Agriculture Reference
In-Depth Information
Parasitism and predation by benefi cial microorganisms in composts has been shown by
various workers to be an important mechanism in disease suppression. Four stages of the
parasitism process can be distinguished, which can be described as: chemotrophic growth,
recognition, attachment and degradation of the host cell walls through production of lytic
enzymes, chitinases and -1,333-glucanases (Elad
et al.
, 1983). For example, antagonists
such as
Trichoderma harzianum, T. hamatum
and
T. viride
have been shown to parasitize
plant pathogens including
Rhizoctonia solani, Sclerotium rolfsii, Phytophthora
spp. and
Pythium
spp. by secreting enzymes which destroy cell walls.
Induced systemic resistance to a broad spectrum of plant pathogens, triggered
by benefi cial microorganisms has been proven to be effective in many plant species
(De Clercq
et al.
, 2004). Induced systemic resistance shows some phenotypic similarities
with plant pathogen-induced systemic acquired resistance, but differences between the
two resistance mechanisms becomes apparent when they are studied in depth (Walters
et al.,
2007; see also Chapter 4). Several workers have recently demonstrated that com-
posts can induce systemic responses in plants, leading to reduced disease severity. For
example, Hoitink
et al.
(1997) found that compost mulches applied under parts of trees
led to improved disease control throughout the root system. Trankner (1992) reported
that compost incorporated into soil reduced the severity of powdery mildew on wheat
and several workers have shown that composts can induce systemic resistance to pythium
root-rot in cucumber when applied to a section of the root system using a split root system
(Zhang
et al
., 1996; Lievens
et al.
, 2001).
Mechanisms of disease suppression can be divided loosely into two general categories
described as 'general' and 'specifi c' (Baker & Cook, 1974). The term 'general' applies
where disease suppression can be attributed to the activity of many different types of
microorganisms. The propagules of pathogens which are affected by general suppression
do not tend to decline rapidly in soil. They are small (<200 m in diameter), do not store
large quantities of nutrients and rely on exogenous C sources such as seed and root exu-
dates for germination and infection (Nelson, 1990). These pathogens are sensitive to the
activities of other microorganisms in the soil, that is they are sensitive to microbiostasis.
It is during the curing or maturation phases of the composting process that composts
become suppressive to soil-borne plant diseases caused by
Pythium
and
Phytophthora
spp.
The exact point during the maturation process at which composts begin to develop sup-
pressive properties depends on various factors (as discussed earlier). Disease suppres-
siveness in this instance is related to the total microbial biomass present in the compost.
The suppressiveness is mediated by diverse mesophilic organisms which recolonize the
compost from the outer, low-temperature layer. General suppression tends to result from
competition for nutrients and eocological niches by numerous bacterial and fungal spe-
cies that adversely affect the activity of, or induce microbiostasis of, plant pathogens.
For example, potting mixes containing composted hardwood bark and light, sphagnum
peat are naturally suppressive to pythium root-rot and damping-off as a result of general
disease suppression (Hoitink & Boehm, 1999).
Specifi c disease suppression occurs where the presence of just one or two microorganisms
can explain suppression of a particular pathogen or the disease that it causes. It has been
suggested that the specifi c microorganism or microorganisms responsible for this effect
can be transferred from one soil to another in order to confer suppressiveness, whereas
