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Facing up to the Stem Canker Challenge
March 2007 - What can we
do to prevent stem canker losses of 1 t/ha or more becoming common in
the face of increasingly warm and wet autumns and a dramatic rise in national
oilseed rape growing intensity ? We find out from ADAS plant pathologist,
Peter Gladders and leading UK breeder, Matthew Clarke of Monsanto.
The second warm, wet autumn in a row coupled with a 10% year-on-year
rise in national oilseed rape plantings is highlighting the increasing
challenge English growers face from stem canker in no uncertain terms.
High August and September rainfall resulted in the first 10-20% spray
thresholds for phoma infections being reached in mid-September this season
– the earliest ever recorded. What is more, stem cankers were already
evident as early as December.
“While stem canker develops from phoma leaf spot , it is not inevitable
that damaging cankers will be produced,” explains Peter Gladders.
There are two types of phoma. Although very common, the small dark phoma
spots with few black fruiting bodies caused by Leptosphaeria biglobosa
are not associated with the penetrating stem base cankers responsible
for almost all the UK disease damage.
“Stem based cankers only develop following infections of Leptosphaeria
maculans which produce large lighter-coloured leaf spots with many black
fruiting bodies. And even then the extent of the leaf spotting doesn’t
necessarily determine the severity of canker damage.
“This depends as much on the timing of infections, the location
of infections, the size of leaves and petioles, the vigour of plants,
the thickness of their stems, and the extent of both their disease resistance
and ability to tolerate cankers.”
To understand how these factors inter-relate it is essential to appreciate
that all phoma infections arise from spores released from previous oilseed
rape residues. There is no cycle of continuous re-infection within the
crop during the season. However, with sufficient rainfall and warmth primary
infections can continue from August through to spring.
Following leaf infection the fungus grows down the leaf vein and petiole
and into the stem at a rate of up to 5 mm/day. Once it reaches the stem
base it causes a canker which progressively gets more severe. Only once
it extends more than half way round the stem is yield compromised.
This pattern of disease progress means earlier infections tend to be
most damaging because when leaves are small there is less distance for
the fungus to travel to reach the stem base and growth is rapid when temperatures
are high. However, if the plant has big enough leaves or grows fast enough
even serious leaf spotting may not be significant because the leaves in
question may have been shed before the fungal growth reaches the base
of the petiole.
Equally, vigorous stem extension delays the progress of infection to
the stem base, thicker stems take a greater amount of canker growth to
damage, and earlier maturity means the damage may not have become severe
enough by the time the crop is harvested.
“So when we’re looking at minimising stem canker damage through
variety selection we have a whole host of mechanisms to consider,”
Matthew Clarke points out. “Varieties like Excel and Hearty actually
resist the original L. maculans infection, so any leaf spotting is likely
to be of the non-threatening biglobosa type.
“Then there are varieties like Canberra which appear to restrict
the movement of the fungus along the petiole. And hybrids like Royal,
Toccata and Excalibur which may limit infection spread by virtue of bigger
leaves and more vigorous growth. These show relatively more leaf infection
but perform very well despite it.
“Finally, in types like Excalibur and Castille, relatively early
flowering and maturity can further limit the actual impact of cankers.”
Valuable though he believes they are, Matthew Clarke stresses this complexity
means stem canker resistance ratings don’t tell the whole story;
even if varieties are compared to one decimal place to produce a more
accurate ranking.
He has no doubt that the group of varieties in the top third of the HGCA
ratings (Table) give growers greater leeway in their agronomy and the
total resistance seen in Excel will be of great interest in situations
where disease pressures are very high. Varieties in the bottom third of
the ratings table will clearly require more robust disease management
in many situations. However, the numerous background factors at work mean
he sees little difference in practice between the varieties in the centre
of the ratings table.
Peter Gladders agrees varietal resistance is a valuable weapon in combating
stem canker, but only when combined with good cultural and chemical control.
“The first essential is effective stubble burial to minimise spore
transfer,” he advises. “I’d avoid autocasting to minimise
the direct stem infection of straggly seedlings. Wherever possible too
I’d plan plantings so they’re a good 200 m from the previous
season’s OSR crops.
“As far as fungicides are concerned, growers need to spray as soon
as the 10-20% phoma infection threshold is reached, making sure the spotting
is from L. maculans not biglobosa. And they must be prepared to spray
again as soon as new infections occur.
“With smaller plants and more susceptible varieties this may mean
three sprays before Christmas to effectively restrict fungal growth into
the petiole,” Peter Gladders notes. “Bear in mind, though,
it isn’t worth spraying once stem extension is underway because
no leaves will then be feeding phoma into the stem base.
“Finally it’s worth appreciating that phoma sprays play a
valuable role, alongside varietal resistance in ensuring light leaf spot
causes few problems across the bulk of England.”
Table: 2007/8 HGCA Recommended Variety Stem Canker Resistance
| Variety |
Type |
Score |
| Excel |
Hybrid |
9.0 |
| Es Astrid |
Conventional |
7.5 |
| Castille |
Conventional |
6.2 |
| Expert |
Conventional |
6.0 |
| Canberra |
Conventional |
6.0 |
| Recital |
Conventional |
5.5 |
| Excalibur |
Hybrid |
5.2 |
| Lioness |
Conventional |
5.2 |
| NK Bravour |
Conventional |
5.1 |
| Fortis |
Conventional |
5.1 |
| Tuscan |
Hybrid |
5.0 |
| Mendel |
Hybrid |
4.9 |
| Disco |
Hybrid |
4.9 |
| NK Grace |
Conventional |
4.8 |
| NK Victory |
Conventional |
4.7 |
| Hornet |
Hybrid |
4.5 |
| Toccata |
Hybrid |
4.5 |
| Barrel |
Conventional |
4.5 |
| Royal |
Hybrid |
4.5 |
| Winner |
Conventional |
4.4 |
| Elan |
Hybrid |
4.3 |
| Es Betty |
Hybrid |
4.1 |
A partial abstraction of the 2007/8 HGCA Winter Oilseed Rape Recommended
List
available at www.hgca.com
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