After the 2007 and 2008 sclerotinia epidemics, last season’s relative non-event could make this year’s control decisions tricky. But with the disease’s potential to slash yields by a third – and more in thin stands – growers cannot afford to ignore it, say specialists.

Sclerotinia remains a serious threat if the weather favours it during flowering, says Syngenta’s Simon Parker.

“Sclerotia, the infection source, can survive in soil for at least seven years.” So there may well be a reservoir from the seasons of high infection lurking, especially after other susceptible crops such as potatoes, peas and carrots.

Although few crops suffered significant damage last year, ADAS’s Peter Gladders expects more to be hit this season, especially where cultivations have returned sclerotia to near the soil surface.

Yield losses in infected plants average 60%, he notes. “So you can use 10% crop infection as an economic treatment threshold.”

Hutchinsons’ technical manager Dick Neale acknowledges the sclerotinia danger, but urges growers to assess the risk on their own farms.

“You must look at all the facts,” he says. “Before the 2007 outbreak the last bad year was 1991.

“Often you can’t get into mature crops, so people seeing dead patches assume they’re due to sclerotinia. But when you inspect the stubble, nine times out of 10, you can see they’ve been caused by summer phoma.”

Mr Neale likens sclerotinia to wheat brown rust. “It’s serious when you get it, but you need to ask how often is it likely?”

Many of this season’s stands are quite variable, which will extend the flowering period when crops are susceptible, warns Mr Parker.

“The differences will be exacerbated in varieties that typically have a long flowering period.

“Any measures that can even up crops and minimise the flowering period will help, including getting fertiliser applications right to keep crops growing strongly.”

Single spray treatments are likely to be optimal for most crops, but timing is critical, says Dr Gladders. “Finding days with good conditions for spraying is difficult, so some growers may opt for routine inputs to protect the crop during most of the flowering period.”

But single sprays won’t protect throughout flowering, he warns. “Often some late sclerotinia infection will occur.

“Fungicides help keep crop canopies green and may have plant growth regulator benefits, so responses of 0.1-0.2t/ha might be expected even in low disease situations.”

Only on low risk farms (without rain to cause petal sticking) is where not spraying might be an option, he suggests.

Protracted flowering increasingly justifies two sprays, says Mr Parker.

“All the fungicides only work preventatively, so spraying at yellow bud will protect from first flowering, with a second spray at mid-flowering topping up protection for later flowering plants and against later leaf diseases like alternaria and botrytis.”

Despite his view that sclerotinia is in danger of being “over-hyped” Mr Neale always employs two sprays at that time of year.

“When it comes to fungicides I think we should be treating oilseed rape more like wheat.” The summer phoma that’s often confused with sclerotinia usually occurs because growers applied only an autumn treatment and failed to follow up in spring, he believes.

“I always advise treatment at green to yellow bud because of the yield response, canopy manipulation and reasonable sclerotinia protection you get. You can then go again three weeks later if the pressure’s high. If it’s dry and there’s no pressure you could leave it another week.”

Mr Parker acknowledges that low sclerotinia infection renders the value of fungicidal activity minimal. “In many instances growers would not have covered the cost of boscalid or prothioconazole treatments last year.

“However, strobilurin based options Amistar and Priori Xtra will give greening effects that consistently increase yield and oil content, and prevent sclerotinia and other later leaf diseases.” That could be particularly valuable in this season’s backward crops, he adds.

Galileo is also a valuable option at early flowering, notes Frontier’s Bob Mills. “It’s a good fungicide with excellent strobe benefits.”

Despite Mr Parker’s view, boscalid was beneficial last season, he adds. “It does give a physiological greening.”

Product choice matters under high disease pressure, but not when it’s low to moderate, suggests Dr Gladders.

“Filan, Compass and Proline were the strongest products for disease control and yield response in an HGCA project, although it didn’t include cost-benefit analysis.

“The need to control light leaf spot and powdery mildew with flowering sprays this year may influence product choice for sclerotinia.”