How dry soil affects sclerotinia infection risks in oilseed rape

Georgia Hassell considers how dry conditions influence sclerotinia infection risks in winter oilseed rape.

Although the infection cycle of the sclerotinia pathogen is complex, a good understanding of the three main risk factors will help you target management:

• Crops in flower
• The presence of spores (inoculum)
• Warm and humid conditions

Recently, oilseed rape has responded to the bright and warm spell, with crops shooting upwards and some forward crops flowering.

Soils are also warming, frequently rising above the 10°C threshold needed for the sclerotinia pathogen to develop in the ground.

However, one condition is generally lacking: soil moisture.

March was one of the driest months on record for some parts of the UK, with rainfall well below average:

• England – 25% of the average
• Northern Ireland – 32% of the average
• Scotland – 64% of the average
• Wales – the driest March on record since 1944

As dry soils slow the maturation of the fungi in the soil, it ultimately affects spore release.

Even when spores are present, dry soils tend to periodically lower relative humidity (RH) under the 80% RH needed for crop infection.

Night-time temperatures also continue to fall below the 7°C threshold.

As a result, virtually all sites in the AHDB sclerotinia infection risk tool are currently green (8 April 2025), meaning that there is a low infection risk.

A few coastal sites across the UK have infection-risk alerts forecast. These are generally associated with higher RH and warmer nights.

The tool highlights the localised nature of infection risk, which emphasises the importance of monitoring conditions in your local area.

After this week, the weather is forecast to turn more unsettled.

Widespread fungicide applications may not be necessary at this time, but sprays may be required in areas flagged as high risk to protect flowering crops – especially at farms with a history of sclerotinia issues.

• Conditions can change rapidly, so monitor the tool daily to stay up to date with infection risks in your area during flowering
• For a comprehensive risk assessment, combine the tool's insights with local spore monitoring or petal test results
• Aim to apply fungicides before infection occurs (as they are most effective as protectants)
• Follow Fungicide Resistance Action Group (FRAG) guidelines to ensure sustainable fungicide use