Agriculture Reference
In-Depth Information
9.7
Ecological considerations
The effectiveness of biofumigation for disease suppression will not only be infl uenced
by the differences in the inherent sensitivity of the pathogen to various ITCs, but the
particular growth stage of the pathogen which is targeted, the capacity for survival and
re-infection in the soil as well as other interactions with non-target organisms.
9.7.1
Pathogen sensitivity and ecology
Inherent sensitivity to ITCs assessed using
in vitro
studies can vary by orders of magnitude
for different pathogens (Brown & Morra, 1997), among isolates of the same pathogens
(Smith & Kirkegaard, 2002), or for different life-cycle stages such as spores, conidia
or mycelia (Mari
et al
., 1993). Thick-walled oospores or chlamydospores of various
oomyces, or sclerotia of
Rhizoctonia
and
Sclerotinia
may be less sensitive to ITCs than
germinating propagules of
Aphanomyces euteiches
or
Fusarium oxysporum
(Smolinska
& Horbowicz, 1999). Recent studies also demonstrated that hyphae of
Rhizoctonia
solani
arising from sclerotia were more resistant to ITCs than those arising from agar
plugs (Yulianti
et al
., 2006). Disease control arising from direct ITC-related suppres-
sion may be expected to be more effective for obligate parasites such as that reported for
the take-all fungus (
Gaeumannomyces tritici
) and powdery scab in potato (
Spongospora
subterranea
) where inoculum recovery requires a host, and more variable for saprophytes
such as
Rhizoctonia solani
. Indeed, pathogens such as some
Pythium
spp. which are less
sensitive to ITCs and can utilize the incorporated
Brassica
amendment as a food source,
can increase after biofumigation (Stephens
et al
., 1999). The non-specifi c nature of biofu-
migation means that non-target organisms may also be affected either directly by the ITCs
and other hydrolysis products or via the pathways related to the general organic matter
addition described in Figure 9.2. These non-target organisms may include non-pathogenic
but functionally important mediators of nutrient transformations, but also pathogen antag-
onists or suppressors which can infl uence disease control. Reported changes in non-target
soil microbial communities include shorter term ITC-related changes (e.g. Rumberger &
Marschner, 2003) but also longer term changes which persist for many weeks and are
unrelated to GSL-hydrolysis products (Mazzola
et al
., 2001; Larkin & Honeycutt, 2006).
In some cases these changes involve organisms known to have disease-suppressive
capability such as
Streptomyces
spp. and fl ourescent
Pseudomonas
(Cohen
et al
., 2005),
or
Trichoderma
spp. (Kirkegaard
et al
., 2004), but also include many as-yet unidenti-
fi ed mechanisms. The complexity of these interactions has been exemplifi ed recently
in studies investigating control of apple replant complex by
Brassica
seed meals (see
Section 9.8.2.2). In a review of biofumigation for potato disease control, Larkin & Griffi n
(2007) concluded that biofumigation through release of GSL-hydrolysis compounds such
as ITCs are not the only mechanism of action for disease suppression by
Brassica
crops,
and in some cases may not be the most important.
9.7.2
Enhanced biodegradation of ITCs
Enhanced microbial biodegradation is a phenomenon in which repeated use of certain
pesticides selects for microorganisms able to degrade the compounds and utilize them
