Sprayer in potato crop© Tim Scrivener

Potato growers can look forward to more accurate blight forecasting after research led to the replacement of the long-standing Smith period with an updated measure more suited to current strains.

With planting already under way in forward areas such as the Suffolk coast and Norfolk brecklands, thoughts have turned to the season-long battle against the top disease in spuds.

In recent seasons, disease outbreaks have frequently been found in crops before a warning has been triggered. 

See also: How to tackle weeds in spuds after linuron withdrawal

The 60-year-old forecasting criteria referred to as a Smith period predicts when plants are vulnerable to blight infection. It is defined as when there has been a minimum temperature of 10C for two days and each day had at least 11 hours of relative humidity at or above 90%.

However, recent years saw its accuracy questioned as Smith periods were failing to predict some infections. It was suspected that new, more virulent blight strains were thriving in colder or drier conditions.

Accurate forecasts enable farmers to opt for more robust chemistry when the risk is high or, conversely, select cheaper options when the risk is low.

This prompted AHDB Potatoes to commission the Scottish-based James Hutton Institute, also home of the Commonwealth Potato Collection, to investigate.

Blight in a potato crop

© Tim Scrivener

After studying the relationship between reported outbreaks and conducting experiments, researchers found adjusting the duration of the required humidity period from 11 hours down to six while keeping the other factors constant produced the most accurate predictions.

Forecasting performance rose from 41% to 69% following the change, when applied to historic outbreaks and removed emerging regional variations in prediction accuracy.

Improved forecasts

The Hutton criteria will be incorporated into the AHDB Blightwatch forecasting system from this season, which runs alongside its Fight Against Blight initiative to provide growers with the best possible data on blight risk.

Both systems work together to provide a complete picture of blight risk in a localised area, based on the level of disease inoculum already present, and whether the conditions are likely to promote its spread.

Improved accuracy of the system will encourage growers who have in recent years lost faith in the ability of forecasting systems after seeing blight in their crops before getting an alert to return to the fold, says Claire Hodge, technical executive with AHDB Potatoes.

The benefit of using the alert data rather than spraying at regimented intervals will allow growers to increase the intervals between sprays where appropriate, and vary their product use depending on the risk, she explained.

Blightwatch is run in partnership with the Met Office, and uses its weather data to give localised alerts throughout the country, both for growers in an area affected by a Hutton criteria and when one is likely.

How to sign up to the disease alerts

  • AHDB – Register online for AHDB Blightwatch and click the “Register” button. 
  • AHDB Fight Against Blight service – look out for reports, as the service is being launched at the end of planting.
  • Visit BlightCast and click the green button, which generates an automated email, asking for simple details such as email address, postcode and name.

The Fight Against Blight initiative, which uses a network of crop-walking agronomists and farmers to complement the weather data with a picture of disease levels on the ground, is set for relaunch at the end of planting this season.

Leaf samples are collected and sent to laboratory testing experts at Fera, which confirm whether blight is present in the crops. Data will also continue to be shared with the James Hutton Institute, which will monitor blight strains and how accurate predictions are.

Syngenta blight forecasting system updated

An alternative free blight forecasting service from Syngenta has also been updated using the Hutton criteria.

BlightCast offers a local prediction based on the postcode of the user, and also suggests when any required spraying will be appropriate based on weather data.

“This is an evolution, not a transformation”, explains Douglas Dyas, potato field technical manager for Syngenta.

Last season, Syngenta modified its forecasting model by lowering the temperature threshold from 10C to 8C and saw improved prediction results, calling it the “New Criteria”.

Syngenta is giving users a choice this season by offering forecasts based on the Smith period, its “New Criteria”, and the Hutton criteria.

Mr Dyas suggests that the Hutton criteria is now likely to be the most accurate model, but growers will have the choice when to phase over to the new system.

Disease alerts will be sent out by email every Monday, Wednesday and Friday from 14 April.