Agriculture Reference
In-Depth Information
should have sufficient residual degrees of freedom to separate treatment means sta-
tistically when the difference between means is also of practical significance.
As compost tea has potential to control more than one plant disease, all plant dis-
eases that appear in experimental plots should be assessed. Arthropod pests should
be controlled biologically or through use of sprays that are unlikely to interact with
the effect of the compost tea and/or confound the results of disease assessment.
Similarly, any foliar applications of nutrients and all other crop inputs should be
recorded. Importantly, the trial should be surrounded by buffer plots or rows to en-
sure adjacent spray operations do not result in spray drifting onto the experimental
treatments.
Applications of compost teas in the first field trials should commence at a time
during the growing season before there is a risk of primary infection and continue
at the minimum, practical spray interval. A small spray interval minimises the risk
of undue crop loss and maximises potential disease suppression. If a weekly spray
interval is selected, then sprays are likely to be applied every 6-10 days in practice,
given advances or delays in spraying due to inclement weather. It would seem rea-
sonable to check the viability of microorganisms before and after they have passed
through the spray nozzle.
Given potential batch-to-batch variation in compost teas, each batch should be
considered a different spray material for the purpose of data analyses. Care needs
to be taken in comparing efficacy of a compost tea treatment relative to a spray
program based on standard commercial practice in which materials and their time of
application may be different to those used in the compost tea program. A treatment
comprising multiple applications of compost teas represents one spray program;
therefore, results should be presented as the effect of one spray program relative
to another, such as a standard spray program or one where water is applied instead
of compost tea. If the objective is to investigate the timing of a spray application,
then the effect of omitting an application of a single batch of compost tea can be
investigated as long as there is another treatment in which all other spray applica-
tions are identical.
Once initial field trials indicate a useful level of disease suppression, trials should
be repeated over multiple growing seasons to understand variation in the extent of
disease suppression across the range of environmental conditions likely to be encoun-
tered in a particular location. Whole-of-block experimentation can potentially shorten
the period of field experimentation because disease suppression is quantified across
fields where disease severity varies across the landscape (Bramley et al.
2011
). This
approach can also address the question of whether or not two treatments, such as a
spray program based on compost tea or the current (standard, organic) spray program,
result in statistically similar or different responses depending on the location in the
field. To adopt this method, the co-operating grower must agree to accept some level
of disease over a wider area of crop than might otherwise be tolerated in small-plot
trials. The trade-off is a rich data set that can inform the grower of site-specific factors
resulting in so-called disease 'hot spots' (Bramley et al.
2011
). Cultural interventions
to reduce disease risk in 'hot spots' can produce more desirable results with compost
teas when overall disease risk in a field has been reduced.
