There has been much debate in recent years on whether Smith periods are still relevant, as the current aggressive strains can infect crops at lower temperatures. So what is being done to improve the forecasting of this disease?

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Smith Periods were developed in 1956 by an agricultural meteorologist, LP Smith, and came into operation in 1975 as the accepted forecasting tool for late blight in the UK.

See also: Farmers Weekly Academy: understanding blight

The Smith period consists of two simple meteorological parameters – at least two consecutive days with:

• Minimum temperatures of 10C
• Duration of humidity of 90% or above for at least 11 hours each day

These are the suitable conditions for sporulation of the potato blight pathogen on the lesions, explains David Cooke. “Leaf wetness is also necessary for infection to occur.”

But questions have been raised for the past few years as to the relevance of the Smith period as current blight strains are able to operate outside of these parameters.

As a result, it has been recognised that there is a need to redefine the Smith period to reflect the changing dynamics of today’s pathogen populations.

Work at the James Hutton Institute to develop a fully validated replacement for the Smith period is in progress, looking at achieving the right combination of temperature and high humidity duration, says Mr Cooke.

It will not be completed in time for this season, which leaves growers with the existing Smith criteria to guide their decision making.

While work continues to fine tune the Smith period, two industry-led forecasting tools, based on the Smith period, have been updated in the last two years to improve decision making.

Blightcast – Syngenta

What is it?

An updated version of the free web-based blight forecasting service from Syngenta, which delivers a five-day localised warning of blight risk via email, enabling growers and agronomists to take a proactive rather than reactive approach to blight management .

How does it work?

Utilising weather data updated hourly from regional met stations and interpolated to postcode level, the tool provides two forecasts, the first based on the traditional Smith period criteria and a second New Criteria and five-day spray application window forecast.

The New Criteria forecast pinpoints forecast temperature to hit over 8C and more than 11 hours at 90% humidity over two consecutive days to trigger a Blight Period or a Near Miss where conditions occur for a shorter period. This New Criteria was borne through observations made by agronomists in 2014, which identified that blight was more active earlier in the season on volunteers before a Smith period had been recorded.

Since its introduction last season, feedback on the updated Blight Cast has been positive, giving an earlier heads up to the disease and start to the fungicide programme, especially in situations where a conventional Smith period would have been marginal.

Blightwatch – Met office, 101Smart Ltd in conjunction with AHDB Potatoes

What is it?

A refreshed version of the free web-based blight forecasting service for use by professionals and amateur growers, run in parallel with the Fight Against Blight (FAB) service provided by AHDB Potatoes. Blightwatch delivers a daily email alert to users when blight risk is identified for up to 10 user-selected postcode areas.

How does it work?

New and existing users need to register or validate their details on the new Blightwatch website, which will be available from the beginning of April 2016. The website has been redesigned to provide a better user experience.

Based on traditional Smith periods, user alerts will be generated when full Smith periods are forecast or when there has been a confirmed FAB outbreak for their chosen postcodes. The new website uses thousands of forecast data points, which will forewarn users 24 hours ahead of potential Smith periods.

Other new exciting features will be integrated into the service over the coming seasons. As with the previous Blightwatch service, the alerts do not constitute a recommendation to spray, but will provide an earlier warning to aid management decisions around spray timing and intervals.