All of the evidence so far suggests that in the absence of the fungicide
Venturia populations resistant to benomyl are as fit ecologically as the
wildtype populations are. However, with some of the other new fungicides,
the resistant biotypes seem to be significantly less fit than the wildtype
in the absence of the fungicide. The simulator allows us to create a
hypothetical fungicide with all the characteristics of benomyl but with
a slightly reduced fitness of the resistant biotype compared with the
wildtype. We can then look at the effect of this "fitness cost" on the rate
of selection of resistance and on the rate of reversion to the sensitive
wildtype. To observe this phenomenon better, we first had better do a run
with no reduction in fitness to resistance to establish a baseline for
comparison and follow the selection of resistance during the course of
one season. (In this case the values at the end of the season do not tell
the whole story.)
Reinitialize the model again with the Venturia parameter set, and then select
both captan and benomyl for application. Bring up the captan spray schedule
and eliminate the first four sprays by entering a "0" in their spray dates.
Eliminate all but the first 2 benomyl sprays in the same manner.
To give us a visible level of resistance at the start, in the Fungus menu,
click on Resistance..., and enter 10% on the benomyl line. Run the
simulation for one season only and observe the pattern of resistance.
Copy and paste the entire season's Log into a text file for later
comparison.
Now that we have a baseline, let us do the same thing, but with a small
fitness cost to the resistant biotype. In the Fungus menu, reset the
Resistance... to benomyl to 10%. Keep the same spray schedule as
before with two sprays of benomyl followed by captan. This time click on
the Fungicides menu, select Characteristics..., and enter the
value of 0.9 in each of the boxes corresponding to "Spore Survival",
"Lesion Development", and "Sporulation". (Note: A value of 1.0 means that
the fitness of the resistant biotype is equal to that of the wildtype.)
Click "Save Settings" and "Done."
How does a small fitness cost affect the rate of selection of resistance?
If there is a fitness cost, what happens when you stop using the fungicide
to which there is resistance?
Look at the characteristics of myclobutanil. How would you expect myclobutanil
to differ from benomyl when it comes to managing resistance?
