Agriculture Reference
In-Depth Information
'Botcast' Threshold 2 rather than the conventional time, when local onion
growers started, disease was controlled with an average of only three sprays per
year rather than seven. Similar models were developed at around the same
time in New York State and Michigan, USA (Lorbeer
et al
., 2002).
Tests of these models in western Europe have resulted in the development of
'hybrid' models. A combination of rules from both Botcast and Blight Alert, the
New York model, reduced fungicide sprays by 27-78% without any loss of yield
over 3 years of trials in France (Huchette
et al
., 2005). A combination of Botcast
for forecasting the initial spray and the SIV model from Michigan reduced the
number of sprays required in The Netherlands by 54% compared with weekly
sprays without any yield loss (de Visser, 1996). In The Netherlands and the UK,
the re-emergence of downy mildew as a serious disease in the 1990s has limited
the practical usefulness of predictions for spraying against
B. squamosa
since
farmers needed to spray routinely with fungicides for downy mildew control (de
Visser, 1998). Therefore, a predictive forecast for downy mildew control
combined with one for
B. squamosa
is required.
Information relating downy mildew risk to the time and duration of
temperatures and humidities conducive to sporulation and relating infection
to the time and duration of leaf wetness along with suitable temperatures has
been incorporated into disease-forecasting models - for example, the perhaps
appropriately named 'Downcast' developed in Ontario, Canada (Jesperson and
Sutton, 1987). This model correctly predicted sporulation on 111 out of 119
nights over two growing seasons in Ontario, and it also proved satisfactory in
Queensland, Australia. However, the model predicted only about 50% of
observed sporulation/infection incidences in The Netherlands (de Visser,
1998) and gave no better than a random prediction of sporulation in the UK
(Gilles
et al.
, 2004). Suggested reasons for these failures of 'Downcast' include
the highly variable weather in European maritime areas, possible differences in
the conditions required for sporulation by pathogen strains from different
geographic locations (Gilles
et al.
, 2004) or inconsistencies in leaf wetness
measurements with the available instruments (de Visser, 1998). A
modification of 'Downcast' (de Visser, 1998) gave better results in The
Netherlands and enabled a 15-29% reduction in the number of fungicide
applications needed in trials in France, with no loss of yield (Huchette
et al.
,
2005).
A quantitative model of spore production by downy mildew, named
'MILIONCAST', was developed in the UK, based on observations of the effects of
temperature and relative humidity on numbers of sporangia produced. This gave
a good prediction of observed occurrence and intensity of sporulation in test plants
outside (see Fig 5.15). In Germany, the model 'Zwipero' was developed to predict
downy mildew sporulation infection events using simulated inputs of tempera-
ture, humidity and leaf wetness within the onion canopy (Friedrich
et al.
, 2003).
These simulated inputs were outputs from another model calculating these
properties within a leaf canopy from standard weather data measured 2 m above
