Oilseed Rape disease update autumn 2022

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Seasonal update from Chris Steele, Crop Diagnostics Product Manager at Microgenetics

After the drought of the summer the establishment of the Oilseed rape crop was in doubt in some regions and the advice was to delay drilling until late August and into the first week of September to hopefully get some moisture into the seedbeds. Unfortunately, the establishment has fallen into two camps this season with those who got moisture at the correct time having a well-established crop compared to those that are still struggling and challenged by a lack of moisture. There is a distinct regional and even local variation as to where those crops got moisture.

So why is moisture so important to Oilseed rape? Obviously, it helps it grow but OSR actually as a two-stage process where adequate moisture is necessary at each stage to allow the seed to germinate.

The first stage relies on the seed to take up at least 40% of its own weight in moisture before it can even start to germinate. The second stage is a period of stabilisation which again requires moisture. Lack of moisture at either stage means no germination or the seed starts to germinate but then withers away.

So, sufficient moisture is needed in the soil to start the process and then more rain following drilling is needed to ensure the crop emerges and becomes established.

Focus on disease

Now It’s November, the focus should be in disease. Crops should be monitored for early signs of diseases (particularly Light Leaf spot and Phoma) and the disease should be controlled.

AHDB’s Phoma leaf spot forecast is predicting a relatively wide range of disease onset dates for winter oil seed rape this winter.

UK rainfall was generally below the long-term average, while temperatures were above average, which usually means low levels of disease. However, post September there has been a significant amount of rain which favoured spore development on stubbles.

The localised nature of this rainfall means that the forecast should be checked for an estimate of risk combined with inspecting crops for signs of Phoma, prioritising susceptible varieties and smaller crops.

The forecast was refined in 2021 to account for damp, rather than wet, conditions. The model now considers daily rainfall events up to 10mm and total rainfall up to 200mm, with additional rainfall having no influence.

The 2022 forecast suggested that a small number of relatively susceptible winter crops may already be approaching the 10% threshold. Conversely, some crops, mainly towards the UK’s north-east coastline, may not breach the threshold until early November.

The model forecasts risk at hundreds of sites across England, Scotland and Wales. The less regional variation, the more reliable the forecast. This year, the forecast is variable, making crop inspections even more important.

Phoma and light leaf spot

The risk of Phoma is higher in smaller, often later-emerging crops, although early-drilled crops will also be at risk, particularly those that have suffered from pest attack. It’s important to prioritise disease monitoring and control on varieties with a Phoma rating of seven or lower, especially if/when it turns wet during the month.

Earlier drilled crops are also more at risk of Light leaf spot infection, and testing is key to identifying the disease at an early stage. SwiftDetect Rapid diagnostic test is now available which allows detection of the Light Leaf spot and gives real time insight into the level of infection before symptoms are visible. This allows farmers/agronomists to optimise fungicide efficacy (allows fungicides to target the disease more effectively and efficiently by using the appropriate product and dose rate) and target applications to specific fields, helping to reduce overall fungicide expenditure and lessen the impact on the environment.

Control of both Phoma and Light Leaf spot is dependent on timing of crop protection products which is critical to achieve good control from either one or sometimes two sprays.

For Phoma the first application should be made to relatively susceptible plants (Recommended Lists rating 7 and below) when 10-20% of plants have phoma leaf spot. A second application should be considered when re-infection is evident (typically, 4-10 weeks later).

For Light Leaf Spot, a typically, well-timed application of an appropriate autumn fungicide – at a sufficient dose – has a relatively large impact on the final light leaf spot levels in a crop. However, when disease risk is low, the effect is much less.

In very low-risk situations (determined by observations of field-level risk and by using the SwiftDetect test), there is potential to reduce fungicide dose or omit some sprays – provided the strategy adopted also controls Phoma sufficiently.

For Autumn 2023 SwiftDetect will also be able detect Phoma and Sclerotinia allowing even better insight into these important diseases.



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