Agriculture Reference
In-Depth Information
sensors and data-loggers. This information can be used in computerized models to
forecast when fungicides need to be applied to prevent disease outbreak. A good
example is the model for
B. squamosa
leaf blight of onions, called 'Botcast',
described by Sutton
et al.
(1986). The principal constraints to the development of
this disease are periods of high temperature (> 30°C), which suppress the
pathogen, and periods of low humidity and consequent lack of leaf wetness, which
prevent sporulation. Based on previous studies of the epidemiology of the disease,
the following algorithm was devised to predict disease severity.
1.
Estimate sporulation or Daily Inoculum Value, DINOV
(DINOV = 0, no sporulation; DINOV = 1, sporulation)
Mean temperature > 30°C for > 4 h on at least one of the preceding 5 days?
→
Yes: DINOV = 0
→
No: Wetness duration on leaves in night < 5 h?
→
Yes: DINOV = 0
→
No: Wetness duration on leaves in night > 12 h?
→
Yes: DINOV = 1
→
No: RH < 70% for > 6 h on previous day and no rain or irrigation occurred?
→
Yes: DINOV = 0
→
No: DINOV = 1
Calculate Daily Infection Value, DINFV.
2.
The value of DINFV for the first two nights following a sporulation (i.e.
DINOV = 1) is calculated as 0,1 or 2 according to temperature and the duration
of leaf wetness, as indicated by Fig. 5.14.
3.
DINFV.
4.
Cumulative Disease Severity Index, CDSI, calculated as: CDSI = sum of DSI
values from the day of onion emergence.
5.
Disease Severity Index, DSI, calculated as: DSI = DINOV
CDSI compared with two thresholds:
CDSI = 21-30, Threshold 1 - some risk of disease, apply fungicide if rain or
irrigation expected
CDSI = 31-40, Threshold 2 - high risk of disease, apply fungicides as soon as
possible.
Implicit in the model are the assumptions that initial inoculum is always
present, that spores produced at night can survive for up to 2 days and that
conditions suitable for spore dispersal occur every day.
In trials in Ontario, Canada good correlations between disease severity and
CDSI were obtained, and Threshold 2 values occurred just a few days before
epidemic disease increases began. By starting fungicidal sprays either at
