Agriculture Reference
In-Depth Information
Brassica green manure
Non-host
/trap
Non-GSL related
suppression
Erosion
control
GSL-related
suppression
Organic matter
ITC-
related
Non-ITC
related
Beneficials
Soil
structure
Nutrients
Disease suppression
Biofumigation
Crop yield and quality
Figure 9.2
Mechanisms by which
Brassica
green manures can infl uence disease levels, and yield and
quality of subsequent crops. The pathway by which ITC-related biofumigation infl uences disease sup-
pression and crop yield is shown by the large arrow. Disease suppression in amended soil can result
from other GSL hydrolysis products, non-GSL-related products, changes in microbial populations asso-
ciated with the organic matter addition. Crop yield can be infl uenced by other system benefi ts of the
green manures (from Kirkegaard & Matthiessen, 2004, with permission from Istituto Sperimentale per
le Colture Industriali).
9.4.1
Trap crops and non-hosting
Perhaps the most well-documented case involving the use of Brassicaceous green manures
as trap or catch crops is for the control of sugar-beet nematodes (
Heterodera schactii
) in
northern Europe (Müller, 1999; Schlathoelter, 2004) Fodder radish (
Raphanus sativus
)
and white mustard (
Sinapis alba
) are grown as green manures preceding sugar-beet
crops. These crops are invaded by the nematodes, which develop within the roots, but
have their sexual differentiation disrupted. This results in very low numbers of females
in the subsequent generation, causing a signifi cant decline in the population and reducing
the infection of subsequent sugar-beet crops. It is a unique example related to specifi c
nematode-resistant cover crops rather than a general non-hosting effect, and a role for
GSLs or their hydrolysis products has not been demonstrated. McCleod
et al.
(2001)
found little evidence that GSLs were involved in the reduced host status (invasion, egg
laying or development) of a range of Brassicaceous crop plants to root knot nema-
todes (
Meloidogyne javanica
). Potter
et al
. (1999) demonstrated reduced hosting of
Pratylenchus neglectus
on canola plants with higher levels of 2-phenylethyl GSL in the
roots, but clearly demonstrated that other non-GSL mechanisms were also involved. For
a green manure to be effective in disease control, it is generally desirable that it does not
host the pathogen in question so that a decline in population or inoculum occurs during its
growth. Indeed, the fact that
Brassica
species are generally moderate hosts to some other
important plant parasitic nematodes (e.g.
Meloidogyne
spp.), has reduced their applicabil-
ity as biofumigant green manures as very careful management is required to avoid popu-
lation increases on the biofumigant crop, particularly in warmer climates (McLeod &
