United Kingdom
October 22, 2007
Source:
HCGA Crop Research News
Project Report No. 420
Understanding Sclerotinia infection in oilseed rape to
improve risk assessment and disease escape
by Caroline Young of ADAS Drayton, Peter Werner of CPB Twyford
Ltd and Jon West of Rothamsted Research. Date: October 2007
Price: £4.50.
Full report:http://www.hgca.com/publink.aspx?id=4090
ABSTRACT
Sclerotinia stem rot in
oilseed rape, caused by Sclerotinia sclerotiorum, occurs in
the UK at an average incidence of 2%, but there are
occasional major outbreaks with high yield loss, which are
very difficult to predict. The aims of this Defra LINK -
HGCA project were: [1] to improve the management of
Sclerotinia and reduce fungicide use, [2] to assess the
potential of polymerase chain reaction (PCR)-based inoculum
detection for risk assessment, and [3] to assess the
potential of apetally as a disease avoidance mechanism,
using near-isogenic lines of oilseed rape. The apetalous
(AP) and full petal (FP) lines used in this project were
developed by CPB-Twyford.
The numbers of petals and stamens sticking to oilseed rape
leaves are a guide to risk of stem rot developing. In this
project, petals, stamens and sepals stuck to leaves and in
axils were counted daily on randomly selected plants in each
plot. However, the numbers counted did not relate directly
to the final levels of stem rot. This was most likely
because of large differences in stem rot on lines that had
similar petal and stamen stick. The difference in stem rot
could be a resistance mechanism, or because sclerotinia is
avoided in some way during its life cycle.
The results of testing flower parts for sclerotinia
infection showed that AP lines could not avoid infection
completely through lack of petals. The percentage of petals,
stamens and sepals infected with Sclerotinia was determined
at early, mid and late flower by agar plating tests (ADAS)
and also by polymerase chain reaction (PCR) tests
(Rothamsted Research). According to agar plate tests, in
general, where there was a moderate to high level of FP
petals testing positive for S. sclerotiorum, there was also
stamen infection on both AP and FP lines, but at lower
levels than for petals on the FP lines. However, more
stamens tested positive on the AP lines than the FP lines,
e.g. in 2004, at early flower there were 37.5% of stamens
testing positive on the AP lines and 24.2 % on the FP lines.
The likelihood is that petals on the FP plants shield FP
stamens from airborne spores which are circulating in the
crop from the source on the ground (fungal fruiting bodies
called apothecia). The same infection pattern was true for
sepals (not tested in 2006), but in general lower levels of
sepals tested positive than for stamens.
PCR tests found much higher levels of infection in all
flower parts than agar tests but did not detect differences
between levels of petal and stamen infection. A quantitative
PCR (qPCR) test developed recently shows good potential for
assessing stem rot risk from airborne spore inoculum. The
qPCR test gives an indication of the peaks of spore numbers
in the air, which correspond to periods of highly infective
flower parts. The qPCR test could in future be based upon a
simple protocol requiring one or two small spore traps to be
operated during flowering and a small number of DNA assays.
Without petals, AP plants are still at risk from infection
via stamens. Flower parts infected with S. sclerotiorum need
to stick to leaves for at least a couple of days to cause
leaf infection, and in each year of the project, there were
similar numbers of stamens stuck on leaves of the AP or FP
lines as petals on the FP lines. However, petals were found
to be approximately six times as infective as stamens, with
about 10% of petals on leaves leading to a lesion, whereas
less than 2% of stamens initiated a leaf lesion.
Our results indicate that there is potential for
sclerotinia-resistant cultivars, and that apetally does lead
to some avoidance of stem rots. In 2006 the near-isogenic
lines AP5 and FP5 differed in stem rot incidence, with 2.2
and 12.6%, respectively. These lines have a background
derived from the cultivar Winner which also had relatively
high level of stem rot, 8.8%. However, the near-isogenic
lines AP1 and FP1 were derived from Nickel which had low
stem rot (2.6%), and both AP1 and FP1 also had low stem rot.
Apetally in winter OSR is a multiple recessive gene trait
and is difficult to breed. It is unlikely to be commercially
viable unless further work is done to develop stable
material in a good genetic background.
In summary, the key messages are:
1. Petals sticking to leaves indicate a risk of infection,
but the numbers of petals sticking may not relate directly
to the final stem rot incidence.
2. Petals and stamens can both cause infection, but petals
are approximately six times more infective than stamens.
3. A quantitative PCR test shows promise for detecting
airborne sclerotinia inoculum within a crop, and could be
used in an inoculum based disease forecasting scheme.
4. There is evidence that some winter oilseed rape cvs may
be more resistant than others to stem rot, and this needs
further investigation. Different cvs may have different
resistance mechanisms.
5. Apetalous lines derived from OSR varieties susceptible to
sclerotinia have lower stem rot incidence than full petal
lines. However, it is currently difficult to breed a stable
line.
Full report:http://www.hgca.com/publink.aspx?id=4090 |
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