Agriculture Reference
In-Depth Information
BOTCAST (used to predict the application of the first spray) and SIV (a sporulation
model used to time subsequent sprays) were tested in the Netherlands (de Visser,
1996). The application of these models reduced chemical sprays by 54% without an
observed yield loss or increase in disease severity. Modifications to BOTCAST are
suggested by de Visser (1996) and comparisons have been made with sporulation
models such as DINOV (Vincelli and Lorbeer, 1988). BOTCAST is used in practice
in the Netherlands to reduce fungicide usage in the control of leaf blight (Meier,
2000).
In the USA integrated pest management programmes have been developed for
dry bulb onions, based on careful monitoring of the development of leaf lesions, and
these have enabled fungicide spray applications to be delayed until a critical disease
level (CDL) is reached (Lorbeer, 1992). In addition, microclimatic measurements
have been used to predict daily inoculum incidence and infection density, from
which disease severity indices are computed. From this, bioclimatological data
thresholds are determined that indicate when fungicide sprays should be applied.
The forecasting system that has been developed is known as BLIGHT-ALERT
(Lorbeer
et al.,
2002) and is available commercially.
Both BOTCAST and BLIGHT-ALERT are weather-based predictive systems.
Field monitoring to determine the CDL is used in BLIGHT-ALERT but not in
BOTCAST (Lorbeer
et al.,
2002). Carisse
et al.
(2003) reported that in Canada bulb
onion leaf blight epidemics began when airborne conidia of
B. squamosa
reached a
concentration of 10 conidia m
-3
air. Following experimentation a critical threshold of
15 to 20 conidia m
-3
air was established and in some years, applying this threshold, it
was possible to manage the disease and reduce the number of spray applications by
20% to zero (Carisse
et al.,
2003).
BOTCAST has been tested in conjunction with the application of
Gliocladium
roseum
spores for the biological control of leaf blight in bulb onions (James and
Sutton, 1996). The fungus was about half as effective as fungicide (chlorothalonil)
in reducing the density of
B. squamosa
leaf spots but it was considered that the
antagonist had potential for controlling the disease sufficiently to avoid economic
yield losses (James and Sutton, 1996).
However, it was noted that germination of
B. squamosa
sclerotia in the soil could
be predicted and may be of use in predicting release of primary inoculum (Clarkson
et al.,
2000). This model might be combined with existing models for leaf blight
development such as BOTCAST (Sutton
et al.,
1986) to provide a more
comprehensive system for predicting leaf blight (Clarkson
et al.,
2000).
19.4.3
Peronospora destructor
(cause of downy mildew)
(a) Pathogen and disease
Peronospora destructor
is a phycomycete fungus. It differs from the
Botrytis
spp.
already described in that it is an obligate biotroph. Otherwise, similar epidemio-
logical principles govern the development and spread of downy mildew.
Downy mildew can cause serious losses in bulb and green (salad) onions and in
onion seed production crops (Gilles
et al.
, 2004). The authors cited bulb onion yield
