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Flexible approach to Smith may pay

8 April 2013

New evidence from the James Hutton Institute shows that the long established blight prediction conditions defined by a ‘Smith Period’ need to be approached with some flexibility.

Speaking at the AHDB Potatoes’s Winter Forums at Sutton Bridge and Harper Adams, the Institute’s Dr David Cooke outlined the findings from experiments that observed the development of GB’s two most prevalent late blight disease genotypes 6_A1 and 13_A2.
“We looked closely at infections on leaves inoculated with either one of the two genotypes, and observed lesion development at a range of temperatures from 6°C, up to 20°C,” he explained.
“We saw infection at 8°C, and even at 6°C there was a tiny level of infection. This clearly shows the pathogen can infect and survive at these lower temperatures and would have the potential to establish once conditions allow. 
“Importantly, disease activity was in evidence below the Smith Period ‘cut off’ temperature of 10°C,” said Dr Cooke.
Relating the findings to field scale conditions, a workshop was held during the afternoon of the Forums hosted by Dr Pete Skelsey, also of the James Hutton Institute. He considered alternative late blight prediction tools, currently in development, and sought audience feedback.
He took a commercial potato field example from June 2011 where there was no Smith Period warning provided, but where a severe late blight outbreak did occur. Subsequent analysis of the conditions showed that whilst the Smith Period criteria was clearly met for one day, the second day recorded a temperature half a degree below the 10°C threshold level, that meant a warning was not triggered.
Such ‘near miss’ observations have led to a new AHDB Potatoes-funded project to look at alternative ways of predicting blight risk that takes into account a greater diversity of influencing factors. Posing the question to the Winter Forum audiences, blight influencing factors other than temperature and humidity were identified. Topography of the land, management practices such as irrigation and fungicide strategy, the crop variety, microclimate, genotype of pathogen and sources of local inoculum were all deemed important. 
A range of such factors is being considered within a new prediction model being developed by the James Hutton Institute. This model is based on maps of potato fields and the past and forecasted weather. Dr Skelsey noted that the model 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.
“This is still at the exploration stage,” he said. “Smith Periods remain an excellent forecasting tool that has withstood the test of time since being developed in 1956 by L P Smith.” 
AHDB Potatoes’s blight specialist Gary Collins advises that Smith Periods should still be considered as an aid to blight management “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 risk.” says Mr Collins. Go to for more information and how to sign up for Blight alerts.
Smith Period
A full Smith Period has occurred if, on each of 2 consecutive days:
  • The minimum air temperature was at least 10°C, and 
  • There were a minimum of 11 hours with a relative humidity of at least 90%
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