Among commercial potato cultivars there is a wide range of resistance to Phytophthora infestans,
from highly susceptible to virtually immune. The current trend in potato breeding is to avoid the use
of strong resistance (major gene or "vertical" resistance), which quickly selects a virulent
P. infestans population, in favor of weaker resistances (polygenic or "horizontal" resistance),
which are likely to be more durable.
While these intermediate levels of resistance are important epidemiologically (as we shall see in
subsequent exercises), by themselves they generally are not sufficiently effective to control a late
blight epidemic. Let us examine three levels of resistance and save the data to compare the effectiveness
of resistance with that of some of the other control measures.
In the simulation window, click on the Potato menu, select Resistance level, and then
select "Low." (This is the default level of resistance.) Next click on the Simulation menu,
select Begin New, and then press the right arrow key on your keyboard
until the season progresses to the end. The vertical blue lines represent rainfall,
the green line represents leaf area index, and the brown line represents the percent of the
leaf area blighted. Click on the Economics menu, and select Show Report. Highlight
the text in the report window, copy it, and paste it in a text editor to prepare your final report.
Repeat these steps with "Moderate" and "High" resistance levels.
Considering the variety with "Low" resistance (most susceptible) as the standard for comparison,
does "Moderate" resistance offer a reasonable disease management option under these conditions?
(Explain.) Does "High" resistance offer an acceptable level of disease control? (Explain.) What are the
risks of high resistance?
