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Plant Disease Epidemiology: Temporal Aspects
Management Simulation
     
     
   

 Note: To run this simulation, you will need the program installed on your computer. (It runs under Windows™, occupying 3 MB on your hard drive and requiring a minimum of 8 MB of RAM.) If you want to do this and have not already done so, click on [INSTALL] now.

In this exercise, you will need to have both the simulation program and your Web browser ("Netscape" or "Microsoft Network Explorer") running simultaneously so that you can toggle back and forth between the two using the [Alt]-[Tab] keys. In these instructions, the symbol will signal you to toggle from this document, complete the given instructions on the simulator and toggle back. Minimize your Web browser now (Click on the in the upper right corner.) and then double click on the Plant Disease Development icon to start the program. Use the the [Alt]-[Tab] key combination to toggle back to this Web page.

The Simulation Exercise

In the Plant Disease Development simulation, you see two epidemics running simultaneously, one in which you have made some changes to the parameters (the "test" epidemic), and a standard ("reference") epidemic for comparison. You may wish to compare the two-dimensional spatial development of the epidemics on a few simulations, but this option drasically slows the speed of execution, so you may wish to select "None" for the spatial dispersion in most of your runs.

  1. What is the epidemiological consequence of reducing the initial inoculum when the apparent infection rate is high?

    For initial disease incidence, enter 0.01 in the Reference epidemic and 0.001 in the Test epidemic. For both epidemics use 0.15 as the apparent infection rate. [Get answer below]

  2. To provide effective control of an epidemic with a high apparent infection rate, how much must the initial inoculum be reduced?

    Keep the apparent infection rates for both epidemics set at 0.15. Leave the initial disease incidence in the Reference epidemic set at 0.01, and change the value for initial disease incidence until the final level of disease rises no higher than 0.1.  [Get answer below] 

  3. What is the epidemiological consequence of reducing the initial inoculum when the apparent infection rate is low?

    For initial disease incidence, enter 0.01 in the Reference epidemic and 0.001 in the Test epidemic. For both epidemics use 0.04 as the apparent infection rate. [Get answer below]

  4. What is the epidemiological consequence of reducing the apparent infection rate?

    For initial disease incidence, enter 0.01 for both epidemics. Enter 0.15 as the apparent infection rate in the Reference epidemic and 0.05 in the Test epidemic. [[Get answer below]

  5. Which has a greater impact on a polycyclic epidemic, reducing the initial inoculum or reducing the apparent infection rate?

    To review the effects of reductions in the initial disease incidence and the apparent infection rate, set the initial disease incidence at 0.01 and the apparent infection rate at 0.15. On two successive runs, reduce first the initial disease incidence then the apparent infection rate to one-tenth its original value. [Get answer below]

    What is the epidemiological consequence of an integrated approach that reduces both the initial inoculum and the apparent infection rate?

  6. For the "Reference" epidemic, set the initial disease incidence to 0.01 and the apparent infection rate to 0.15. Imagine a hypothetical epidemic where it is possible by crop rotation to reduce the initial disease incidence to 1/10th of the original. Run the simulation with 1/10th the original initial disease incidence.

    The remaining inoculum comes from infected seed, which in our hypothetical example can also be reduced to 1/10th of the original by means of a clean seed program. The overall reduction in initial disease incidence, therefore, is 1/100th of the original. Run the simulation with this level of initial disease incidence (0.0001).

    Intercropping with a nonsusceptible crop is not generally considered an effective means of disease control, but it can reduce the apparent infection rate by about 1/3 in some cases. Reduce the apparent infection rate from 0.15 to 0.10 and run the simulated epidemic first with an initial disease incidence of 0.01 and then with 0.0001.

    Partial resistance often is not considered adequate for effective disease control, but nevertheless it can reduce the apparent infection rate significantly. Suppose that we have partial resistance that can cut the apparent infection rate in half. Run the simulation first with an initial disease incidence of 0.01 and and an apparent infection rate of 0.075 (half that of the Reference epidemic). Then run the epidemic with an apparent infection rate of 0.05 (to account for the 1/3 reduction gained by including intercropping in the system). Finally run the epidemic with initial disease incidence of 0.0001 and an apparent infection rate of 0.05. For the final run, change the "Dispersion" from "None" to "Cluster" to get an idea of the spatial development of the epidemic. [Get answer below]

 

ANSWERS

What is the epidemiological consequence of reducing the initial inoculum when the apparent infection rate is high?

A reduction in initial inoculum (or initial disease incidence) results in an apparent delay in the epidemic. Note that in the early part of the epidemic, before the logistic factor (reduced amounts of susceptible tissue) has much of an influence, every point along the disease curve is reduced by the proportion of the reduction in initial disease incidence.

To provide effective control of an epidemic with a high apparent infection rate, how much must the initial inoculum be reduced?

To have any significant impact on disease development in a polycyclic epidemic with a high apparent infection rate, the initial inoculum must be reduced to exceedingly low levels.

What is the epidemiological consequence of reducing the initial inoculum when the apparent infection rate is low?

The reduction in disease incidence throughout the epidemic is proportional to the reduction in initial inoculum, just as it is for the early part of the epidemic when r is high.

What is the epidemiological consequence of reducing the apparent infection rate?

Unlike reducing initial inoculum, which merely delays the onset of the epidemic, a reduction in the apparent infection rate reduces the rate of disease development throughout the epidemic.

Which has a greater impact on a polycyclic epidemic, reducing the initial inoculum or reducing the apparent infection rate?

A reduction in the apparent infection rate has a far greater impact on the epidemic than a comparable reduction in the initial inoculum. 

What is the epidemiological consequence of an integrated approach that reduces both the initial inoculum and the apparent infection rate?

Small reductions in the initial inoculum or the apparent infection rate may not seem adequate to control an epidemic when used individually, but when combined into an integrated program, satisfactory disease control can be achieved.  

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