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Battling blight

20 June 2013

Warm and humid conditions last year resulted in the highest number of confirmed incidences of late blight since AHDB Potatoes’s Fight against Blight (FaB) monitoring began in 2003. So far in 2013 there’s been no confirmed blight incidence in GB, but following a season of extreme blight pressure with more crops emerging and temperatures rising, vigilance is vital.

Blight Scout activity plays a crucial role in the monitoring and management of the disease and 2012 was the 10 year anniversary of this important activity. Continually improving our technical understanding wouldn’t be possible without the team of AHDB Potatoes Blight Scout volunteers, who walk potato fields regularly during the growing season and send in suspected foliage samples to Fera for analysis.
“In 2012, of the 385 foliage samples received, 96% were confirmed as infected with late blight”, explains AHDB Potatoes’s blight specialist Gary Collins. “This is the greatest percentage of positive samples received since 2003, which is a credit to how the Scout team’s understanding and identification ability has improved over the last decade.” 
Last year the first positive blight sample was recorded over a month earlier than in 2011 and was from a discard pile of crisping potatoes collected on 16 May near Ross-on-Wye in Herefordshire. The season continued until 18 September, when the last positive sample was sent in from a conventional crop of salad potatoes in Suffolk. 
“High levels of blight in 2012 could affect disease levels this season,” added Gary. “Hopefully, the weather this season will be less conducive, but there may be higher levels of inoculum in volunteers, outgrade piles and seed. It is important to keep an eye on these sources as planned fungicide and aphicide programmes can be jeopardised by a failure to tackle outgrade piles. 
“Blighted plants release millions of air-borne spores which can travel, and remain viable, for miles under dull and damp conditions. The closer an outgrade pile is to your crop, the greater the risk of infection.”
Industry vigilance is vital, stem blight was reported at the end of May in a crop in Northern Germany and although GB has not recorded a positive sample this season to date. AHDB Potatoes received its first request for a new scout recording kit via our twitter feed (@PotatoCouncil) 23 May from the South East.
The kit was sent out to Kent based agronomist and Blight Scout Graeme Skinner, who posted an photo of suspected foliar blight on his twitter feed (@GraemeSkinner1). The image was taken at one of two outgrade piles in Kent, which were burnt off with Reglone straight away. This mitigates the potential risk to nearby crops.
Scout samples are sent to FERA for analysis and if confirmed as positive, a blight alert is sent to local growers who have signed up for AHDB Potatoes’s free FaB/Blightwatch service at
The FaB tools ( provide access to the latest, localised information on Smith periods and blight outbreaks and levy-payers can register for free automatic alerts for up to 10 postcodes that arrive by email and /or a text message to your phone.
Latest technical information
A pre-print version of a brand new AHDB Potatoes guide, ‘Managing the Risk of Late Blight’ is now available at This new publication updates AHDB Potatoes’s previous suite of technical information with the latest scientific research on new genotypes. The guide helps growers formulate a robust strategy for blight management, keeping blight out of a crop and minimising the impact if an infection does occur. 
Tackling all sources
“Building knowledge amongst allotmenteers and gardeners is also a key part of the Fight against Blight; tackling potential sources which ultimately reduces disease pressure on commercial crops,” says Gary.
There are a growing number of allotments that check for blight, and blight monitors have taken on the role of distributing specific guidance to gardeners and allotmenteers. The ’What is potato blight?’ guide has been a major download at in the last couple of years and has proven very popular. An updated 2013 version has been reviewed by Alys Fowler (Gardeners Question Time) and contains information tailored to gardeners on understanding late blight and how to control it.
The Royal Horticultural Society, Garden Organic, the National Association of Allotment and Leisure Gardeners, the Sarvari Research Trust, WCF Phoenix, and the National Trust have provided support for this publication and at the end of May the online guide was publicised by the National Trust to its 55,000 gardening members. 
“Helping gardeners to be vigilant and carefully dispose of any affected plants, helps protect the whole potato crop,” noted Gary. “Therefore if there are allotment sites near you, please make them aware of this useful resource. Our message is simple, if you spot blight on your potato plants it’s important to ‘bin it, bury it, or burn it’.”
The future of blight forecasting
Recently completed AHDB Potatoes-funded research at the James Hutton Institute (JHI) evaluated the response of commonly occurring blight genotypes to temperature and humidity. This was to understand more about why some genotypes are more successful than others and to discover if the population changes have affected the validity of blight predictions based on Smith Periods. 
A new PhD studentship, due to begin this autumn, will continue this work with the aim of developing a better prediction tool for late blight risk across GB. The work will be supervised by David Cooke and Pete Skelsey at JHI. Dr Skelsey has previously developed models of late blight infection and spread and these will be used in conjunction with data for GB blight genotypes. Ultimately it is hoped that the approaches will lead to blight risk predictions based on weather forecast data with lead times, of for example 2 day forecasts.
The Smith Period has remained the stalwart method of blight forecasting, since it was developed in 1956 by L P Smith. A full Smith Period is recorded if, on each of 2 consecutive days the minimum air temperature is at least 10ºC, and there’s a minimum of 11 hours with a relative humidity of at least 90%. The recent research has demonstrated that blight infection can occur outside the defined Smith Period criteria, therefore continued investigation is crucial.
“The new work is still in the exploratory stage and may have the potential to offer an additional 24 hours planning time, and could be used to predict risk for a following year’s crop, by modelling different field situations, crop rotations and meteorological scenarios.
“Smith Periods should still be considered as an aid to blight management. But in light of the new evidence, it is important to monitor near misses. Anyone can register for Blightwatch alerts through the AHDB Potatoes website. A full Smith period is still a high risk period, but the new research will help to define lower parameters that are also high risk. Viewing the local data on the Blightwatch site can help with managing fungicide programmes and evaluating the risk.” said Gary.
Gary will be on the AHDB Potatoes stand at BP2013, Harrogate, 27 and 28 November to discuss blight best practice with levy-payers. For more details about the BP2013 event and industry dinner go to
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