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About Smith Periods |
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About Smith Periods
A full Smith Period has occurred when:
At least two consecutive days where min temperature is 10ºC or
above and on each day at least 11 hours when the relative humidity
is greater than 90%.
A 'near miss' occurs when one or both of the above two consecutive days has only 10 hours
when the relative humidity is greater than 90% and the temperature is 10ºC or above. Note
that a near miss is not recorded when temperature values are close to but not at the threshold
value. Temperature and humidity data is shown in all charts to allow growers to see
The Interpretation of Smith Periods.
Smith Periods for blight control are calculated from hourly
temperature and relative humidity values. The weather records on
which the post coded Smith Periods are based are supplied by the Met
Office.
A full Smith Period has occurred if, on each of 2
consecutive days:
the minimum air temperature was at least 10oC, and
there were a minimum of 11 hours with a relative humidity of at
least 90%
Within the calculation there is a provision for a 'near miss'.
This occurs when the temperature criterion has been satisfied but
the number of hours with a high relative humidity totalled only 10
hours on one or both days.
Smith Periods were developed empirically i.e. the relationship
between specific weather criteria and blight development was
identified, but the underlying mechanism(s) was not known. Smith
Period criteria have been used by plant pathologists as an aid to
the assessment of blight risk in conjunction with knowledge about
the presence of blight in a locality and the stage of crop
development.
Smith Period conditions are conducive for sporulation of the
potato blight pathogen on lesions - leaf wetness is also necessary
for infection to occur. If Smith Periods occur at frequent and
regular (7-10 day) intervals, there is a greater chance of blight
development. This is because the generation time i.e. from original
infection through to lesion development to subsequent sporulation is
around 7-10 days. Smith Periods at these intervals would prove ideal
for the blight pathogen.
It is important also to take notice of minimum temperatures.
Although Smith criteria will not have been satisfied at say 9.9
oC, the activity of the blight pathogen will only be
marginally affected. Also, conditions in sheltered areas of fields
may well have satisfied Smith while conditions in the area more
generally failed to satisfy the criteria.
The above caveats should be considered carefully when using Smith
Periods in decision making. Smith Periods are only an aid to
decision support and do not in themselves always indicate the need
for immediate application of a fungicide.
Weather Records
The Met Office routinely collects hourly weather information from
a network of over 100 automatic and manual stations in the UK.
Traditionally, Smith Periods for potato blight warnings have been
calculated from these station values.
In contrast to this, the system used by blightwatch.co.uk involves
the interpolation of these station values down to post code areas.
Around 650 of these areas have been established to cover the whole
of the UK. This approach has several benefits.
Firstly, more weather records can be used when compared to the
traditional system. This leads to improved precision in the
interpolated values. Secondly, and more importantly, as Smith
Periods are now calculated for post codes, this removes the biggest
complaint that growers with crops situated between two weather
stations were left to guess the likely local implication of the
warnings.
Clearly, this improvement makes the Smith Period warning system
more relevant to growers throughout the country. Despite this, it
must be emphasised that Smith Periods are only one of many
management tools and they do not predict the weather conditions on a
field by field basis. At this level, the slope, aspect and the
effects of local microclimate have an influence. This must be borne
in mind when deciding on optimum blight spray programmes. For this
reason, the information on the Blight Watch pages must not be used
in a legal context.
While every effort has been made to ensure that the
information is accurate, no liability can be accepted for any error
or omission in the content or for any loss, damage or other accident
arising from the use of the fungicides for the control of potato
blight in individual crops.
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