Agriculture Reference
In-Depth Information
Smith also records that “The differences in effectiveness of the two systems,
however, would appear to be small in practice and there would be little point in
altering an established system unless the benefits are likely to be considerable”. It
was not until 1975 that the Smith Period came into full operation and formed the
basis of blight forecasting in the UK. The Meteorological Office provided data
fulfilling the Smith criteria from its synoptic network to help growers time the
beginning of their routine spray programmes.
The Smith Period is still used widely in the UK and data from the
Meteorological Office's synoptic network is now spatially interpreted and made
available to the industry via the internet (Barrie and Bradshaw, 2001).
The Smith period, like the Beaumont, period consisted of two simple
meteorological parameters - at least two days (ending at 0900) with:
•
a minimum temperatures of 10°C or above;
•
duration of humidity of 90% or above for at least 11 h each day.
9.6.6 Negative Prognosis Model (Schrödter and Ullrich, 1967)
Developed from laboratory studies
on how temperature and humidity regulate the
life cycle of the late blight fungus, Schrödter and Ullrich (1967) demonstrated that
weather conditions exert differing influences on the likelihood of infection,
sporulation and mycelial growth. These data were quantified in order to generate a
daily severity figure. The daily figures were accumulated until a specific threshold
was reached after which routine spraying should begin. The exact magnitude of the
threshold could be varied depending on locality but once it passed a figure of 180,
blight was most likely to be found. During highly favourable weather, the daily
figure could be as high as 20 while, in hot dry conditions, there could be a negative
value. This scheme had the advantage for the grower that he can have confidence
that blight is unlikely to occur until the threshold is reached and sprays need not be
applied.
9.6.7 Blitecast (Krause
et al
., 1975)
Unlike the previous schemes, Blitecast (Krause
et al
., 1975) attempts to advise not
only on the date to begin spraying but also the timing of subsequent applications.
Data required for the scheme were daily maximum and minimum temperatures, the
number of hours when relative humidity was equal to or above 90%, the maximum
and minimum temperature during the period when the RH was 90% and above and
the daily rainfall figure to the nearest 1 mm. The RH and temperature sensors should
be sited between the potato rows and within the potato canopy. The system is
basically a combination of two programmes used in the USA, one (Hyre, 1954)
based on rainfall and maximum and minimum temperatures, and the second (Wallin,
1962) based on relative humidity and temperature, and uses an accumulation of
arbitrary severity values based on the relationships between the duration of relative
humidity and temperature.
