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. 75:1204-1207.

In addition, further laboratory studies will be conducted (Spotts et al.) to help validate the new formulation of the model in terms of degree-hours (new degree-hour algorithm and implementation by Len Coop, see below). Degree-hours 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 2001-2002 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 degree-hours relevant to pear scab. During periods of leaf wetness, temperatures above 32 F contribute to the accumulation of degree-hours. 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 degree-hours:

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 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 Degree-Hours 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
###############

This model is similar to the Pear scab model but was derived from currently used modified Mills tables, and takes the same degree-hour 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: degree-hours - 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 }