There is a bit of a disagreement among practitioners about whether fungicide
resistance evolves faster when a fungicide is used to bring a severe outbreak
under control than it does when the fungicide is used to maintain control
when the inoculum level is already low. Those who have observed in the field
the gradual failure of a fungicide to control plant disease tend to argue that
a fungicide that is "vulnerable" to resistance should not be used when the
level of disease is high because it will "fail" much faster than if it were
used only to maintain a low level of disease. Those who are steeped in
epidemiological theory tend to argue that the most effective fungicides
(and coincidently the most vulnerable to resistance) should be used only
when the disease levels are high and to replace them with the old standbys,
such as captan, as soon as the severe epidemic has been brought under control.
The reason for these diametrically opposing views may arise from the
perception of failure of the fungicide and the apparent
evolution of resistance. Let us try to use simulation to test this idea.
We can consider the simulation that we have just completed (an initial
inoculum of 5000 spores/acre) a very high level of infection resulting
from poor scab control the previous season. Let's compare it to a season
that starts with only 1/10 the level of initial inoculum.
Again reload the Venturia dataset by copying it and pasting it in the
Load Data File window. Don't forget to click on the "Load Data File"
bar at the bottom of the window.
Still using the default benomyl spray schedule, click
on the Fungus menu and select Inoculum..., change 5000 to 500,
and click "Save Settings." Click on Begin New in the Simulation
menu, and run the season to its completion. Compare the final number of lesions
and the final percent resistance with those at the end of the first season
in the previous exercise (the same benomyl spray schedule but 10 times as
much initial inoculum). Continue the simulation for 3-4 seasons, each time
comparing the lesion number and the percent resistance at the end of the season
with the comparable values for the severe disease simulation. As in the
previous exercise, copy and paste the summary of the year-end values in the
Log into a text file as a record for later reference.
Do we risk a higher evolution (rate of selection) of resistance if we use
benomyl as a "rescue" treatment (that is, to bring an epidemic under control
after the inoculum has been allowed to rise to a high level) than if we use
it only to maintain control of a low level of infection? (Explain.)
Would you arrive at a different conclusion if instead of monitoring resistance
you were simply observing the number of lesions as as indication of "failure"
of control? (Explain.)
