Scab respray model
Use the appropriate primary scab respray guidelines
instead of this table until ascospore releases are completed.
This table estimates fungicide respray intervals for
standard surface protectant fungicide (e.g. captan, mancozeb, Polyram, but not
sulfur) during the transitional period after the end of primary scab ascospore
releases and before 2nd generation scab lesions have had time to begin showing.
If there is doubt about the success of primary scab
control, it is prudent to maintain enough fungicide protection to prevent the
spread of secondary scab during this period so that if secondary scab does
appear, it will not have had a chance to spread to unprotected fruit and foliage
prior to detection.
After 2nd generation scab lesions from the season's
final ascospore release have had time to begin appearing, if scouting confirms
that scab control has been successful, then there is no need to keep using this
table. In that case, subsequent fungicide applications can be planned according
to the extended intervals adequate to prevent
flyspeck and other summer diseases.
Green text and yellow background in the table indicates
dates after the final primary scab control spray until the date scab control can
be confirmed by scouting for presence of 2nd generation scab lesions.
The Protection End Dates shown in this table are
based on informal 'rules of thumb', as follows:
For spray dates prior to fruit set, a full dose,
good coverage protectant fungicide application is counted as protection against
spread of secondary scab for up to 2 inches of rain in the 9 'average growth days' following
application, or up to 1.5 inches in days 9-11. With less than 1.5 inch of
rain, coverage ends at 11 growth days.
'Growth days' are counted as days of
average cumulative growth units (degree day base 43F), not calendar days.
With hot weather, 9 growth days might accumulate
in less than 7 calendar days.
With no rain and hot weather,
minimum coverage is set at 7 calendar days . Conversely, even with very cool weather and no rain, coverage ends at a maximum of 14
For spray dates after McIntosh fruit set, but at least
14 days before estimated terminal bud set, a full dose protectant fungicide
application is counted as protection against secondary scab for up to 2 inches
or rain or 14 days average growth, whichever comes first. Terminal bud set
averages about 6 weeks after petal fall, so this group includes spray dates from
fruit set until about 4 weeks after petal fall.
With no rain and hot weather,
minimum duration of coverage from these spray dates is set at 10 calendar days . Conversely, even with very cool weather and no rain, coverage ends at a maximum of 18
For spray dates within 14 days of terminal bud set
until the end of August (i.e. from about 4 weeks after petal fall until the end
of the spray season),
growth days are replaced by calendar days. For this group, coverage ends at 2 inches
subsequent rain or 14 calendar days, whichever comes first.
Brown text with grey background
in the table indicates
dates when 2nd generation scab lesions from the final ascospore
release have had time to begin appearing. By this point, growers will have
had opportunity to confirm the status of primary scab control. Estimates for these dates are provided only
for those situations where scab control has not been successful, and there are enough
active scab infections present to cause an
unacceptable risk of fruit scab infection.
In that case, fungicide protection is needed for at
least 4 weeks until active scab lesions exhaust their supply of spores.
Depending on scab severity, market sensitivity, temperatures, rain frequency, and
proximity to harvest; shorter or
longer fungicide intervals to suppress spread from active scab lesions may be
more appropriate than the estimates shown in the table.