Disease Severity Value calculation methods

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

Criteria for changing spray recommendations:

RISK LEVEL	DSV ACCUMULATION	LATE BLIGHT
GREEN	7-Day Total <= 3 - AND - Season Total < 30	- AND - LATE BLIGHT NOT SEEN
YELLOW	7-Day Total >= 3 - OR - Season Total > 30	- AND - LATE BLIGHT NOT SEEN
ORANGE	7-Day Total >= 21	- OR - ISOLATED OUTBREAK IN COUNTY IN < 500 ACRES
PINK	DSV ACCUMULATION NOT RELEVANT	ISOLATED OUTBREAKS IN COUNTY IN 500-1000 ACRES
RED	DSV ACCUMULATION NOT RELEVANT	LATE BLIGHT WIDESPREAD IN COUNTY ON > 1000 ACRES

Spray Recommendations:
For details on particular fungicides and rates to use at each risk level see the Application Rates page.

Risk level: GREEN

Lowest labeled rate
of protectant fungicide recommended
-- minimum 7 day application interval

Risk level: YELLOW

Highest labeled rate of protectant fungicide recommended
-- minimum 7 day application interval
-|- Protectant fungicide plus Supertin at 2 oz/acre if near end of season at early senescence -|-

Risk level: ORANGE

Highest labeled rate of protectant fungicide recommended
-- minimum 7 day application interval
-|- Protectant fungicide plus Supertin at 2 oz/acre if near end of season at early senescence -|-
-|- if late blight within 2-5 miles, CLICK HERE for application rate -|-

Risk level: PINK

Systemic fungicides recommended at full application rate
or protectant fungicide plus Supertin at 3 - 3.75 oz/acre
-- minimum 7 day application interval
-|- Latest Alerts and Emergency Procedures -|-

Risk level: RED

Systemic fungicides recommended at full application rate
or protectant fungicide plus Supertin at 3 - 3.75 oz/acre
-- minimum 5 day application interval
-|- Latest Alerts and Emergency Procedures -|-

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