In this issue..

One spray or two?
“
The debate
is always whether to
put on a second
treatment. ”
Technical
Sclerotinia
Sclerotinia has the potential
to decimate yields. CPM
seeks advice from experts
in a county where the
threat is often highest.
By Lucy de la Pasture
It’s a sight that makes the heart sink. The
oilseed rape crop has been looking full of
promise all season, you’ve been feeling
quietly confident that things have gone
well and then you spot them –– patches
of white stems, ‘rat-droppings’ and
lodging –– the tell-tale signs of sclerotinia
infection.
Not many OSR diseases have the visual
impact of sclerotinia and it’s high on the list
of grower priorities when it comes to
fungicide applications, says Dr Julie Smith,
senior research scientist based at ADAS
Rosemaund in Herefordshire.
Although sclerotinia is a slightly sporadic
disease and doesn’t occur in every season,
it made an unwelcome return in 2016,
catching some growers unawares. The
question is why was this, and how can we
guard better against a repeat performance?
According to Julie Smith, weather data
revealed more infection events than ‘normal’
took place last spring. For infection to occur,
three factors have to happen simultaneously
–– ascospore presence isn’t enough on its
own, she explains.
Relative humidity
“For plants to become infected, a minimum
temperature of 7 0 C, with relative humidity at
80% or above for at least a day is required.
Petals need to be carrying ascospores, then
the infected petals have to stick to the
leaves of OSR plants to enable the fungus to
infect the plant and for sclerotinia to become
a problem in the crop.”
And although the disease is widely
associated with wet springs and moisture
is required for infection to occur, it needs to
be the right kind of moisture.
“Heavy rain suppresses ascospore
release and under these conditions, petals
are often washed off the leaves and don’t
have the opportunity to stick to them.
In contrast, light rain or even just heavy
dews offer perfect conditions for petals to
stick and ascospore germination,” says
Julie Smith.
As petal fall is the main culprit when it
comes to sclerotinia infection, apetalous
or reduced petal varieties have been
suggested as a way of circumventing
disease spread. Defra-funded work
carried out by ADAS has investigated this
According to Julie Smith, sclerotinia control is
more dependent on getting the timing right than
the fungicide applied.
by tracking flower opening and petal fall in
apetalous specimens and found the sepals
and stamens can also be a source of
sclerotinia inoculum, rather blowing the
theory.
Further work revealed the leaf layers
that were most likely to catch petals
were situated between the bottom and
mid-canopy, leaf layers 7-9. In the study,
sclerotinia infection subsequently came in
on the stem at the same level confirming the
importance of petal-stick in the infection
process, as well as the need for good
fungicide coverage and penetration of
the canopy.
18 crop production magazine arable extra march 2017