Agriculture Reference
In-Depth Information
Blitecast is computer-driven programme, like the negative prognosis model, also
based on severity values. An initial blight warning is given 7-10 days after either an
accumulation of 10 rain-days (after Hyre, 1954) or 18 severity values (after Wallin,
1962) followed by a spray application programme based on the number of rain-
favourable days and severity values accumulated during the previous seven days.
Data are supplied to a central computer operator by the grower from in-crop
thermohygrographs. Four recommendations are issued: no spray; a late blight
warning; a 7-day spray schedule; and a 5-day spray schedule (recommended during
severe blight weather). Recommendations are provided over the telephone and are
completed within 3 minutes from the data being supplied. Fohner
et al
. (1984), using
a computer simulation model found that Blitecast was no more effective in
scheduling spray applications that a standard 7-day programme. However, they
suggested that improvements could be made to the effectiveness of scheduling with
better fungicides and weather forecasting.
9.6.8 Sparks's Risk Criteria (Sparks, 1984)
The Sparks model (Sparks, 1984) was developed as a direct result of the severe blight
year of 1983, when major blight epidemics developed in the eastern counties of the
UK despite there being no warnings under the Smith system. Early season
temperatures were below the 10°C required to trigger a Smith period but there were
exceptionally long periods of high humidity. Also, the plants were affected at a very
early development stage, in many cases as they were appearing through the ridges.
Potatoes are known to go through stages of varying degrees of susceptibility. This has
been associated with the carbohydrate ratio (Grainger and Rutherford, 1963). Where
the ratio between total carbohydrate of the plant and the non-carbohydrate of the shoot
was above 1.0, the plant is more susceptible to blight. This occurs both early and late
in the season.
In 1983, crops were at their first susceptible stage when conditions were extremely
favourable for infection. Sparks tried to correct the errors he perceived in the Bourke
and Smith schemes by taking account of lower temperatures and the development
of separate generations of blight. The Sparks system was computerised and
recommendations were issued as a single figure ranging from 0 (no risk) to 3 (high
risk). Sparks also took account of canopy cover, open or closed, so that adjustment
to risk depending on crop growth could be made at the start of the season. The scheme
was never officially published and was considered to be too complex by the
Agricultural Development and Advisory Service (ADAS) (formerly the Government's
extension service for England and Wales). Keane (1995) compared the Sparks model
with that of Bourke and concluded that the closed canopy model greatly over-
estimated blight risk and was therefore not of practical use under Irish conditions.
9.6.9 NEGFRY (Hansen
et al
., 1995)
Developed by the Danish Institute of Plant and Soil Science, NEGFRY is a
combination of the negative prognosis scheme of Schrödter and Ullrich (1967) and
