The Potato Lateblight Model
Disease Severity Values (DSV’s) are calculated using a modified version of the Wallin model. See the following articles for further details:

Wallin, J. R. 1951. Forecasting tomato and potato late blight in the northcentral region. (Abstr) Phytopathology 41: 37.
Wallin, J. R. 1962. Summary of recent progress in predicting the late blight epidemics in United States and Canada. American Potato Journal 39: 306-312.
Wallin, J. R., and P. E. Waggoner. 1950. The influence of climate on the development and spread of Phytopthora infestans in artificially inoculated potato plots. Plant Disease Reporter Suppl. 190: 19-33.
Kathleen M. Baker, William W. Kirk, Jeffrey M. Stein, Jeffrey A. Andresen, 2002. Web-interactive integration of regional weather networks for risk management of late blight in potato canopies.

Input variables
Hourly temperature and relative humidity values are collected from Michigan Agricultural Weather Stations (MAWN) around Michigan.
Minimum and maximum temperature during periods of relative humidity >= 80%. Number of hours of relative humidity >= 80% are then calculated.

Model Description
The model predicts the first occurrence of potato late blight and its subsequent spread based on seasonal accumulation of severity values. These severity values are based on various combinations of the hours with a relative humidity of 80% or greater and the average temperature during these periods. Accumulation of severity values should start at plant emergence. In our model we start accumulating severity values on May 1st, but you can select your own emergence date using the Calculate your own emergence page.

The following table indicates how severity values are assigned:

Relationship of temperature and relative humidity (RH) periods as used in the Wallin late blight forecasting system.

Average Temp Range Severity values for the number of hours relative humidity > 80%
0 (none) 1 (minimal) 2 (slight) 3 (moderate) 4 (severe)
45-53 F
<16 Hours 16-18 Hours 19-21 Hours 22-24 Hours n/a
54-59 F
<13 Hours 13-15 Hours 16-18 Hours 19-21 Hours 22-24 Hours
60-80 F
<10 Hours 10-12 Hours 13-15 Hours 16-18 Hours 19-21 Hours