After sclerotinia hit many oilseed rape crops harder this season
than expected after the dry April, a Rothamsted researcher believes
spore trapping may offer growers a better handle on whether they
need to treat.
"We're trying to get a better understanding of sclerotinia to
help reduce the unnecessary use of fungicide," explains Jon
West.
Overall this year about 5% of plants were infected at
Rothamsted, but some crops in the west of England showed much
higher levels, he notes.
The Rothamsted work* has been examining the possibility that
petal-less varieties might "escape" infection.
It has also been exploring the value of trying to assess the
amount of sclerotinia spores in the air around the critical
flowering period when infection occurs.
Spore release from the fungus' over-wintering fruiting bodies or
sclerotia is governed mainly by fluctuations in soil moisture and
temperature over winter and in the spring.
"If it's dry the sclerotia effectively go into a state of
suspended animation," explains Dr West.
"But there is a lot of variation in the way different isolates
of the disease respond."
One conclusive finding, using digital photography to track
sources of infection at CPB Twyford, is that varieties having flowers
without petals are still vulnerable to the disease.
But this is rare and they may escape the bulk of infection
spread by infected petals compared to conventional types.
It has also become clear that it is not just rainfall that
triggers infection, which usually occurs when petals and stamens
trap airborne spores and stick them to stems.
"Petal (and stamen) stick is crucial, but a lot of things have
to come together for it to occur. If it's very windy the petals dry
quickly and can get dislodged, and if it's very wet they can get
washed off. It has to be the 'right type of rain' for heavy
infection."
If the flowering period is short the chances of the right
conditions occurring are reduced. If it is extended, as in this
season, the risk is increased.
"But from our work it now also seems that it's not necessarily
rainfall alone that's required."
Dr West believes the mild winter and plenty of soil moisture
from February's deluges encouraged earlier than normal spore
release. And despite April being dry, it seems heavy dews could
have been enough to allow the plentiful spores to "infect" the
petals and stamens and become stuck to the stems.
"In central and eastern England it seems infections took place
mainly following rain in May, but this was also unusual after the
dry April."
Early work with Burkard traps, based on waxed tapes to collect
the spores, appears promising, especially when backed by the PCR
technique, he says.
"It can quantify the amount of sclerotinia spores in the
air."
But because the PCR technique also detects dead, unviable
spores, more work is required to develop a cheap practical system
which could be used to advise growers.
"The HGCA was reluctant to fund further work, thinking there
wasn't much interest from farmers. After this year's experiences
perhaps it may change its mind," says Dr West.