofumigants
In a practical sense, the mechanism of disease control from Brassicaceous soil amend-
ments may be of little interest to farmers provided it is effective, predictable and repeatable.
However the strategies required to improve disease control will vary greatly depending on
the principal mechanism/s operating within particular systems (Figure 9.2). In that regard
it is important to briefl y consider some of the notable non-ITC related mechanisms of
disease suppression shown in Figure 9.2 before a more detailed consideration of the ways
in which ITC-related biofumigation can be maximized for disease control.