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CROPPING:
DISEASE MANAGEMENT
CSIRO Plant Industry
Stem canker control helps
manage root rot
Mark
Filmer
Susan
Sprague
for CSIRO PLANT INDUSTRY
Windborne spores: The blackleg
fungus is most commonly spread
by windborne spores. Blackleg is the
most devastating disease of canola
in Australia and worldwide.
At a glance
Root rot caused by the
blackleg fungus is
widespread in commercial
canola crops.
Windborne spores infect the
leaves and the fungus then
grows down the stem and
into the roots.
Current strategies to
minimise blackleg stem
canker such as using
resistant varieties and
fungicides also minimise root
rot symptoms.
There appears to be little or
no reduction in crop yield
due to root rot in addition to
that caused by stem canker.
A CSIRO study has shed new light on root rot in canola crops — one of the
symptoms of the potentially devastating fungal disease blackleg.
Researchers now have a better understanding of how the fungus enters the
root, how root rot relates to other symptoms of blackleg such as stem canker,
methods of control and the effect of root rot on crop yield.
Farmers can help control root rot in canola
by using the same strategies they adopt to
minimise leaf lesions and stem canker,
according to the findings of a joint CSIRO
and University of Melbourne study.
The study, which focused on the problem
of root rot, will help farmers to better
understand, manage and control the highly
prevalent fungal disease blackleg.
A devastating disease
The fungus Leptosphaeria maculans,
commonly known as blackleg, is the most
devastating disease of canola crops.
Symptoms of blackleg typically include
leaf lesions and stem cankers. Leaf lesions
occur during late autumn or early winter
and stem cankers during spring.
Another symptom, first reported in
Australian canola crops during 2000, is root
rot. Symptoms appear at early flowering
and increase in incidence and severity as
the plant matures.
Windborne spores are the most common
means of spread of the blackleg fungus.
Spores released from infected stubble
germinate and invade leaves causing
lesions. The fungus spreads to the stem
base where it forms stem cankers that can
circle the stem base causing yield loss and,
in severe cases, plant death.
Up until now, little information has been
available about how the fungus enters the
root, how root rot relates to other symptoms
such as stem canker, methods of control
and the effect of root rot on yield.
Root rot symptoms appear at
early flowering and increase
in incidence and severity as
the plant matures.
Background to the study
Scientists set out to determine the
prevalence of root rot in commercial canola
Farming Ahead May 2007 No. 184 www.kondinin.com.au
53
CROPPING:
crops and factors that influence the
development of root rot. They also wanted
to discover the pathway of root infection,
investigate the effect of root rot on yield and
identify control methods.
There was a positive relationship between
the mean severity of root rot and stem
infection in the paddocks surveyed — the
severity of root rot increased with an increase
in the severity of blackleg stem infection.
There was a positive
relationship between the
mean severity of root rot
and stem infection in the
paddocks surveyed.
Progression of disease symptoms
Commercial crops sampled
During 2003 and 2004, more than 100
commercial canola crops were sampled
in southern New South Wales before
windrowing to determine the incidence
and severity of root rot. Stem canker
symptoms were also assessed to investigate
the relationship between stem and
root symptoms.
Plants were randomly sampled from five
locations in each paddock. Whole plants
were removed from the ground and
assessed for the incidence and severity of
internal infection at the base of the stem
and in the roots.
Stems were assessed by cutting across
the base of the plant and observing the
internal stem surface (0–100 per cent) that
had evidence of blackening. The severed
root portion was then cut length-wise and
scored on a scale of 0 (no infection) to 5
(severely infected) based on the area of the
surface that was blackened.
The Australian blackleg rating (ABR) of
the varieties assessed ranged from 5.5–9.0.
Commercial crop survey results
The survey revealed that all paddocks had
plants with root rot symptoms during 2004,
while 86% of paddocks did during 2003.
Although the root rot symptom was
present in 42% of plants during 2003 and
49% during 2004, the average severity of
infection (using the 0–5 scale) was low
during both 2003 (0.85) and 2004 (1.1).
Despite this, some paddocks had scores
of more than 2.5 and more than 80% of
plants infected.
Root rot: This canola root has been infected by
blackleg root rot. The tell-tale symptom is the
dark discolouration of the roots. Severe cases
of root rot can lead to plant death.
The expression of root rot symptoms was
monitored in a paddock experiment at
Galong, NSW, (annual rainfall of 650
millimetres) during 2004.
Untreated seed of canola variety Grace
(ABR 6.5) was sown into wheat stubble
during June.
Plants were assessed for leaf lesion
incidence at the cotyledon (the embryonic
first leaf) to one leaf stage and at the 2–4
leaf stage by counting the number of plants
with one or more leaf lesions out of 20
consecutive plants in each plot.
Plots were sampled for root rot and
stem canker incidence and severity six
times between stem elongation and
plant maturity.
Observation of symptoms
Plants were exposed to a high level of
disease as the incidence of plants with one
or more leaf lesions was 60% at the
cotyledon to one leaf stage and increased to
88% at the 2–4 leaf stage.
Root rot symptoms started to appear at
early flowering. Incidence and severity
increased slowly through the flowering and
early pod filling stages then dramatically
during late pod fill.
The development of stem canker during
plant growth followed a similar pattern with
disease symptoms starting to appear at
early flowering and increasing in incidence
and severity as the plant matured.
Plants with severe stem
canker had significantly
reduced seed yield but root
rot did not appear to cause
any additional reduction in
crop yield.
Pathway of root infection in the paddock
Paddock experiments in which soil was
fumigated to remove organisms were
carried out to determine the pathway of root
infection under conditions of natural
inoculation. The experiments were carried
out at Wallendbeen (650mm annual
rainfall) and Ardlethan (450mm annual
rainfall) in NSW.
After fumigation, the plots remained
undisturbed for at least two weeks
before sowing untreated seed (Rainbow
variety, ABR 6.5).
Fifty seedlings from each plot at
Wallendbeen were assessed for blackleg leaf
lesions at the cotyledon to one-leaf stage.
At both sites, plants were assessed at
plant maturity for the presence of internal
infection in the roots.
54 Farming Ahead May 2007 No. 184 www.kondinin.com.au
CSIRO Plant Industry
DISEASE MANAGEMENT
Close-up view: This image, taken through a
microscope, shows the blackleg fungus (green
lines) spreading through the cell walls (the
purple–pink areas) of a canola plant.
Results of paddock experiments
The incidence of seedling leaf
lesions was similar between plants in
fumigated (69%) and untreated (73%) plots
at Wallendbeen.
At plant maturity, root rot was present in
both fumigated and untreated plots at
Wallendbeen and Ardlethan (see Table 1).
Growth of blackleg within the plant
A detailed study of the path of infection
through the stem and into the roots was
carried out in the glasshouse to understand
how blackleg grows within the plant.
A strain of the fungus with a gene from a
jellyfish which produces a protein that
fluoresces bright green when viewed with a
microscope was used to infect seedlings.
Blackleg was followed during plant growth by
observing cut sections under a microscope.
Blackleg grew from the stem into the
root within xylem vessels (the cells in the
plant that transport water from the roots to
the shoots).
Although the fungus was present in the
stems of seedlings, it did not enter the roots
until flowering and continued to spread in
the roots as plants matured.
These observations were consistent with
the paddock experiments where root rot
symptoms were initially observed at the
start of flowering and were most severe at
plant maturity.
TABLE 1 Severity of root rot*
Location
Fumigated
Untreated
Wallendbeen
3.3
2.4
Ardlethan
2.0
1.5
* On a scale of 0–5. The soil in these paddock
trials was fumigated with methyl bromide.
Source: CSIRO Plant Industry.
CROPPING:
DISEASE MANAGEMENT
1
0
*
60
*
UTC Jockey Impact
Treatment
40
20
0
5 2004
4
Blackleg root rot
Stem canker
3
2
*
1
0
UTC
80
40
*
Jockey Impact
Treatment
5
100
60
*
FIGURE 2 Varietal resistance
20
0
Root rot severity
2
80
Root rot severity
3
100
Stem canker severity
5 2003
4
Stem canker severity
Root rot severity
FIGURE 1 Effect of fungicides on the severity of blackleg root rot and stem canker*
4
3
2
1
0
2
4
6
8
Australian Blackleg Rating
10
* Effect of fungicides on the severity of blackleg root rot and stem canker before windrowing at Galong,
New South Wales, during 2003 and 2004. Treatments marked with an asterisk (*) are significantly different
from the untreated control.
* Effect of varietal resistance (Australian Blackleg
Rating) on root rot severity. Root rot was scored
using a 0 to 5 scale where 0 = healthy root and 5
= severely diseased.
Source: CSIRO Plant Industry. Illustration: Kondinin Group.
Source: CSIRO Plant Industry. Illustration: Kondinin Group.
Fungicides and resistant varieties for control
Paddock experiments were carried out at
Galong, NSW, during 2003 and 2004 to
determine the effectiveness of fungicide
treatments currently recommended for
stem canker control for reducing root rot.
The fungicide treatments were flutriafol
(Impact-in-Furrow) applied to the starting
fertiliser at 400 millilitres per hectare and
fluquinconazole (Jockey) applied as a seed
dressing at two litres per tonne of seed.
Seed of Rainbow variety was used during
2003 and Grace was used during 2004.
Fungicides inhibit invasion of
the leaf and prevent the
fungus from establishing
within the plant.
Paddock experiments were carried out at
sites in NSW, Victoria and South Australia
to determine whether canola varieties with
resistance to blackleg stem canker were also
resistant to root rot.
Varieties with an ABR of 2.5–9.0 were
sown at each site and assessed for the
severity of root rot at maturity. Just before
windrowing, plants from each fungicide
treatment and variety were assessed for root
rot and stem canker severity.
Fungicides and resistant varieties effective
Flutriafol (Impact) and fluquinconazole
(Jockey) reduced the severity of blackleg
root rot (see Figure 1). During 2003,
flutriafol reduced the severity of root rot by
26%, while fluquinconazole had no effect.
During 2004, fluquinconazole and
flutriafol reduced root rot severity by 22%
and 49% respectively but only flutriafol
reduced the severity of stem canker.
Both fungicides inhibit invasion of the
leaf and prevent the root rot fungus from
establishing within the plant.
Leaf infection
Since the root rot results from infection
through the leaf, it is expected that a
reduction in leaf penetration would also
reduce the severity of stem canker and root
rot by reducing the amount of fungus
entering stem and root tissue.
Root rot and stem canker severity were
greater in varieties with low levels of
resistance to stem canker (ABR 2–3) but
declined with increasing levels of host
resistance (see Figure 2).
Highly prevalent
The presence of root rot symptoms in
most commercial canola crops suggests
root rot is not a new symptom in the
infection of canola by blackleg but has
probably not been reported previously.
Root rot was present in several paddocks
that had never grown canola previously —
indicating infection of the roots occurred
via infection of leaves by airborne spores.
This was supported by the presence of
root rot symptoms in plants grown in field
plots where blackleg inoculum had been
removed from the soil by fumigation,
thereby eliminating the possibility of
infection from soil-borne inoculum and by
observations of plants inoculated with the
green fluorescing blackleg isolate.
Impact of diseases
Plants with severe stem canker had
significantly reduced seed yield but root rot did
not appear to cause any additional reduction in
crop yield.
Sowing canola varieties with moderate to
high levels of resistance to stem canker will
also help reduce the severity of root rot.
Control measures similar to those
recommended for stem canker (the use of
fungicides Jockey and Impact) are effective
in controlling root rot.
Root rot has been found in
paddocks that had never
grown canola previously —
indicating airborne spores on
leaves caused infection of
the roots.
Acknowledgments: Susan Sprague received
a Grains Research and Development
Corporation (GRDC) scholarship to carry out
this research as part of her doctorate; GRDC
and Graingrowers Australia for funding the
project; FarmLink and the Best Bet Canola
Group for access to paddock trials; Chris Duff,
John Sykes, Greg Condon and Geoff Pitson;
Peter Hamblin, Agritech Crop Research; and
John Graham, CSIRO Plant Industry.
CONTACT Susan Sprague
(02) 6246 5387
(02) 6246 5399
[email protected]
Paddock experiment: The CSIRO study found
root rot was reduced in canola varieties with
resistance to blackleg stem canker.
56 Farming Ahead May 2007 No. 184 www.kondinin.com.au