One strategy for the control of apple scab is simply to spray by the calendar, that is, to apply
a fungicide every week throughout the period during which the apple trees are susceptible. This
period begins at green tip, when the tips of the sepals first emerge from the flower buds. Protecting
against sepal infection is particularly important, because these infections easily spread to the
developing fruit and cause severe quality losses. As the cluster leaves emerge, these newly exposed
tissues are also highly susceptible to infection. Weekly applications of fungicides are needed
to cover this new growth and to replenish residues that have weathered away. The susceptible period is
generally considered to be over when the elongating shoots stop producing new leaves and set a terminal
bud.
Of course, the weekly schedule has to be modified somewhat, depending on the weather. If a rain occurs
while the fungicide residues are still wet, they are far more vulnerable to washing off than
if the residues have dried on the leaves. Most of the fungicide applied in the rain is simply wasted,
so it is important for the manager to keep an eye on the weather and modify the weekly schedule
accordingly. Also because of the risk of affecting the bee activity, it is wise to avoid spraying
during bloom. It is generally a good idea to apply a fungicide during the pink stage to be sure to have
adequate coverage until petal fall.
Run the simulation with the default values for cultivar susceptibility, weather, and inoculum
(highly susceptible cultivar, moderate temperatures and moderate rainfall, and 1010
ascospores). In the simulation window, click on the Simulation menu and select Begin New.
Note that at the start of the simulation the trees are in the dormant stage. Close the current growth
stage window by clicking on the "X" in the upper right-hand corner of the window. Press the right
arrow button on the keyboard to advance the simulation one day at a time, and note the change of dates
in the line at the bottom of the window. You should also see small vertical blue lines appearing on the
graph. These represent rainfall events. In the Management menu, select Weather report
to see a three-day weather forecast. Use this to help you plan your next spray.
(If the graph disappears as you toggle from one window to another, click on
Refresh in the View menu.)
Advance the simulation through time until the green tip stage of growth. Now is the time to begin
your fungicide sprays. A delay of even one day can allow enough infection to have a significant impact
on subsequent scab development. Apply a fungicide by selecting Spray... in the Management
menu. In the window that opens, note the default rates of application for the three fungicides. Leave
the rate for Protectan at 6 lb/A (6.67 kg/Ha) and set Combocide and Eradican to zero. Note also that the fungicide
spray will be applied tomorrow. Apply the spray by clicking on the "Spray" button in the lower right
corner of the window.
(Since the simulation requires that the decision to spray be made the day before
the spray is actually applied, a warning is given if it is raining on the spray day--simulating what you
would see if you went out to spray. Note the amount of rain. Rains less than 0.1 in (0.2 cm) do not
have a significant effect on the spray deposition.)
Advance another 7 days, and note the green line rising above the horizontal axis. This is the total leaf
area. The brown line represents scab lesions. To make the fungicide residue appear on the graph,
in the View menu select Protectan Residue and click on Refresh. Note how quickly
it weathers away following the application. Apply another fungicide spray, and continue until terminal
bud set, applying Protectan at roughly weekly intervals. (If possible, avoid applying sprays between
pink and petal fall.) After terminal bud set, discontinue fungicide applications and advance to the end
of the season.
In the Economics menu, select Show Report. Highlight and copy the report and paste it into
a text editor to compare the results with other runs of the simulation and to prepare your final
report for this exercise.
Repeat the simulation under dry conditions by returning to the
Simulation menu and selecting Begin New. In the Environment menu, select
Weather, and click on Moderate and Dry. Proceed as before, and at the end of the season
copy and paste the Economic Report into the document that you have started.
What are the advantages of a calendar spray schedule with weekly applications of a protectant fungicide?
What are the drawbacks?
