Physiological Days

Physiological Degree Day Program and Early Blight Forecasting:

The "physiological degree day" (PDD) program is used to predict the development of the potato plants or top growth, the potential for disease infection, and potato sprout emergence when the soil temperature is above 50 F. It was modified from the "growth degree day" (GDD) model except that the PDD has cut-off points at both minimum and maximum temperatures and partially takes into account hourly variations in temperatures. It is a tool to help in pest management and is part of the potato crop management software.

The PDD will not substitute for walking the field. Nothing can substitute for looking at weed populations, diseases, insects, and soil wetness. The PDD is a gage, another management tool. It will let the grower know what to look for in the field and what the crop should be like from year to year. It is really a weather-driven model. Recently, it has helped relating disease thresholds to fungicide applications, eliminating some unnecessary treatments.

It is based on a sequence of formulas for differing periods of time. First one needs collect daily high and low air temperatures. These are then entered into the equations. The following is an SAS program that I prepared for degrees in Fahrenheit based on this model:

data pday;
input date tmaxF tminF;
PT1 = tminF;
PT2 = ((2*tminF)+tmaxF)/3;
PT3 = (tminF+(2*tmaxF))/3;
PT4 = tmaxF;
If PT1<45 then PT1m = 0;
If PT1>44 and PT1<70 then PT1m = 10*(1-(((PT1-70)**2)/625));
If PT1>69 and PT1<86 then PT1m = 10*(1-(((PT1-70)**2)/256));
If PT1>85 then PT1m = 0;
If PT2<45 then PT2m = 0;
If PT2>44 and PT2<70 then PT2m = 10*(1-(((PT2-70)**2)/625));
If PT2>69 and PT2<86 then PT2m = 10*(1-(((PT2-70)**2)/256));
If PT2>85 then PT2m = 0;
If PT3<45 then PT3m = 0;
If PT3>44 and PT3<70 then PT3m = 10*(1-(((PT3-70)**2)/625));
If PT3>69 and PT3<86 then PT3m = 10*(1-(((PT3-70)**2)/256));
If PT3>85 then PT3m = 0;
If PT4<45 then PT4m = 0;
If PT4>44 and PT4<70 then PT4m = 10*(1-(((PT4-70)**2)/625));
If PT4>69 and PT4<86 then PT4m = 10*(1-(((PT4-70)**2)/256));
If PT4>85 then PT4m = 0;
Fpday =(1/24)*((5*PT1m)+(8*PT2m)+(8*PT3m)+(3*PT4m));
accum + Fpday;
put accum =;
title " accumulated physiological days (P-Days)";
proc print; var date Fpday accum; run;

Daily values accumulate from the date of emergence and these sums are the physiological days (P-Days) for a particular plant, field or area depending on the method of obtaining the temperature data. The recommendation for applying fungicides for early blight depends on the cultivar's maturity rating; see table below.

Table. Physiological-Day Accumulation Favorable for Early Blight.

Maturity Low Medium High
Early 300 350 400
Middle 300 350 400
Late 300 400 500

There are computer programs available such as WISDOM from University of Wisconsin - Madison that are available to do these calculations for you.


  • Madden, L., Pennypacker, S.P. and MacNab, A.A. 1978. FAST, a forecast system for Alternaria solani on tomato. Phytopathology 68:1354-1358.
  • Pscheidt, J.W. and Stevenson, W.R. 1988. The critical period for control of early blight (Alternaria solani) of potato. Amer Potato Jour 65:425-438.
  • Sands, P.J., Hackett, C. and Nix, H.A. 1979. A model of the development and bulking of potatoes (Solanum tuberosum L.). Field Crops Res 2:309-331.