Sclerotinia infection risk alerts (forecast) for oilseed rape

Use this tool to understand sclerotinia infection risk and target control.

About the sclerotinia infection risk alerts

Sclerotinia stem rot is usually the main disease to consider during the flowering stages of oilseed rape. Although the infection cycle of Sclerotinia sclerotiorum is complex, a good understanding of the three main risk factors – the presence of sclerotinia inoculum (spores), warm and humid conditions, and crops in flower – will help you target control.

Sclerotinia stem rot in oilseed rape

Sclerotinia infection risk tool delivers a high level of accuracy (news item)

Top tip: Click on the page arrows <-> to toggle between page views.

  • < 1 of 4 > = Tool home page
  • < 2 of 4 > = Look back at how the tool has performed
  • < 3 of 4 > = Risk map
  • < 4 of 4 > = Spore trap data (five sites)

Weather and infection risk

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 (red circle). Near misses are also highlighted (amber circles). Where infection risk is low, this is shown (green/white 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 amber/red).

Forecast forensics (2021 season overview)

The 2021 risk period was relatively cool. This 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.

Weather overview

The first crops started to flower in early April, with petal fall occurring on the most advanced crops by the second week of May. Typically, lateral branches were at early flowering stage towards the end of May.

Generally, the flowering period was prolonged by cooler temperatures – with many crops flowering into June.

April was the fourth driest on record and coldest (average minimum temperatures) since 1922. The dry and cool (particularly night-time) April conditions meant thresholds were rarely met for sclerotinia infection.

May was wet – in fact, the UK experienced the fourth wettest May on record. In the first week of May, twice the average UK rainfall fell. However, milder conditions helped trigger infection alerts in the south west of England on 6 May. Alerts continued through the rest of May, which covered most regions and coincided with the main flowering period.

Final forecast analysis

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.

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 (see 'look back' page on the tool).

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.

91.5%



Correct alerts

Forecast

Days with alert Days without alert
Observed

Days with alert 1,336 true positives 724 false negatives
Days without alert 464 false positives 11,000 true negatives

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.

Sclerotinia stem rot in oilseed rape

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