Philip Walker
The focus for disease control is now mainly sclerotinia as the majority of crops are at the mid- flowering stage.
Light leaf spot continues to be reported mainly from existing infection.Warm weather and rain splash will facilitate the spread of the disease and checking for signs of infection on the stem is usually a good indicator of whether the disease is spreading within the crop canopy. Infection on the developing pods can cause pod distortion with resulting yield losses. Where light leaf spot is still a problem, a good strategy when selecting fungicides targeted at sclerotinia is to select flowering spray products which also have activity against light leaf spot.
Oilseed rape crops across the country are now flowering with most reported at or close to mid-flowering (Figure 1). Backward crops are only just starting to flower, with variations in flowering seen across areas of individual fields. For these crops, plants with few lateral branches will reach flowering sooner than crops with multiple lateral branches that will flower later, therefore these will need to be monitored to determine the best timing for spray applications. When significant petal fall begins sclerotinia infection can occur if the weather is sufficiently mild and humid, spore inoculum is present and there is sufficient petal stick on the leaf canopy. The thresholds for a sclerotinia risk alert are temperatures >7oC and humidity > 80%, for 23 or more consecutive hours, and usually light rain is also required to allow the fallen petals to adhere to the canopy. Weather conditions have been relatively dry to allow adequate petal stick, although there are variations from field to field where there has been morning dew or frequent rain showers.
The latest AHDB sclerotinia forecast is mainly showing low risk of infection as overnight temperatures have remained below threshold, however there are a few coastal areas in south west England and Wales where temperature and humidity are above threshold (Figure 2). The forecast for next week is expected to see an increase in temperatures so the risk will depend on humidity. Variations in humidity are more localised than variations in temperature, and lower lying fields next to rivers are often at higher risk for sclerotinia infection than nearby fields which are dryer.
Figure 6. AHDB weather-based alerts, Wednesday 19 April, indicating all sites high and low risk for Sclerotinia infection risk in oilseed rape. https://ahdb.org.uk/sclerotinia-infection-risk-alerts-for-oilseed-rape
Petal testing at ADAS sites in Herefordshire sampled at GS62 has shown a low level of inoculum present with 10% positive, however higher levels of 33% positive were detected nearer the coast in Llandeloy, Pembrokeshire. A positive petal test for a site with a weather-based alert will help to confirm it as high risk for sclerotinia infection. These results show that sclerotia inoculum is present and that sclerotinia ascospores have been released, but there is likely to be localised risk depending on the history of sclerotinia infections in particular fields. Spore monitoring data from AHDB still shows there is a low presence of airborne inoculum, although this has increased in the past couple of weeks. The recent warmer temperatures have likely stimulated further germination of sclerotia which has resulted in more spore release and for crops in areas of high risk, a spray application maybe considered for fields with a previous history of sclerotinia close to or at mid-flower.
