Agriculture Reference
In-Depth Information
greenhouse conditions (Stensvand
et al.
, 1997). It is likely the Mills system has been
used for decades without problems, except for temperatures below 8°C, because
minimum infection times in the field are longer than minimum infection times in
growth chamber experiments. It is probable that when false positives and false
negatives are considered, there will be little if any improvement in the identification
of scab infection periods by substituting the revised table for the modified Mills
table; however, the two systems must be compared rigorously over several growing
seasons to establish if revised Mills is a significant improvement.
The revised Mills table proposed by MacHardy and Gadoury (1989) and by
Stensvand
et al.
(1997) made no distinction between light, moderate and heavy
infection periods. Mills noted that the severity of an infection period would increase
as the hours of wetting increased because potentially more inoculum could reach and
infect the host. Data from volumetric spore trapping studies in the field indicate that
some daytime discharge events are prolonged requiring six to eight hours of rain
before 50% of the ascospores are trapped while other discharge events are
concentrated requiring just two hours of rain before 50% of the ascospores are
trapped (Rossi
et al.
, 2003). As the duration of a wetness period increases beyond
minimum infection times, more ascospores, particularly delayed-release ascospores,
can infect thereby increasing disease severity. Mills's severity level system is often
criticised because a light Mills period may result in more disease than a moderate or
severe Mills period. However, inoculum dose often accounts for the difference in
disease severity between two apparently similar infection events. It is often forgotten
that the severity of a particular infection period is determined not only by
favourability of the physical environment as judged by Mills, but also by the
potential ascospore dose (PAD; see Gadoury and MacHardy, 1986) which is rarely
known. A prediction for light infection means that the minimum conditions for
infection have been met and disease will develop provided inoculum is present
(inoculum was assumed to be present by Mills) and no control is attempted. Severity
predictions are important because, based on circumstances beyond Mills, a grower
may respond to a prediction for light infection differently than to a prediction for
moderate or heavy infection.
18.3.3 Other scab predictive systems based on physical environment
Trapman (1994) and Xu
et al.
(1995) have developed simulation models for
detecting scab infection periods and have incorporated them into personal computer-
based warning systems called RIMpro and Ventem™, respectively. Research on
these systems was stimulated by work on microprocessor-based instruments for
disease prediction in the early 1980s (Jones
et al.
, 1980). Unlike scab warning
systems based on the Mills criteria, simulation models can deal with the dynamic
nature of weather and give growers real-time predictions of disease. Rainfall,
surface wetness, temperature and relative humidity data are used to determine, for
each wetting period, the proportion of spores that successfully infect the leaves,
referred to as relative infection measure (RIM) by Trapman (1994) and as infection
efficiency (IE) by Xu
et al.
(1995). Scab control obtained in 1992 with three curative
