24/10/2017
How to simulate pest and Disease Damage using DSSAT
(Question Asked by Entomologist from AARI)
Working with File X and T:
You have already learned how to use ATCreate to enter data into the *. crT file. You can enter observed pest damage into the FileT, and modify the FileX, so as to turn on disease simulation, and read the pest damage from the FileT, and output the disease simulations.
Modifying File X: Using XBuiild, select the UFGA7601 experiment. Give it a new experiment number, UFGA7603, and name it “Pest Simulation.” First, under Simulation OptionsManagement-Harvest change from scheduled harvest to simulate to the normal end of its life cycle. Then add a second (new) treatment and name it “plus leaf spot disease” Select this treatment 2, and then add a new level in Simulation Options. In level 2 of simulation options, modify the Options to turn Disease and Pest Routines On (Y). This will allow the model to read the defoliation rating that you will enter into FileT. Also, under Output, make sure disease output files are output. Save your new FileX under the new name, and exit from XBuild.
Entering pest damage into FileT: Typically, one would visually rate percent leaf defoliation and leaf disease with some standard rating. For this exercise, we will compute the percent defoliation that occurred (by computing the percent decline in LAI), assuming that the one-time peak LAI that would have been maintained to maturity. Do this for each date of LAI sampling in the present FileT, recording both the day of year and computing the percent defoliation. Then use ATCreate to enter this percent defoliation into the FileT, under the variable header (PCLA). Before you do this, duplicate the experimental data to create two treatments in the FileT, to allow you to simulate the experiment, with and without the pest damage. Treatment 1 will be pest-free (because Disease was turned “off” in Simulation Control), but Treatment 2 will have entries for calculated defoliation and Disease was turned “on” in Simulation Control.
Now simulate the experiment: Observe the LAI, leaf mass, stem mass, pod mass, total crop mass, and pod harvest index. Did the simulations of growth and pod yield improve after accounting for the leaf defoliation? Do this account for enough yield loss?
Adding percent necrotic area (PDLA): So far, we have not accounted for the leaf necrotic area. This could additionally be done, by entering percent values under the header, PDLA. We do not have actual records. However, the percent necrosis on remaining leaves from leafspot typically starts at 1-2% at the time defoliation begins and reaches about 10% of total leaf area about two-thirds of the way through the leaf defoliation cycle (to 100% defoliation). You could try entering this as well, to see if it makes an additional effect. One additional point here. Look at the PNGRO040. PST file in the PEST directory. Note the definitions of other types of damage, and also the ability to modify the amount of photosynthetic reduction from leaf necrosis, PDLA. We (Bourgeois et al.) have measured leaf photosynthesis data that indicates that observed leafspot necrosis contributes four times more decrease in leaf photosynthesis (virtual lesion effect, acts like four times as much leaf area is affected than the actual percent necrotic area). Try changing this scalar from 1 to 4 if you want to.
