Apple scab is a perennial disease on a perennial crop. The lesions present on the leaves at the end
of one season produce the ascospores that infect the new leaves the following spring. The management
of the disease in the current season can therefore affect its management in the subsequent season.
In this simulation, the default value for the initial inoculum (1010
ascospores per acre) simulates the ascospore load from a high level of disease the previous season.
It may take a season of heavier spraying than usual to bring this epidemic under control and reduce
the number of sprays to a more reasonable level in subsequent seasons.
As in the previous exercises,
run the simulation with the default values for cultivar susceptibility, weather, and inoculum (highly
susceptible cultivar, moderate temperatures and moderate rainfall, and 1010
ascospores). Manage apple scab with roughly weekly sprays of Protectan, as you did in the first exercise.
(Leave the rate for Protectan at 6 lb/A (6.67 kg/Ha) and don't forget to set the rates
for Combocide and Eradican to zero.) After terminal bud set, discontinue fungicide applications and
advance to the end of the season.
Highlight and copy the Economic Report and paste it into a text editor to compare the results with
those of subsequent seasons.
Continue the simulation for another season by returning to the Simulation menu and selecting
Continue. This simulates the overwintering of the infected leaves and the production of the
appropriate numbers of ascospores the following season. After closing the pop-up window showing the
dormant bud stage, click on Inoculum, Ascospores... and note the numbers of ascospores
available for the current season. (In the E notation, "E" stands for exponent of the base 10, so E10
would be 1010, E9 would be 109, etc.)
Proceed as before, applying Protectan at roughly weekly intervals. If you see no apple scab lesions
by the time you reach the pink stage of bud development, try dropping off one or two sprays from the
spray schedule. At the end of the season, copy and paste the Economic Report into the document that
you have started.
Continue the simulation for a third season and check the numbers of ascospores available as before.
Again try to eliminate another spray or two from the spray schedule.
What effect does good contol of apple scab during the current season have on the inoculum available
the following season? What effect does the initial inoculum available at the start of the season
have on the spray program for that season?
