A new formulation of the model of Spotts et al. 1991 is being tested using the Hood River 15 minute data.
The model is based on the accumulation of degreehours during times of sufficient leaf wetness.
It is believed (during this testing of the model), that 320 degreehours are needed for an infection event to occur.
The primary reference for this model is:
Spotts, R. A., and L. A. Cervantes. 1991. Effect of temperature and wetness on infection of pear by Venturia pirina and the relationship between preharvest inoculation and storage scab. Plant Dis. 75:12041207.
In addition, further laboratory studies will be conducted (Spotts et al.) to help validate the new formulation of the model in terms of degreehours (new degreehour algorithm and implementation by Len Coop, see below). Degreehours have been used for other plant diseases including Fire blight, and should be a simple and familiar way of tracking pear scab infection risk, once the model is properly validated. Disclaimer: use this and all models at your own risk. This model is new and some aspects will change within the next few months or years  feel free to ignore it. We ask that you simply TEST it during the 20012002 seasons, and report your findings (see below).
To use the model, check the current 15 minute data files, for example, MCAREC. Currently, these files have 3 extra columns indicating model status:
scnt:
This column keeps track of the number of 15 minute periods for which no moisture (leafwetness < 2) was recorded.
When the count reaches 48 (12 hours), any pear scab cumulative degree hours (CUM_DH) is reset to zero. Be sure to note whether the leaf wetness sensors are working correctly, and whether they are representative for your own orchard blocks. Moisture conditions can vary greatly locally.
cum_DH
This column keeps track of the cumulative degreehours relevant to pear scab. During periods of leaf wetness,
temperatures above 32 F contribute to the accumulation of degreehours. Temperatures above 66 are kept at 66 because studies indicate the development rate of pear scab ascospores does not increase above this temperature.
message
These are the text messages that depend on the cumulative degreehours:
if CUM_DH < 250 the message is "No scab risk "
if CUM_DH > 250 the message is "Scab cycle nearing"
if CUM_DH > 320 the message is "Scab cycle started!"
if CUM_DH > 350 the message is "Scab cycle ongoing!"
The text labels (or cum_DH) may be monitored for today or up to five days ago to check whether an infection event has occurred. At this time, no (future) forecasting of infection events is conducted.
The testing and comments for the model are being coordinated by Robert Spotts, MCAREC,
robert.spotts@orst.edu. You might
track any infection events versus this and any other models you are using and
report your findings back to Dr. Spotts.
To document the model algorithm, we include it below:
###############
#
# New Pear Scab model based on DegreeHours accumulated during moist
# periods of leafwetness  uses 15 minute temperature and leafwetness readings
#
###############
DH = current degree hours relevant to pear scab
cumDH = cumulative degree hours
nomoistcount = no. 15 minute periods that leafwetness < 2
ctemp = current temp
pstemp = temp relevant to pear scab
Every 15 minutes:
if leafwetness < 2
add 1 to nomoistcount
DH = 0
if nomoistcount > 47
cumDH = 0
else
nomoistcount = 0
if ctemp < 32
DH = 0
else
if ctemp > 66
pstemp = 66  32
else
pstemp = ctemp  32
DH = pstemp/4
add DH to cumDH
if cumDH < 250
risklabel = "TEST:no_risk"
if cumDH > 250
risklabel = "TEST:infection_cycle_approaching"
if cumDH > 320
risklabel = "TEST:*infection_cycle_started!*"
if cumDH > 350
risklabel = "TEST:*infection_cycle_ongoing!*"
display nomoisturecount, cumDH, risklabel
###############
Apple scab model  brief documentation.
This model is similar to the Pear scab model but was derived from currently used modified Mills tables, and takes the same degreehour approach as with pear scab above. Apple scab responds to temperauture more rapidly than does pear scab, and has slightly different thresholds.
Algorithm:
########
# Apple Scab model inverted Mills Table: degreehours  infection risk only
# hourly version: expecting good hourly data
########
if ( $lfwetness < 1 ) {
$Anomoistcount++;
$ADH = 0;
if ( $Anomoistcount > 8.8 ) { $AcumDH = 0 }
}
else {
$Anomoistcount = 0;
if ( $temp < 32 ) { $ADH = 0 }
else {
if ( $temp > 66 ) { $ADH = 66  32 }
else { $ADH = $temp  32 }
$AcumDH += $ADH;
}
}
if ( $AcumDH < 170 ) { $Alabel = $gre . "no app_scab" . $stp }
if ( $AcumDH >= 170 ) { $Alabel = $yel . "scab near " . $stp }
if ( $AcumDH >= 195 ) { $Alabel = $red . "APPLE SCAB!" . $stp }
if ( $AcumDH > 275 ) { $Alabel = $red . "SCAB cycle!" . $stp }
If you have comments or corrections to make, send them to:
coopl@science.oregonstate.edu.
