Agriculture Reference
In-Depth Information
shoots until leaf emergence has finished, adjustments for changing fruit
susceptibility with age are usually not made until shoot growth stops. The model is
particularly valuable for evaluating the importance of long continuous wet periods
late in the season.
18.3.5 Latent period predictions
The Mills table can also be used to predict the approximate time in days for
development of scab lesions on young leaves. This information is a valuable guide
to advisers and growers on when to inspect orchards for lesions following predicted
infection periods. Lesions with conidia may be visible 9 to 17 days after the onset of
a wet period suitable for infection (Mills and LaPlante, 1951). The time required for
visible lesions is a function of temperature and relative humidity. The mean
temperature for the first five days of the infection period is used to predict days from
infection to visible lesions. High relative humidities favour sporulation, low
humidities delay lesion development and can extend the latent period for several
days (Tomerlin and Jones, 1983).
18.4 PREDICTING APPLE SCAB RISK BASED ON PRIMARY INOCULUM
LEVELS
After the Mills system, perhaps the greatest advancement in timing of fungicide-
based strategies for controlling apple scab has been the development of both indirect
and direct methods for estimating the potential release of ascospores during periods
of environmental risk. Fungicide applications for scab control are initiated at bud
break (green tip) in the spring to coincide with the beginning of the ascospore
release season. Both application frequency and the quantity of fungicide per
application are often relaxed at the end of the ascospore release season.
Mills's assumption that primary inoculum is abundant during all environmental
risk periods is often incorrect today because of the high level of scab control
achieved with modern fungicides. Primary inoculum may not be important at times
due to year-to-year variation in when the release season begins or ends, suppression
of ascospore maturity, often drought related, during the release season, and variation
among orchards in potential ascospore dose. Knowledge of inoculum risk is as
important as knowledge of environmental risk in designing scab control programmes
aimed at reducing fungicide use on apples.
18.4.1 Estimating ascospore maturity and discharge
Visual assessments of pseudothecia are useful for estimating the beginning and end
of the ascospore release season and have been used for nearly sixty years in the
United States to time the initiation and the end of control programmes for primary
apple scab (Gilpatrick and Szkolnik, 1978; Gadoury
et al.
, 1992). During the
ascospore release season, visual assessments are useful for estimating the potential
for ascospore release in the event of rain. Although the green tip stage of tree
