Use our sclerotinia alerts to target oilseed rape fungicides

Our sclerotinia infection risk alerts help you optimise sprays for more cost-effective control and to manage fungicide resistance risks in oilseed rape. Jason Pole explains.

Oilseed rape is at various development stages across the UK, but it will not be long before green buds start to open on the most forward crops.

When crops flower, sclerotinia infection risk needs to be considered.

When soils are moist and warm (above 10°C), sclerotinia’s resting bodies (sclerotia) germinate, develop pale brown fruiting bodies and release sclerotinia spores.

Spores that land on oilseed rape petals or pollen use them as a nutrient source.

When these fall and stick to plants (especially on damp surfaces), infection can occur.

While bright yellow flowers are easy to spot, it’s not easy to know when the weather is right for infection.

This is why our alerts use forecast and observed data for relative humidity (RH) and air temperatures to track when UK sites are at or above critical thresholds (80% and 7°C, respectively) for sustained periods.

We use coloured circles to show infection-risk status at UK sites:

• Dark blue circles show conditions are likely to support infection (in the next two days)
• Light blue circles show that conditions are close to becoming suitable for infection
• Green circles show the infection risk is currently low

Sclerotinia stem rot control products are protectants, with little or no curative activity.

This means accurate timing – based on infection risk – is extremely important.

The best timing for a single spray is usually just before mid-flowering on the main raceme and before significant petal fall.

Typically, treatments provide good protection for about three weeks.

You can use the alerts to assess the need for follow-up sprays. This is particularly useful when the flowering period is extended in relatively cool seasons.

Our website includes historic sclerotinia control data from our fungicide performance trials (product efficacy at a range of doses).

Our website also hosts Fungicide Resistance Action Group (FRAG) group guidance.

As per cereal crops, the guidance recommends using a range of mode of action (MoA) groups across the spray programme. Several options are available for oilseed rape.

Strains of the pathogen with decreased sensitivity to SDHI fungicides have been found in mainland Europe for several seasons, particularly in France.

Although an SDHI sensitivity shift was detected in a single UK strain in a laboratory screen, this was nearly a decade ago.

There are no current issues with the field performance of SDHIs in the UK, but resistance mutations are a threat. Fungicide programmes need to account for this risk.

A key principle of fungicide resistance management is to minimise the number of sprays.

When infection risk is low for a sustained period, there is potential to omit sprays, especially on farms with no history of the disease or where oilseed rape has not been grown for several years.