Agriculture Reference
In-Depth Information
18.3.1 Using Mills's criteria for scab prediction in practice
The Mills table uses the number of hours of continuous wetness from the beginning
of rain and the average temperature during the wet period for predicting scab
infection. In practice wet periods are not always continuous resulting in split wetting
periods. Several procedures have been suggested for how to deal with dry periods
between rains in the Mills system. Mills initially suggested that wetting periods
separated by no more than four hours be added together but later he changed it to
twelve hours (half a day) or more with sunny weather. Studies in South Africa with
ascospores indicated that wetting periods separated by dry intervals of sixteen hours
or less should be combined (Schwabe, 1980). Work in The Netherlands indicated
that the twelve hour dry interval proposed by Mills should be shortened to eight
hours (Rooseje, 1963), an interval used in some warning systems (Jones
et al.
,
1980). With conidial inoculum, several researchers (Schwabe, 1980; Becker and
Burr, 1994) have suggested that wetting periods should be added together when dry
intervals are shorter than 32 to 48 hours. Becker and Burr (1944) proposed a series
of regression equations for determining the proportion of viable conidia remaining at
the start of secondary wetting periods. Of these various suggestions, the eight-hour
rule has worked reasonably well when predicting infection from ascospores.
Mills's table (Mills and LaPlante, 1951) indicated that predicting infection from
conidia would require one-third less wetting time than for ascospores; this has
prompted numerous studies on the time required for infection by ascospores and
conidia. Results from greenhouse and moist chamber studies conducted in the
United States (Keitt and Jones, 1926; Stensvand
et al.
, 1997), The Netherlands
(Roosje, 1963), Belgium (Sys and Soenen, 1970), UK (Moore, 1964) and South
Africa (Schwabe, 1980) all indicated that infection times for conidia are similar to or
slightly longer than those for ascospores when spore suspensions are applied directly
onto the host. Although these results suggest that the infection times used for
ascospores and conidia in the Mills system should be the same (MacHardy and
Gadoury, 1989; Stensvand
et al.
, 1997), none of the studies address the question of
whether ascospores require more time than conidia to reach host tissue. Again, it is
generally forgotten that Mills had reasoned, based on the spore release data available
at that time, that it would take about three hours for a significant number of
ascospores to be deposited on susceptible apple tissues while the deposition of
conidia would be nearly instantaneous with the onset of rainfall.
The significance of night-time release of ascospores and how to utilize this
information in advisory services and predictive models based on Mills has been
debated since Brook (1969) first demonstrated that the ascospore release response
for
V. inaequalis
was stimulated by far red light and suppressed by darkness. When
day-time and night-time rain events were compared, ascospore release was often
delayed until sunrise during night-time rain events while release was initiated soon
after the onset of day-time rain events. The cumulative number of ascospores
trapped at night in orchard air was found to represent a small percentage of the total
number trapped each season. Spore trapping studies conducted in several countries
indicated that 80-95% of the ascospores were trapped during the daylight hours
(Brook, 1966; MacHardy and Gadoury, 1986; Villalta
et al.
, 2002). However, a
