Agriculture Reference
In-Depth Information
complicate and limit the potential use of this scab control strategy (Köller
et al.
,
1997; Schnabel and Jones, 2001).
Overwintering of scab in lesions on apple twigs should also be considered as part
of the inoculum assessment process. The risk of overwintering conidial inoculum is
greatest in orchards where scab was a problem the previous autumn (Becker
et al.
,
1992). Conidia of
V. inaequalis
are detected in the spring in overwintering scab
lesions on shoots and on the interior and exterior of bud scales in some areas of
Europe (McKay, 1938; Cook, 1974; Hills, 1975; Moosherr and Kennel, 1995) and
the United States (Becker
et al.
, 1992). The viability of the overwintered conidia on
the outer surface of twigs and bud scales is very low and therefore this inoculum
source is unlikely to cause scab outbreaks (Holb
et al.
, 2004). However, the viability
of conidia found on inner bud tissues is much higher and these conidia are the ones
likely to cause early season scab outbreaks (Holb
et al.
, 2004). In orchards with
overwintering conidial inoculum, the release of conidia would be likely to occur
with the onset of rain. Because of the risk of conidial infection at night, the
assumption in revised Mills that the release of ascospores from night-time rains
begins at sun-rise, should be ignored.
18.4.5 Integrating inoculum and environmental risks into warnings
The Mills table provided extension agents in New York State with a tool for
developing scab warnings based on environmental risks, and inoculum assessments
were soon incorporated to reinforce these warnings. Extension agents, who were the
integrators, worked closely with local meteorologists so that warnings were based on
the best available weather information, and with local radio station and post-office
personnel to ensure that the warnings were disseminated quickly. As scab warning
systems proved successful in reducing problems with apple scab control, they were
quickly adapted for use in other countries. Because of grower acceptance, there is
still a demand for these systems, but they are harder to maintain because of
declining local resources.
Scab warning systems continue to be improved and should enhance the
competitiveness of apple growers in the future. Microprocessor technology and
personal computers have stimulated the development of new models and instruments
for scab prediction (Jones
et al.
, 1984; Trapman, 1994; Xu
et al.
, 1995). In Europe,
as part of local IFP efforts, modern scab warning systems are being developed in
several countries (Butt, 1994). There are indications that scab warning systems will
continue to advance as new knowledge on apple scab and new technologies improve
their reliability, affordability, convenience and relevance to current and future scab
control techniques.
18.5 SUMMARY
Research on the epidemiology of apple scab during the last thirty years has been
dominated by studies aimed at improving the Mills criteria for predicting apple scab
infection. It is a tribute to Mills and to the robustness of his table that, after more
