Sclerotinia infection risk alerts (forecast) for oilseed rape

If spores are present, conducive weather is required for infection to occur.

> Based on observed (past 24 hours) and forecast (next 72 hours) weather data, the interactive maps track the period of time that relative humidity (RH) and air temperatures are at or above threshold (80% and 7°C, respectively).

> If these thresholds are exceeded for 23 continuous hours (the time needed in order for sclerotinia to infect the crop), a weather-based infection alert is issued at the corresponding site (dark blue circle). Near misses are also highlighted (light blue circles). Where infection risk is low, this is shown (green circles).

Click on a circle (any colour) to show an hour-by-hour chart of the sclerotinia infection risk for that site.

Sclerotinia control products are protectants and should be applied prior to infection (before circles turn blue).

An overview of the 2021 season

Our weather-based sclerotinia infection risk alerts support decisions. When thresholds for temperature and humidity are met, a risk-alert is triggered. However, forecast weather data is not always accurate.

Here we analyse how good the predictions were during the 2021 season, in relation to risk alerts.

The 2021 risk period was relatively cool, which delayed flowering and extended the flowering period. It also resulted in a relatively low number of weather-based infection risk alerts and, generally, low spore levels.

Our analysis shows that forecast alerts were highly (91.5%) accurate, when compared to actual weather during the period 15 March to 23 June 2021.

Differences tend to be due to relative humidity (which is harder to forecast, due to localised showers and local factors) rather than temperature, and this is particularly true in costal locations.

Spores and petal fall

Light rain during the petal-fall period increases petal stick. From infected petals, spores can germinate and infect the crop (providing weather is conducive). However, heavy downpours can actually reduce infection risk, reducing the airborne inoculum and washing petals straight to the ground.

The relatively cool weather over most of the winter and spring resulted in low levels of spores recorded in the AHDB-funded network of six spore traps. The wetter and milder May weather saw inoculum levels increase across the month. The results are similar to those revealed by ADAS petal tests. These reported:

• Zero-to-moderate levels during the early flowering period
• A decline through the mid-flowering period
• Moderate inoculum levels in the late-flowering period

Spray programmes

Fungicides targeted at sclerotinia must be applied ahead of infection (as protectants). A spray normally provides around 3 weeks protection. The 2021 season highlighted the value of the sclerotinia risk alert tool. With risk low for many through the early flowering stage, there was an opportunity to delay treatment and, where applied, reduce the requirement for a second treatment.  However, the protracted flowering period in many crops – due to cooler conditions – meant a second spray will have been required in some crops, if weather conditions triggered an alert.