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GETTING MORE FOR LESS: EXPLORING THE MAIN SOURCES OF
VARIATION IN MEASURING CROWN ROT
S.E.H. Fletcher, C.D. Percy and A.M. Kelly
Department of Agriculture, Fisheries and Forestry, Toowoomba, Queensland. Susan.Fletcher@daff.qld.gov.au
ABSTRACT. Adult plant phenotyping of crown rot symptoms has in recent years been under scrutiny. There is concern
regarding the large amount of variation that may be expressed in a single genotype. We show the results from three separate
methodologies that use different sampling strategies. Results indicate the main source of variation is between plants or
between tillers, depending on the sampling method. To increase accuracy and robustness of crown rot phenotyping,
investigation into stratified sampling is currently being undertaken.
INTRODUCTION
Crown rot (caused by Fusarium pseudograminearum) is a
major disease of winter cereal crops, particularly in northeastern
parts of Australia. The extent of crown rot infection
is driven by the environment, where favourable conditions
early in the season promote infection, with stress induced by
a dry finish.
Symptom expression is typically measured as basal
stem browning on fully mature plants grown in fields. This
measure has been shown to vary significantly between tillers
of a single plant, increasing error variance when tillers are
sampled randomly (2). This study explores the source of
variation in crown rot phenotyping and offers some simple
improvements to crown rot screening methodology. The
dominant source of variation occurs between tillers within a
plant and also between plants, rather than between the tested
genotypes.
MATERIALS AND METHODS
Three field trials in northern Australia were grown to assess
crown rot resistance of 28 varieties (Table 1).
Methods for assessing crown rot in adult plants assign a
rating for the extent of browning along the internode
according to an underlying scale of 0-4 (Wellcamp,
Narrabri), 0-3 (Tamworth). Disease recordings were taken
on inoculated plots for all trials and this recording is
analysed for this subset of plots in each trial.
All methods used a different design and sampling
strategy across plants and across tillers within plants. One
major differentiation between methods is that tillers are
bulked across 20 plants per plot in the Wellcamp trial. For
the Tamworth trial an average rating across tillers were
given to each plant.
Table 2. Percentage of variation accounted for by the
sampling hierarchy of three methods of crown rot
assessment in field trials.
Percentage of variance
Source Tamworth Wellcamp Narrabri
Genotype 22 13 3
Replicate 0 2 4
Plot 2 5 21
Plant 77 1
Tiller 80 68
DISCUSSION
Reducing the non-genetic variation will improve the
genotypic discrimination, for example, scores may be taken
on only the primary tiller. To measure the variation, scores
could be made on all tillers, but the type of tiller should be
recorded and accounted for in the analysis. It appears
crucial to consider a modification to sampling of tillers to
improve the precision of phenotyping. To increase accuracy
and robustness of crown rot phenotyping, investigation into
stratified sampling is currently being undertaken at DAFF.
ACKNOWLEDGEMENTS
The field trial at Narrabri was conducted by the University
of Sydney, the trial at Tamworth was conducted by NSW
Industry and Investment, and the Wellcamp trial was
conducted by Qld Department of Agriculture, Fisheries and
Forestry. We would also like to acknowledge our
colleagues Damian Herde, Steven Simpfendorfer and
Richard Trethowan for trial conduct.
Statistical analysis. A linear mixed model was fitted to the
disease severity data from each trial with terms for 1.
genotype, replicate and plot, and additional terms for
sampling of plants and tillers within plants as appropriate.
Variance components were estimated using residual
REFERENCES
Malligan, C.D. (2009) Crown Rot Symptom Development
and Pathogen Spread in Wheat Genotypes with Varying
Disease Resistance. (PhD thesis, University of Southern
Queensland, Toowoomba).
maximum likelihood (REML) (2).
2. Patterson, H.D., and Thompson, R. (1971) Recovery of
interblock information when block sizes are unequal.
Table 1. Trial parameters and sampling strategy for the Biometrika 31:100–109.
crown rot trials
Number of Tamworth Wellcamp Narrabri
Replicates 3 3 2
Genotypes 28 26 26
Plants assessed 25 5
Tillers 40