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| Rural Industries |
Objective
To develop fundamental standards
that would assist in the production and use of potting media that
are consistently suppressive to Pythium and Rhizoctonia
diseases.
Background
It became clear that the
major organic compost components, composted pine bark fines (CPBF)
and composted eucalypt hardwood forest sawdust (CHWSD) had the
potential to provide good sustained suppression to both diseases.
However in a study of five commercial composters, suppression
varied considerably from batch to batch of component and the component
batches also varied in suppression ability when aged in potting
media. No one composter produced a consistently suppressive material
to both diseases over a five batch trial, although the potential
existed to do so.
Australian standards exist
for potting mixes and are a valuable quality control to ensure
good plant growth. The standards are of necessity independent
of the types of components used. Disease suppression standards
could in theory be added to these standards, but it would be also
necessary for compost producers to work to component based standards
with quality assurance guidelines to meet these standards.
It is also necessary for
growers to appreciate that their handling, storage and fertiliser
treatments can change potting media to disease conducive media.
It is also necessary to understand that mix suppression varies
with time and that different component combinations give disease
suppression over different periods of time.
Outcomes
Some key parameter were found
to be important in maintaining disease suppression in potting
mixes. Electrical conductivity (EC) over 1.0 mS/cm negated Rhizoctonia
suppression, whilst pH below 5.5 generally improved Rhizoctonia
suppression. Excessive nitrate levels may negate Pythium
suppression.
Pythium
suppression requires an actively degrading compost and a measure
of microbial activity assists in determining this. Microbial
activity alone is not sufficient measure to determine the longevity
of Pythium suppression in a mix. Components of mixes vary
in their initial degradation level and degradation rate. No simple
measure was found to predict this phenomenon, but ageing data
on each component type and ageing data on a diverse range of mixes
allowed for some predictive information.
Rhizoctonia
suppression involves different mechanisms of action and no correlation
was found between the two diseases.
Implications
This study builds onto other
overseas findings to assist in developing guidelines for composters.
Some unknown variable factors still exist in obtaining consistent
suppression. Microbial growth successions in other studies suggest
that greater attention to compost production guidelines and the
possible addition of specific microorganisms or crude "effective
compost" at maturation would improve consistency.
RIRDC Project No: DAN-50A
RESEARCHER: Dr Peter Fahy
ORGANISATION: NSW Dept of Agriculture
BCRI
RMB 10
RYDALMERE NSW 2116
PHONE: 02 683 9777
FAX: 02 630 4475
Last updated: 10 October 1996
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