The use of composts and compost extracts in plant disease control - Disease Control in Crops - page 104

four composts prepared from different feedstocks were associated with improved growth
of tomato, whereas the fourth compost signifi cantly depressed tomato plant growth.
Aryantha et al . (2000) showed that fresh chicken manure or chicken manure composted
for 5 weeks, suppressed root rot caused by Phytophthora cinnamomi . They also found
that chicken manure composted for 2 weeks was less suppressive and that composts made
from cow, sheep or horse manure did not suppress root rot caused by P. cinnamomi . The
nature of microbial populations has been shown to depend on the nature of the feedstock.
For example, composts prepared from lignocellulosic substances such as tree barks tend
to become colonized primarily by Trichoderma spp. (Kuter et al. , 1983), which have been
shown to control Rhizoctonia solani (Grebus et al. , 1994). In contrast, grape pomace,
which contains high concentrations of sugars and relatively low levels of cellulosic sub-
stances tends to become colonized by Aspergillus and Penicillium spp., which have been
shown to suppress Sclerotium rolfsii (Hadar & Gorodecki, 1991).
Several consultants and researchers (mainly in the United States) are now suggesting
that it is possible to produce specifi c composts for particular crops, soils and host/
pathogen combinations (e.g. www.soilfoodweb.com). The work to support these recom-
mendations is not yet widely published in the scientifi c literature and further work is
required to fully develop these theories into practice for different cropping systems.
Most composts that have been used successfully to prevent and control diseases have
been produced aerobically. Even composts produced under aerobic conditions contain
small pockets of anaerobic material. However, it is recognized that composts produced
under predominantly anaerobic conditions contain a range of toxic end products includ-
ing low molecular weight organic acids. Some of these composts can remain toxic to
plants for months or years (Hoitink, 1980). The presence of these toxic products is less
critical for composts used in fi eld soil if incorporation occurs well ahead of planting.

5.7.4.2 Organic matter decomposition level (and compost maturity)

The quality of the organic matter with respect to available substrate will determine
whether benefi cial organisms and/or facultative saprotrophic pathogens can multiply
in it. The effects of partially decomposed organic materials such as composts on soil
microbial populations (and plant disease) are different from those of fresh plant materi-
als. The addition of fresh or barely decomposed organic residues to soil has often been
shown to cause temporary increases in populations of facultative saprotrophic pathogens
such as Rhizoctonia and Pythium spp., since they can reproduce easily in such material.
For example, damping-off caused by Pythium spp. was most severe 7 or 10 days after
incorporation of cover crop residues (van Bruggen & Termorshuizen, 2003).
Reports of disease suppression by fresh or stacked manures are relatively rare in the
literature. This may be partly due to the fact that some manures tend to provide unsuitable
conditions for growth and proliferation of antagonistic microorganisms due to high salt
content, high ammonium concentrations and in some cases lack of oxygen within manure
stacks (Aryantha et al ., 2000). They can also cause phytotoxicity, particularly to sensitive
horticultural crops (Roy & Newhook, 1970; Aryantha et al ., 2000).
The degree of maturity of composts is crucial in determining their disease suppressive-
ness. Fresh organic matter does not usually support biological disease control, even if it
is inoculated with microbial species/strains of proven effi cacy (De Ceuster et al. , 1999).