Agriculture Reference
In-Depth Information
18.4.2 Monitoring ascospore release in orchards
Once infection periods are predicted based on the physical environment, the number
of ascospores released into the orchard air (dose) largely determines the number of
spores subsequently deposited on leaves and fruit. Because of the nature of a
particular wetting period or because of changes in ascospore maturation after
sampling, actual release of ascospores in the orchard may not always reflect
laboratory estimates. To obtain real-time estimates of total spore dose during each
wetting period, some advisory programmes use spore samplers to measure the aerial
concentration of ascospores in the orchard (Sutton and Jones, 1976).
Burkard volumentric spore traps and Rotorod samplers are instruments used to
monitor and estimate airborne ascospores in orchard air during natural rain events
(Aylor, 1993; Sutton and Jones, 1976). The Burkard trap, because of its high cost
and complex design, is used primarily as a research tool. Rotorod samplers, because
of their lower cost, efficiency and ease of operation, can be used to determine total
ascospore dose during each wetting period. Due to the highly aggregated pattern of
ascospore distribution across an orchard (Charest
et al.
, 2002), spore traps and
samplers are normally placed in locations likely to yield high numbers of
ascospores. Sometimes overwintering infected leaves are placed at the trapping site
to increase the potential for airborne ascospores. Used in this way, spore trapping
devices are useful for detecting discharge events to time fungicide applications. In
Michigan, field advisers monitor airborne ascospores in orchards with Rotorod
samplers and make information on ascospore release available to growers via self-
answering telephones, fax networks, electronic mail and radio (Jones, 1995). The
main advantage of using spore samplers is that the variable effects of weather and
environment on release are reflected in the monitoring data.
18.4.3 Models of pseudothecial development and ascospore maturity
Initial efforts to predict ascospore maturity involved the use of degree-day
accumulations starting from the date of 50% leaf fall for a New York model (Massie
and Szkolnik, 1974) or from the time when asci began to develop for a North
Carolina model (James and Sutton, 1982b). The New York model predicted mature
ascospores much earlier in North Carolina than they were observed in nature (Sutton
et al.
, 1981), while predictions with the North Carolina model were in close
agreement with observations in Italy on the maturation of ascospores (Mancini
et al.
,
1984). These studies were the first to show that the rate of ascospore maturation in
nature was directly related to temperature.
Degree-day accumulations starting with the first appearance of mature ascospores
in nature or with the green-tip stage of bud development are very useful for predicting
ascospore maturity using daily temperatures. The maturity of ascospores of
V.
inaequalis
can be estimated from the following model:
Y
= 2.51 + 0.01
X
, where
Y
is
the probit of proportion of matured ascospores and
X
is accumulated degree-days
(base 0°C) from the first appearance of mature spores (Gadoury and MacHardy,
1982b). Transformation of estimated probit values to percentage values can be
accomplished using a graph (Gadoury and MacHardy, 1982b), a table of standard
