Exercise 2: Benomyl
Benomyl belongs to a chemical family called benzimidazoles, which also inludes
the fungicides carbendazim, thiabendazole, and thiophanate-methyl.
Benomyl is far more effective than captan in suppressing the development
of the fungus and reducing the carryover of initial inoculum the following
season. When benomyl first became available in the early 1970s, it was widely
acclaimed for its ability to combat apple scab under severe disease pressure.
Unknown at the time was the vulnerability of benomyl to a phenomenon that was soon
to be widely recognized: fungicide resistance.
To be sure that Resistan is properly initialized, reload the Venturia dataset
by copying it from the Venturia dataset window (which should still be open) and
clicking on Load Data File in the Simulation menu. Paste
your copied text there as before, and click on the "Load Data File" bar at the
bottom of the window.
Again click on the Fungicides menu and click Select....
In one of the boxes, select "benomyl" and then click on "Done."
The default spray schedule is to spray captan every 14 days,
beginning on day 1, for a total of 8 applications at a dose of 0.5 pounds
per acre. You can confirm this by selecting Schedules... in the
Fungicides menu.
As before, select Begin New in the Simulation menu, and click
the arrow button at the right of the scroll bar until the season progresses
to the end. Note how few lesions there are at the end of the season compared
with the first season when captan was used. This means far fewer lesions to
produce carryover inoculum for the following season. Continue the simulation
for another season by clicking Continue in the Simulation menu.
During the second season, note the line rising exponentially from near zero
at the beginning of the season and leveling off near 100% toward the end
of the season. This is the percent of the fungus population that is resistant
to benomyl. Note also that the last 3 or 4 sprays of benomyl were no longer
very effective and that the season ends with many more lesions than the
previous season (but still fewer lesions than after the second season with
captan). Continue until no further simulation is possible. Click on Log
and note the year-end summary at the bottom of the log file. Copy and paste the
summary to a text editor to keep a record for comparison with the results
of the exercises to follow.
What happened?
In order to detect a problem of resistance, the grower could simply wait
for failure of control, or she/he could routinely monitor fungicide resistance
at the end of each season by plating spore samples on benomyl-amended media.
When would the grower become aware of a resistance problem in each case
(with and without monitoring resistance)?
....proceed to EXERCISE 3
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