Agriculture Reference
In-Depth Information
effects of the incorporated tissues could still reduce the population to manageable levels,
but it is generally desirable to select biofumigants that are poor hosts, or to grow them
at times of the year when pathogens do not build up. In strawberry production systems
rotations with
Brassica
vegetables such as broccoli have been shown to provide effective
control for pathogens such as Verticillium wilt (
Verticillium dahliae
), but not for
Pythium
spp. (Subbarao
et al
., 2007). The greater reduction of
V. dahliae
microsclerotia and higher
vigor and yield of strawberry following broccoli or brussel sprout rotation crops com-
pared with lettuce occurred at both infested and non-infested sites indicating benefi ts
other than suppression of those diseases were also involved.
9.4.2
Non-glucosinolate or ITC-related effects
Several studies have demonstrated signifi cant suppression by incorporated
Brassica
amendments that are not associated with the GSL concentration of the tissue. A study by
Potter
et al
. (1998) showed clearly that the signifi cant suppressive effects (60-95%) of
leaf tissues from six diverse
Brassica
species to the nematode
Pratylenchus negelectus
were unrelated to either the total or specifi c GSL content of the tissues. Similar results
have also been shown for root knot nematodes (
Meloidogyne javanica
) by McLeod &
Steele (1999). Mazzola
et al
. (2001) and Cohen
et al
. (2005) have also demonstrated that
the suppression of the microbial complex associated with apple replant disease which
includes the fungal pathogen
Rhizoctonia solani
using
Brassica
seed meal was unrelated
to the GSL levels. There are usually two explanations proposed for these observations,
but these may occur simultaneously. Firstly, the incorporation of organic matter itself
can increase the populations of antagonistic organisms in the soil, as was demonstrated
by the involvement of
Streptomyces
spp. and nitric-oxide producing bacteria in the sup-
pression of
Rhizoctonia solani
in seed meal amended soil (Cohen & Mazzola, 2006). The
stimulation of soil microbial activity or alteration of the communities to suppress specifi c
disease in potato crops has also been demonstrated for
Rhizoctonia
(Larkin & Honeycut,
2006) and
Verticillium
wilt (Davis
et al
., 1996), and in other pathosystems by Smolinska
(2000). These mechanisms are summarized in Figure 9.2 via the 'organic matter' and
'benefi cials' pathways. Secondly, many potentially biologically active, non-GSL com-
pounds such as methanethiol are released from
Brassica
amendments (Bending &
Lincoln, 1999), and other products of microbial decomposition of tissues including fatty
acids or ammonia can also be biologically active (Bailey & Lazarovts, 2003). These
are summarized in Figure 9.2 under 'non-GSL suppression'. Irrespective of the mecha-
nisms responsible, these disease control mechanisms that are unrelated to GSLs, and
can occur using non-
Brassica
amendments signifi cantly confound the interpretation of
biofumigation studies if appropriate controls are not included in the experiments.
Although ITCs are generally considered the most toxic of the GSL hydrolysis
products, a range of other potentially toxic compounds including nitriles, epinitriles,
and ionic thiocyanates can also be released (Brown & Morra, 1997; Morra, 2004; Rollin
& Palmieri, 2004; Palmieri, 2004) (Figure 9.1). Generally they are less toxic, although
Morra (2004) has shown that much of the weed suppression noted following incorpora-
tion of Brassicaceous seed meals is likely to arise from the ionic thiocyanate rather than
the ITCs. To resolve the question of ITC-related suppression it is generally desirable to
correlate the level of pest suppression with measured levels of GSLs in the tissues or ITC
