Agriculture Reference
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Most benefi cial microorganisms are also killed during the high-temperature phase of
composting. However, some remain in the outer low temperature parts of the compost
pile/windrow (Hoitink et al ., 1997). The disease-suppressive properties of composts are
usually induced during the curing phase, because most biocontrol agents re-colonize
the compost after peak heating. A wide range of species has been identifi ed as biocon-
trol agents in compost-amended substrates. These include Bacillus spp., Enterobacter
spp., Flavobacterium balustinum 299, Pseudomonas spp., other bacterial genera and
Streptomyces spp. as well as fungal species including Penicillium spp., Gliocladium
virens , several Trichoderma spp. and others (Chung & Hoitink, 1990; Hoitink et al .,
1997). Compost moisture content (ideally 40-50% moisture) is critical if the compost is
to be successfully colonized by disease-suppressive microorganisms after peak heating.
Compost produced in the open, in an environment which is high in microbial species
diversity has been shown to be colonized by a greater variety of microbial species than
the same produced in an in-vessel system (Kuter et al ., 1983). This may be partly because
the survival of a wide range of benefi cial microorganism species is less likely in in-vessel
systems, because the entire contents of the vessel will reach consistently high temperatures
at the same time. Composts made in enclosed systems may require to be cured for longer to
improve suppressiveness, incorporated into soils for several months prior to planting or they
may require inoculation with specifi c biological control agents (Hoitink et al ., 1997).
Microfl ora and fauna associated with suppressive composts
The types of microorganisms isolated from disease-suppressive container media are
similar to those studied by scientists working on biological control in fi eld soils. It has
been demonstrated that fungal populations in composted hardwood bark media
suppressive and conducive to rhizoctonia damping-off differ signifi cantly (Kuter et al .,
1983). Although no single species dominated in all the media tested, Trichoderma and
Gliocladium virens were abundant in all suppressive media. Trichoderma spp. were also
identifi ed as important fungal antagonists in composts prepared from larch bark for con-
trol of Fusarium brown rot in Chinese yam (Sekiguchi, 1977).
There is little information available on the activity of bacterial antagonists in composts
or compost-amended soils. Bacterial antagonists recovered at random by baiting with
propagules of Rhizoctonia solani or plant roots from suppressive batches of compos-
ted hardwood bark include isolates of Pseudomonas aeruginosa, P. putida, P. stutzeri,
Xanthomonas maltophilia, Janthinobacterium lividum, Flavobacterium balustinum,
Enterobacter cloacae, E. agglomerans, Bacillus cereus, B. mycoides and B. subtilis . It is
not known which of these bacterial antagonists predominate in suppressive composts or
what their relative contributions are. However it can be concluded that the bacterial and
fungal colonists isolated are generally rapid, primary colonizers of organic matter.
Microarthropods (springtails and mites) may play a role in the suppression of
soil-borne plant pathogens in compost-amended substrates, although specifi c reports on
their role in composts are few. It is known that they are most active in soils that contain
high levels of organic matter (Brady & Weil, 1999). Long-term organically managed soils
tend to have higher organic matter levels and have also been shown to contain higher
populations of soil fauna (including collembola, predatory nematodes and mites) than their
conventionally managed counterparts (van Bruggen & Termorshuizen, 2003).
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