Combining a single-fungicide with a multi-site one in tank mixes or in formulated combinations
is one approach to reducing the dose of the "at risk" fungicide and reducing the rate of selection
of resistance. Another is to use the fungicides at full dose but reducing the number
of applications of each in some kind of integrated spray schedule.
Let us try a simple such schedule with benomyl and captan, applying each at the full,
recommended dose but applying only half the number of sprays of each.
Reinitialize the model as before, using Load Data File to load the
Venturia data file. And as before select both benomyl and captan in the
Fungicides, Select... dialogue box. Then click on
Fungicides, Schedules... and bring up the captan spray schedule.
Eliminate the first eight of the sixteen sprays by entering a "0" in the first eight spray dates.
(Don't worry about the dose. No spray will be applied if it has a "0" for the date.)
Then bring up the benomyl spray schedule and eliminate the last four benomyl sprays in the same
manner. This should leave you with four benomyl sprays at biweekly intervals followed
by eight captan sprays at weekly intervals.
Run the simulation for five consecutive seasons, an examine the year-end
summary in the Log.
What is the effect of on the selection of benomyl resistance of applying fewer benomyl
sprays and maintaining control of apple scab for the rest of the season with captan?
In the field, resistance of Venturia to a fungicide is determined by
swabbing a lesion across an agar medium amended with the fungicide and
observing spore germination after 10-12 hours of incubation. To even
detect apple scab in the field with reasonable certainty, there would have
to be more than 100 lesions per acre, and to get enough lesions for a
resistance test would probably require ten times that number. Typically the
resistance test would be done at the end of the season or when one noticed
an apparent "failure" of the fungicide because an unexpectedly high number
of lesions. What would be the level of resistance when it would first be
detected when benomyl is sprayed alone (Exercise 2)? What would it be if 4 sprays of benomyl were
followed by 8 sprays of captan each season?
