Agriculture Reference
In-Depth Information
cooling phase of composting when the internal windrow temperature was approxi-
mately 50 ˚C.
Escherichia coli
,
Listeria monocytogenes
and
Bacillus cereus
were
not detected (< 1 cfu per 100 g) in this compost. ACT was inoculated with
E. coli
M23 strep
r
at 1 × 10
7
cfu ml
−1
at the beginning of extraction. This
E. coli
strain,
although non-pathogenic, has growth characteristics similar to strains of
E. coli
pathogenic to humans (Brown et al.
1997
; Salter et al.
1998
). No significant change
in the number of
E. coli
M23 strep
r
was observed up to 72 h later. However, there
was a significant increase in
E. coli
M23 strep
r
numbers by 72 h when 0.8 % fish
hydrolysate or 1 % molasses were introduced to ACT 24 h after extraction com-
menced. Introduction of 0.5-2 % liquid kelp or a mixture of 1.7 % liquid kelp and
0.8 % fish hydrolysate lead to a decline in the number of
E. coli
M23 strep
r
. There
was no relationship between the number of
E. coli
M23 strep
r
and the abundance
of culturable bacteria and fungi in ACTs amended with nutrients, although a low
oxygen concentration, pH and high conductivity was associated with increased
E.
coli
M23 strep
r
numbers in an ACT amended with 1 % molasses. The results im-
ply that methods should be identified and imposed to assure that human enteric
pathogens do not contaminate amended ACTs, during or after preparation, and/or
are prevented from attaining levels that pose a risk to humans consuming fruit and
vegetables treated with them.
9.7
Methods for Evaluating Disease Suppression
Most studies for evaluating potential crop protectants, including compost tea, begin
with in vitro or in planta tests to assess direct activity against the plant pathogen of
interest. Commencing research with field trials is potentially costly and risky if the
developmental product fails to control disease during commercial crop production.
This phase of the work usually occurs after pathogen and/or disease suppression has
been demonstrated in controlled-environment experiments.
9.7.1
Controlled-Environment Experiments
The detached-leaf bio-assay of Weltzien and Ketterer (
1986
) illustrates the type
of rapid technique needed to reliably evaluate different teas prior to testing in the
glasshouse or field. Other authors commence experimentation using growth media
amended with different types or concentrations of compost tea to evaluate growth
inhibition of target plant pathogens that can be cultured in the absence of the host
plant (Koné et al.
2010
). Growth media and controlled incubation conditions typi-
cally favour colonisation by the plant pathogen. It follows that the same media will
favour growth and metabolic activity of a sub-set of culturable microorganisms
within the compost tea if their growth rate and nutritional needs do not vary sig-
nificantly from those of the plant pathogen. A large proportion of microbial taxa are
non-culturable (Amann et al.
1995
). Compost tea is likely to contain non-culturable
