Agriculture Reference
In-Depth Information
to action of these last. Almost all published papers on this topic are based on direct
suppressivity observations. Nevertheless, compost may also indirectly counteract
the development of disease, for example, by improving the vegetative status of
plants or by stimulating the native soil inhabiting antagonists before cultivation.
8.2.4
From Potential to Multiple Suppressivity
The set of characteristics of compost that are relevant for plant disease control in-
cluding, for example, In-vitro antagonism, fungitoxicity, organic matter pathogen
availability, microbial activity indexes, etc., define the potential of suppressivity.
Therefore, compost may be evaluated as potentially suppressive. But, only the chal-
lenge with a suitable set of pathosystems can ascertain it's real in host-plant effec-
tiveness. In the majority of literature reports, the study of this functional property is
focused only on one phytopathogen/host plant system. Less frequently, the composts
are tested against two or more diseases, revealing their significant variability. Com-
posts able to control at least two different diseases are defined multisuppressive.
8.3
The Mechanisms Governing Compost Suppressivity
Compost suppressivity is caused by the totality of physical, chemical, biochemical
and microbiological dynamic interactions between all its parts and plant-pathogen
system. This complexity expresses univocally the suppressive ecosystem function,
so that externally appears as a super-organism (Fig.
8.1
). Behind all of this, there are
a number of precise mechanisms of action, whose full understanding has been and
to date still remains the main challenge for researcher worldwide.
8.3.1
Antagonistic Models
The major role in compost suppressivity is played by microbes directly responsible
for antagonistic interactions with pathogens. The category includes a variety of po-
tential antagonistic microbial groups including various genera of bacteria, such as
Bacillus
and
Pseudomonas
, other than fungi, such as
Trichoderma
. These microbes
exert their antagonism by microbiostasis, antibiosis and hyperparasitism (Fig.
8.2
),
and/or for the stimulation of systemic resistance in host plants (Zhang et al.
1996
).
8.3.1.1
Microbiostasis
The close competition for nutrients induced by compost-inhabiting microbial pop-
ulations, results in the suppression of pathogen spore germination, in decreased
