Agriculture Reference
In-Depth Information
sprays following periods with high RIM values was similar to control obtained with
nine curative sprays following Mills infection periods (Trapman, 1994), while
Ventem™ identified infection periods more accurately than the Mills system in each
of three years (Xu
et al.
, 1995).
In the same way that many scientists have questioned the accuracy of the Mills
criteria for predicting scab infection periods (MacHardy and Gadoury, 1989;
Trapman, 1994; Xu
et al.
, 1995), scientists are likely to question the accuracy of
these new simulation models. The problems of measuring leaf wetness are well
known (Huber and Gillespie, 1992), yet scientists rarely questioned the accuracy of
wetness duration measurements. Even study to study variation in the placement of
wetness sensors, from within trees to between tree rows to outside the orchard, can
make a big difference in the duration of wetness readings. In most of the cases
reviewed by MacHardy and Gadoury (1989), it was not possible to determine
whether the greater sensitivity of the warning system relative to Mills's table is due
to increased sensitivity of weather recording instruments or to a modification of the
Mills criteria. Consistently underestimating wetting periods would result in the
conclusion that infection actually occurs in fewer hours than Mills's table indicates.
Jones
et al.
(1980) and Xu
et al.
(1995) both incorporated provisions in their models
to compensate for possible errors in determining the length of the wetting period.
Until wetting periods can be evaluated consistently from region to region, models
developed in one region may or may not work in another, depending on the amount
of error associated with wetting period measurements.
18.3.4 Modifying Mills's criteria for changing fruit susceptibility
In the Mills system, the relationship between leaf wetness, temperature and infection
of fruit and of young apple leaves is considered to be identical. However, as apple
fruit mature, infection periods that result in infection on young leaves often fail to
result in infection on fruit (Tomerlin and Jones, 1983). The hours of continuous
wetting needed for fruit infection increases exponentially in the 1-23 week period
following full bloom (Schwabe
et al.
, 1984). A model was developed for estimating
the percentage of fruit infection based on temperature, wetness duration and fruit
age. The percentage of fruit infection (
P
) can be estimated from the equation:
P = (-6.7240 + 0.3407W + 0.7265T - 0.0043W
2
- 0.0228T
2
+ 0.0070WT)
2
(18.1)
where
T
is temperature (°C) and
W
is an estimate of the hours of wetting needed for
infection at that stage of fruit development. Estimates of
W
are from the following
equation:
W = W/{(-6.482 + 119.4X
0.5
)/120}
(18.2)
where
X
is the number of weeks past full bloom,
120
is a scalar that adjusts the
duration of wetness according to the age of the fruit, and
W
is the hours of wetting
observed during the wet period. Since highly susceptible leaves are present on
