Agriculture Reference
In-Depth Information
inorganic abiotic molecules. It responds to the question: what causes the suppressiv-
ity and in what proportions? There are a number of examples in which both biotic
and abiotic elements of compost, have relevance in disease control. On the one part,
the biological component, represented mainly by fungi, bacteria, actinomycetes,
protozoa, nematodes and their products, play the crucial role in determining biotic
suppressivity. On the other hand, compounds showing significant antifungal and
disease suppressive bioactivity are indicated as determinants of abiotic suppressiv-
ity. In this category enters also physical compost features conditioning the patho-
genesis. Obviously, the biological suppressivity is completely loss following com-
post sterilization by heating, fumigation or other non-thermic antibiotic treatments
(Malandraki et al.
2008
). Whereas, the suppressivity can be restored after cooling
when the antagonistic micro-flora naturally recolonize the compost or by re-adding
to the sterilized null-effective one, a minimal amount of the raw compost or its wa-
ter extracts. The total or partial persistence of suppressive properties in biologically
vacuum composts, highlights unequivocally the role of the abiotic component.
8.2.2
General and Specific Suppressivity
Based on the amount and diversity of the involved actors, the compost suppressivity
can be divided into two major categories, general and specific. In particular, general
suppressivity is that generated by the sum of the activities exerted by the overall po-
tential agents of suppressivity. In this case, the responsibility of the effect is distrib-
uted on all elements that can be biotic and abiotic, including the groups of microbial
compost community and/or the whole spectrum of the chemical and physical fea-
tures. The specific suppression, instead, is considered to be generated through the
actions of one or few particular determinants. Often, this effect is established by few
specific microbial groups or single microbes exhibiting species-specific antagonis-
tic activity against phytopathogens. But, also single abiotic character of compost
can act according to a specific model. Apparently, this contraposition between the
general and specific models may seem to be connected to the effectiveness of the
system in diseases controlling. But it is not true. Actually, as we shall see later, the
levels of suppressivity in compost depend from the challenge between the mode of
action and the specific pathosystem physiology. In other terms, limited to the objec-
tives of this section, it is sufficient to say that some pathogens and/or diseases are
more manageable with general suppressivity model, while others are better control-
lable with specific ones.
8.2.3
Direct and Indirect Suppressivity
The suppression of diseases can be mediated by compost in both direct or indirect
way. It is direct if exist a clear causal-effect linkage between the pathosystem and
the compost suppressive determinants. Thus, disease suppression is closely related
