Agriculture Reference
In-Depth Information
normal curve areas, or the standard normal cumulative distribution function (such as
NORMSDIST(Z) in Microsoft's Excel). When using tables or a distribution function,
the estimated probit value is reduced by a scaling factor of five. This model uses an
easy-to-identify biofix or starting point and predictions are relatively accurate since
degree-days are accumulated for a few weeks, compared with a few months in the
New York and North Carolina models. However, moisture can limit ascospore
maturation (James and Sutton, 1982a) and prolonged dry periods during the spring
may temporarily interrupt maturation, thereby reducing the accuracy of this degree-
day model (Rossi
et al.
, 1999; Schwabe
et al.
, 1989).
A method for estimating the risk of airborne ascospores in Italian apple orchards
was developed by Giosuè
et al.
(2000). The model was designed to replace or
augment visual assessments of ascospore maturity and discharge. Daily changes in
pseudothecial development were estimated using a model developed by James and
Sutton (1982b) and a modification by Mancini
et al.
(1984). Once pseudothecia
were predicted to contain the first mature ascospores (stage st = 9.5) a second model
was used to predict the number of ascospores expected to be discharged during each
wetting period. The model used a cumulative airborne dose measured with
volumetric traps in a previous season to quantify the expected discharge. The
driving variables, hourly temperature and leaf wetness, were used to accumulate
degrees above 0°C when leaves were wet. A Mills table was used to determine the
risk of infection on rainy days with predicted ascospore discharge.
18.4.4 Assessing differences in inoculum density among orchards
Perhaps the research on inoculum density by MacHardy and co-workers has had the
greatest effect on scab control strategies of any work on scab in the last 30 years.
Prior to this research, scab programmes were initiated based on estimates for first
ascospore release using leaves taken from locations with high levels of over-
wintering inoculum, which meant that spray programmes were normally initiated no
later than green tip. Gadoury and MacHardy (1986) developed an assessment
method, based on the incidence and severity of apple scab just before leaf fall and
the amount of leaf litter present at bud break, for establishing the potential inoculum
density (PAD = potential ascospore density) in orchards. In orchards with a low
PAD, it was found that the initiation of scab spray programmes in spring could be
delayed for two to three weeks after budbreak compared to orchards with high levels
of inoculum (Wilcox
et al.
, 1992; MacHardy
et al.
, 1993). Thus, in low inoculum
orchards, the first scab sprays coincided with the tight-cluster to pink stage of
flower-bud development rather than green tip. Although a model was developed for
estimating potential ascospore dose in individual orchards (Gadoury and MacHardy,
1986), it is somewhat complicated and seldom used in practice. To reduce possible
risk from unprotected infection periods before the first sprays were applied, Wilcox
et al.
(1992) proposed that a series of sprays with sterol inhibitor fungicides with
post-infection control activity be used in orchards where early season sprays had
been omitted. However, the selection of scab strains with reduced sensitivity to the
sterol inhibitor groups of post-infection fungicides in several areas of the world will
