Exercise 1: Captan
Captan belongs to a chemical family called dicarboximides, but to distinguish it
from the newer systemic dicarboximides, it is better to call it more specifically
a phthalimide.
No resistance to captan has yet been observed in
V. inaequalis, despite its use for almost 50 years, while resistance
to benomyl was reported in some orchards after only 3 years of use. The
parameters in this data set correspond to rates of fungicide weathering and
rates of infection that would occur on a very susceptible cultivar in an
unusually rainy season, and the initial inoculum (5000 ascospores/acre)
corresponds to a very high level of infection the previous season. Under
these conditions captan does not adequately control apple scab, and the
quantity of ascospores to begin each successive season creeps upward slowly.
In the simulation window, click on the Fungicides menu, and then click
on Select.... Note that the default is to apply no fungicides.
In one of the boxes, select "captan" and then click on "Done." In the dataset
that has just been loaded, the default spray schedule is to spray captan every
7 days, beginning on day 1, for a total of 16 applications at a dose of 5 pounds
per acre. You can confirm this by selecting Schedules... in the
Fungicides menu.
In the Simulation menu, select Begin New, and click the arrow
button at the right of the scroll bar until the season progresses to the end.
Note the saw-tooth graph of the fungicide residue, which spikes with every
spray application and then declines as the residue weathers away. The black
line rising exponentially toward the end of the season represents the number
of apple scab lesions. Note that as the graph hits the top, the line breaks,
and the scale is reset by a factor of 10. The first line, therefore,
represents 0 to 100, the second 100 to 1000, the third 1000 to 10,000, etc.
If the graph disappears as you toggle from one window to another, click on
Refresh in the View menu. Click on Log to see the text
output of what appears on the graph. Any portion of the log can be highlighted,
copied, and pasted into a text editor to facilitate your preparation of your
report on this exercise. The log file keeps a record of the current season's
output plus a summary that is cumulative from one season to the next.
Go back to the Simulation menu and click on Continue. This
simulates the overwintering of the fungus and the carryover of inoculum
from one season to the next. Advance the simulation through time as before
by clicking on the arrow button at the right of the scroll bar. Continue the
simulation for 5-6 seasons. Notice how the scab begins to develop earlier
with each successive season. Notice also the losses to apple scab (the red
line on the graph) creeps higher each season. Check the cumulative summary
at the bottom of the log file.
Explain what is happening.
....proceed to EXERCISE 2
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