Oilseed rape growers are being urged to be ready to apply early to mid-flowering sprays.

Tighter rotations raising the risk of sclerotinia, and earlier than normal activity of the disease in this season’s crops, mean this could be the year to spray.

Germination of the fruiting bodies which produce infective spores from the soil-borne resting sclerotia rose rapidly last week, ADAS says.

Although sclerotinia levels vary markedly across the country and yield responses to light infections can be relatively small, treatment helps prevent the disease building up in the soil as a threat to future crops such as peas, potatoes and carrots, explains pathologist Peter Gladders.

“There is a risk this year and spraying is likely to be justified.”

According to CSL surveys the average level of infection in oilseed rape is only 1-2%.

And with the average yield loss of 0.5% for every 1% infection, the initial penalty may seem low.

But farm history of the disease is an important driver, says Dr Gladders.

“There are hot spots where it can easily reach 10%, and in bad attacks it can be 20-50%.”

Given that some fungicides, such as Amistar (azoxystrobin) and Filan (boscalid), also offer physiological greening effects which can boost output, treatment is well worth considering, he believes.

“All sclerotinia products give 90% control provided the spray timing is spot-on.”

That can present difficulties in crops left patchy from last autumn’s pigeon predations and other problems, he acknowledges.

Because all the control products are protectant-only they need to be on before the petals carrying the inoculum of the aerial spores fall and stick to the leaves.

“You really need to go by what the bulk of the crop looks like, and mid-flowering is when you have about 20 flowers open on the main raceme.”

Controlling sclerotinia is becoming increasingly important as rotations tighten, says Masstock’s David Neale.

“This year there are some very good plant stands and plenty of yield potential which are very good reasons for treating for it.

“Crop development is 10 days to a fortnight later than last year though.”

Filan has performed well in the firm’s trials.

“We’ve had 0.3-0.5t/ha responses, and in high risk situations we’ll be pushing people in that direction.”

An alternative is Amistar Priori Xtra (azoxystrobin + cyproconazole) which also offers yield-enhancing greening, he notes.

Where alternaria is also a threat, notably south of the M4 motorway, it is important to maintain robust doses, Dr Gladders points out.

“And there have been signs that light leaf spot may be coming back where people missed out on earlier sprays.”

In those cases it could pay to add a triazole, he suggests.

andrew.blake@rbi.co.uk