There is also disagreement among practitioners about the wisdom of reducing
the dose of a fungicide, especially of those fungicides vulnerable to resistance. Many
of those who have observed fungicide failures under different conditions
in the field will tell you that using any dose lower than the highest
recommended dose simply invites problems with resistance. On the other hand,
selection theory says that the higher doses select resistance faster, which
suggests that we should use only as high a dose as necessary to achieve
satisfactory control.
Again, this may be a matter of the perception of fungicide failure based
on the numbers of lesions that we see, rather than the measured buildup
of resistance. We can also use simulation to answer this question.
The default spray schedule used in Exercise 2 is based on the recommended
dose of benomyl in each spray. Let's see what would happen if we used half
the recommended dose.
Reinitialize the model by copying and pasting the Venturia dataset into
Load Data File once again. Select "benomyl"
in the Fungicides menu again. Then select Schedules... and
find the spray schedule for benomyl. In the "Dose For All" box, enter .25
and click "Apply Dose." (Do not worry if .25 appears in all the dose boxes;
if the day is 0, no spray will be applied.) Click "Save Settings" and "Done."
Run the simulation to the end of the season and compare the final number
of lesions and the percent resistance to benomyl with those of the full dose
of benomyl (Exercise 2). Continue to run the simulation for several consecutive seasons
until the disease control appears to fail. Compare the number of lesions and
the benomyl resistance at the end of each season with those of the simulation
with the full dose of benomyl (.5 lb/A).
What effect does reducing the dose of benomyl to half the standard dose
have on the rate of selection of resistance?
What effect does reducing the dose have on the effectiveness of the disease
control?
What can be done to improve the effectiveness of benomyl applied at the
reduced dose?
