Spore traps could suss out sclerotinia

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.