Exercise 1 is to acquire proficiency in graphing DPCs and calculating AUDPC, relative AUDPC, and r. Exercise 2 is to understand the concept of relative AUDPC. Exercise 3 is to highlight some key issues when calculating AUDPC and r.

1.  Graph a DPC and calculate AUDPC, relative AUDPC, and r

      IMPORTANT: if you have problems with activities 1.1 to 1.5, skip to activity 1.6.

1.1.       Prepare the data 

1.1.1.Break into groups (number to be decided by group and facilitator). Each group will be assigned data from an experiment – or use data from an epidemic they brought. There are data from 4 experiments carried out in Peru: Comas 1999, Oxapampa 1999, Oxapampa 2000, and Huancayo 2000. In these experiments 3 potato cultivars were tested with several application systems of the protectant fungicide chlorothalonil. The number of repetitions was 4.

1.1.2.Copy the row of days after emergence and paste it in a new EXCEL worksheet (in the Edit menu, select Paste Special and then Values).

1.1.3.Select a treatment (the combination of a cultivar and an application system, e.g., Amarilis with no fungicide). Copy the severity data of this treatment and paste them below the corresponding cells of days after emergence.

1.2.  Graph a disease progress curve 

1.2.1.Calculate the mean blight severity and its standard deviation for each evaluation.

1.2.2.Plot the mean blight severity on days after emergence. Then, add vertical lines representing the standard deviation of the mean blight severity.

1.3.  Calculate AUDPC and relative AUDPC

1.3.1.Calculate the AUDPC for each pair of readings within each repetition. Then, sum them up for the total AUDPC for that repetition. Use equation 1.

1.3.2.Calculate the mean and standard deviation of AUDPC for the treatment.

1.3.3.Calculate the relative AUDPC for each repetition.

1.3.4.Calculate the mean and standard deviation of relative AUDPC for the treatment.

1.4. Calculate r

1.4.1.Transform severity into logits according to equation 2. Be careful not to consider severity values equal to 0% and 100%.

1.4.2.For each repetition, plot logits on days after emergence, and estimate the slope of the linear regression. This is the parameter r of the logistic model.

1.4.3.Calculate the mean and standard deviation of r for the treatment.

1.5. Summarize your results

1.5.1.Prepare a table with the means and standard deviations of AUDPC, relative AUDPC, and r for the  treatment.

1.5.2.Make a “Materials and Methods” section and write something about how you calculated the AUDPC and r.

1.6. See an example. You can see an example on how to graph DPCs, and calculate AUDPC, relative AUDPC, and r in the file AUDPC and r (student) [*], Sheet 1. Severity data of 2 treatments (cultivar Amarilis with no fungicide and Amarilis with fungicide applications every 7 days) of an experiment carried out in Comas (Peru) have been processed. Figure A shows the DPCs and Table A shows the means and standard deviations of AUDPC, relative AUDPC, r for these treatments (figures and tables in the file AUDPC and r (student) are coded with capital letters).

[*] If you are a facilitator, you may want to use file AUDPC and r (facilitator).

2.     Understanding the concept of relative AUDPC

2.1. In the file AUDPC and r (student), Sheet 2, go to Table B and calculate AUDPC and relative AUDPC for epidemics E1 and E2. Also calculate the following: (relative AUDPCE1 / relative AUDPCE2 x 100), and compare the resulting value with relative AUDPCE1. Figure B shows the DPCs of epidemics E1 and E2.

2.2. In the same page, go to Table C and calculate relative AUDPC for epidemics E3 and E4.

3.      Key issues for calculating AUDPC and r

In the file AUDPC and r (student), Sheet 3, there are severity data from 2 epidemics: cultivars Amarilis and Tomasa with no fungicide. Severity was evaluated 19 times in 98 days and AUDPC and r were calculated. This set of conditions was coded as ‘Scenario 1’. Four more scenarios were created by deleting some severity readings (Table 1) (Figures C to G). Calculate AUDPC and r for each scenario. Table D and Figures H and I will be automatically done.

Table 1. Scenarios created to highlight key issues for calculating AUDPC and r.