Wet weather heightens risk of stem rot in oilseed rape

An early warning scheme for the potentially devastating disease sclerotinia stem rot in oilseed rape is being refined to alert growers to this threat, which can cut yields by up to 80%.

The disease, which is encouraged by warm, showery weather during flowering, can be more of a problem in the wetter West than further east, and this has prompted more work to improve prediction.

In the bad disease year of 2007, yields in crops not treated with fungicides fell by four-fifths in Herefordshire trials. And even in a moderate year for the disease, a fungicide treatment led to a 0.7t/ha yield increase up to 4.2t/ha.

See also: Sclerotinia life cycle and biology

Julie Smith, senior researcher at crop consultant Adas, is working with colleagues on improving the traffic light warning system that is on offer to growers through the AHDB and is driven largely by weather conditions in the spring.

High risk

  • A history of sclerotinia on the farm
  • Short rotations of susceptible crops (such as peas, potatoes and certain weeds such as shepherd’s purse, as well as oilseed rape)
  • Warm and unsettled weather during flowering

Low risk

  • No history of sclerotinia on the farm
  • Dry and settled weather during flowering

“Last year we saw more sclerotinia around than in the previous two years because of warm, showery weather at flowering,” she says.

The disease is one of the hardest for researchers to predict, but in high-risk situations it can be controlled by a number of fungicides such as Proline, Pictor and Amistar.

The risk prediction system is based on four factors:

  1. Tracking the germination of overwintered sclerotia, which usually starts in March.
  2. Assessing the airborne ascospore release from germinating sclerotia
  3. Testing petals of the flowering crops for ascospores
  4. Using weather forecasts covering the flowering period

The disease fungus survives in the soil for a number of years as resting bodies or sclerotia. In the spring, rising soil temperatures encourage sclerotia near the soil surface to germinate.

Pale brown fruiting bodies (apothecia) form to produce airborne spores and when these ascospores land on oilseed rape petals, they may adhere to stems and leaves.

In wet weather, these petals will stick more readily to stems and leaves. These petals provide nutrients for the fungus to survive. Light rain and humid conditions are best for petal stick, while heavy rain can wash the petals away.

Infection event

A so-called infection event can occur when the temperature is above 7C and the air humidity is more than 80% for a minimum of 23 hours from the mid-flowering stage when petals start to fall.

The AHDB warning scheme looks to monitor all this information and give growers an idea of the possible severity of the disease in their area.

Good control can be achieved with a single fungicide application at mid-flowering, but in a high-risk situation there may be a need for the first spray when 20-30% of flowers are out, and then another one some three weeks later.


  • Proline – azole prothioconazole
  • Pictor – SDHI boscalid + strobilurin dimoxystrobin
  • Amistar – strobilurin azoxystrobin

‘Go early’

“If there is a choice, go early rather than later as all the fungicides recommended are protectant in action,” Ms Smith says.

There is no genetic resistance to the disease, but there are some indications that early flowering varieties may be at a slightly lower risk than later-flowering ones.

“The biggest thing that growers can do is to watch the weather, as showery conditions and high temperatures will lead to a high risk of the disease,” she says.

Growers should be monitoring the crop and the weather, and also refer to the AHDB website for its sclerotinia risk report.