Agriculture Reference
In-Depth Information
9.6.3 Beaumont period (Beaumont, 1947)
The Dutch rules were initially used in England but there was an unacceptable period
between the predicted outbreak of blight and its actual occurrence. Beaumont (1947)
recognised the importance of high humidity and its duration. The Dutch Rules relied
upon dew. Beaumont reported that dew does not readily form on potato leaves and
that there were also difficulties in measurement, as dew point is calculated rather
than observed, and therefore the length of the actual period of dew present on the
crop could not be determined with any degree of accuracy. Beaumont considered
humidity to be more important as, during a period of high humidity, there is no wind
and the nightly drop in temperature was most likely to result in dew formation and
leaf wetness.
Beaumont combined the dew, cloud cover and rainfall requirements into a period
of high humidity. His rules were that there should be at least two consecutive days
with:
a minimum temperature not less than 10°C;
a relative humidity not below 75%.
When these conditions were met, growers in the vicinity were advised to begin their
routine spray programmes. This became the standard blight warning scheme for the
UK until the mid-1970s.
These rules were evaluated by Large (1956) over a period of five years (1950-
1955). He concluded that “an 'operations-chart' method of interpreting the
occurrence of Beaumont periods in screens at standard weather stations (about 40
synoptic weather stations were used) enables broad regional forecasts of the date of
blight outbreaks to be made successfully in England and Wales, provided that the
indications from the whole network of stations were taken into account and due
regard paid to the seasonal and regional differences in the forwardness of crops”.
This is interesting as it illustrates precisely the problems with forecasting systems
that will be discussed in more detail later. However, it is important to interpret the
data intelligently. This statement basically means that the important element is a
number of synoptic sites reporting Beaumont periods rather than relying on a single
station for decision making for an individual locality.
It was also found that, with the exception of the UK's extreme south-west where
crops were generally more advanced, Beaumont periods occurring before July could
be ignored (Large, 1953; 1959b). This again demonstrates the importance of
intelligent interpretation of a fixed set of rules.
9.6.4 Irish rules (Bourke, 1953)
The Irish rules (Bourke, 1953) differ from the Dutch rules and Beaumont period in
that the relative importance of the duration of shorter spells of humid weather is also
assessed. Also a forecast of blight-favourable weather is included in the forecast.
The work of Crosier (1934) on the aetiology of the pathogen was used in defining
the criteria.
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