Agriculture Reference
In-Depth Information
development is often used as a biofix for the beginning of the ascospore release
season, maturity assessments can be used to establish in a particular year whether
ascospores are maturing earlier or later than green tip.
Microscopic examination of the contents of pseudothecia crushed on glass
slides is a widely used method for assessing ascospore maturity (Szkolnik, 1969;
Gadoury and MacHardy, 1982a) but such examinations require considerable skill
in interpretation (Gadoury
et al.
, 1992). The maturity status of each pseudothecium
is judged, based on classification of ascospore maturity in each ascus into five
categories. Data from 20 to 25 pseudothecia taken from several freshly collected
leaves in each orchard are summarized and provide an estimate of the percentage
of mature ascospores. Pseudothecial maturity can also be determined by sectioning
fixed specimens (James and Sutton, 1982a) but this method is very time consuming.
Laboratory methods for estimating ascospore maturity are often supplemented
with ascospore discharge tests. Several methods have been developed for assessing
the potential release of ascospores of
V. inaequalis
from infected leaves or leaf discs
freshly collected from the field and include spore release towers and tunnels (Hirst
and Stedman, 1962; Brook, 1969; Gilpatrick
et al.
, 1972), aerated or agitated water
in bottles or flasks (Hutton and Burchill, 1965; Kollar, 1998), or simply mounting
moist leaf tissue on the top inside surface of Petri dishes (Szkolnik, 1969).
Ascospore discharge tests are easy to conduct and they provide a more accurate
estimate of release potential than pseudothecial assessments. Approximately 10 to
15% of the asci need to be rated as mature before an ascospore release season is
initiated (Gadoury
et al.
, 1992) and ascospore discharge tests with spore release
towers may overestimate the number of spores likely to be airborne in an orchard
late in the season (Aylor, 1996). Also, cumulative ascospore release when measured
by microscopic examination of crushed pseudothecia and discharge tests lags behind
cumulative estimates made with volumetric spore traps in the field (Gadoury
et al.
,
2004).
The results from ascospore discharge tests indicate that ascospores can be
discharged from leaves freshly collected over a 10 to 12-week period but the exact
duration of the discharge period is governed by temperature (Gadoury and
MacHardy, 1982b; James and Sutton, 1982b) and can be limited by lack of moisture
(James and Sutton, 1982a). Peak discharge occurs between the pink and bloom
stages of apple flower bud development, with the discharge pattern approximately
normally distributed between green tip and four to six weeks after petal fall
(Gilpatrick and Szkolnik, 1978; Villalta
et al.
, 2002). The accumulated number of
ascospores discharged from leaves approximates a sigmoid curve. Ascospore
discharge data, when combined with meteorological data, can provide usable
estimates of aerial spore concentrations (Aylor and Qiu, 1996). By knowing the
concentration of ascospores in air above an orchard and the number of lesions
resulting from deposition of ascospores on susceptible tissue (Aylor and Kiyomoto,
1993), it will become possible to evaluate the need for fungicide applications based
on inoculum and environmental risks combined.
