The tool that flags sclerotinia infection risks in oilseed rape

We recently switched on our sclerotinia infection risk tool for the 2025 season. Georgia Hassell explains how it can help target fungicide sprays to manage this oilseed rape disease.

The recent arrival of spring and spring-like weather will boost crop growth and focus attention on disease control. For oilseed rape, this includes the key disease during the crop’s flowering period – sclerotinia.

When soils are warm and moist, sclerotinia’s resting bodies (sclerotia) germinate. ‘Warm’ is above 10°C. In the last few weeks, some UK soils got near this mark.

It will not be long before soils are consistently above this temperature, encouraging the development of pale brown fruiting bodies and the release of sclerotinia spores.

In addition to spores, two more ingredients are required for crop infection: consistently warm and humid conditions and crops in flower.

While bright yellow flowers are easy to spot, it is not easy to know when the weather is conducive to infection.

This is where the sclerotinia infection risk tool comes in.

Using forecast and observed weather data, the tool tracks when relative humidity (RH) and air temperatures are at or above thresholds (80% and 7°C, respectively) for sustained periods.

Infection-risk status is indicated by coloured circles on a UK map (for hundreds of sites), where:

• Dark blue circles indicate that conditions are likely to be conducive to infection (in the next two days)
• Light blue circles mean near misses
• Green circles show low infection risk conditions

Based on an analysis of over-season performance, the tool has demonstrated a high rate of accuracy (greater than 90%) when forecast alerts are compared to alerts based on observed weather.

This means that the tool can guide fungicide timings. As sclerotinia stem rot control products are protectants (with little or no curative activity), this is extremely important.

The tool can also help gauge whether a spray is needed.

Certainly, the risk of yield loss is extremely low on farms with no history of the disease, where gaps between successive oilseed rape crops are sufficiently long and infection risk conditions are low.

Omitting a spray is also consistent with Fungicide Resistance Action Group (FRAG) guidance, which advocates the use of fungicides only when necessary.

FRAG also recommends that a range of mode of actions are used across the spray programme (all modes of action for sclerotinia control can be used elsewhere in the programme).

Historic data from our fungicide performance trials can help you identify suitable products and doses. Typically, treatments provide good protection for about three weeks.

We started the sclerotinia-risk-reporting service a decade ago (in 2015) and have often included data from a limited network of spore traps. Because spore pressures can vary widely across the UK, this element will no longer be included as part of the service.

Petals provide essential nutrients for sclerotinia spores to germinate. Some laboratories can test whether sclerotinia inoculum is present in petals.