In the default simulation, all of the initial inoculum comes from a cull pile located only
10 meters (32.8 feet) from our field of potatoes. (A cull pile is where the potatoes discarded from the packing
line are dumped outside the packing house.) The cull pile is likely to contain potatoes from the previous
season that were infected with Phytophthora infestans, and sprouts from those infected tubers
contain lesions that produce sporangia that can be carried by air currents to the newly emerging potato
plants. Let us try to reduce the inoculum load by moving the cull pile a bit farther from the field.
In the Inoculum menu, select Sporangia..., change the cull pile distance from field
to 100 meters, and click "Save." Run the simulation without applying any fungicides and compare the
results with those of the default run (cull pile 10 meters from the field). What value would you put
on keeping the cull pile an adequate distance from the field?
Another potential source of initial inoculum is infected volunteer potato plants emerging from
the previous season. The default simulation does not have any inoculum from this source, but let us
add some to compare with the previous simulation. Leaving the cull pile distance from field unchanged
at 100 meters, in the Inoculum menu, select Infections..., enter 50 in the "Volunteers"
box, and click "Save." Run the simulation again and compare the results with those of the previous run.
The usual potato plant density is on the order of 50,000 per hectare, so this represents only about
one plant in a thousand potentially infected at the start of the season. Is this enough to initiate
a significant epidemic? What value would you put on removing volunteers at the start of the season?
