Model BliteCast for P. infestans

BliteCast for modelling infection by Phytophthora infestans

Sensor needed: Precipitation, temperature, relative humidity, leaf wetness BLITECAST is used to model the first possible infection by P. infestans BLITECAST is an integrated computerized version of both the Hyre and the Wallin model. The first part of the program forecasts the initial occurrence of late blight 7-14 days after the first accumulation of 10 rain-favorable days according to Hyre's criteria, or the accumulation of 18 severity values according to Wallin's model. The second part of the program recommends fungicide sprays based on the number of rain-favorable days and severity values accumulated during the previous seven days. Accumulation of rain-favorable days and severity values begins when distinct green rows can be seen in the potato field, and ends at vine kill. The first spray is recommended when the first late blight forecast is given. Subsequent sprays are recommended according to an adjustable matrix which correlates rain-favorable days with severity values.

Threshold for applications

First spray is recommended when the first forecast is given. Subsequent treatments are based on the following table:

Adjustable matrix used to relate severity values and rain-favorable days and generate spray recommendation for Blitecast.
Total number of rain-favorable days during the last 7 days Severity values during the last seven days
  <3 3 4 5 6 >6
  Message number
<5 -1 -1 0 1 1 2
>4 -1 0 1 2 2 2
Meaning of message numbers is:
-1 = No spray is recommended.
0 = A late blight warning (treat or review conditions in 2 to 3 days. If the short range forecast is for blight favorable weather, follow a 7-day spray schedule)
1 = A 7-day spray schedule is recommended.
2 = A 5 day spray schedule is recommended.
Note: Some researchers have advised reverting to a regular spray schedule at 1% disease severity.

Model description

Severity Value Accumulation Using Wallin's System of Forecasting Late Blight (Blitecast) Hours of RH > 90% 

Average Temp. Range F* 0-9 10-12 13-15 16-18 19-21 22-24 25 +
 45-53 (7-12°C)  0  0  0  1  2  3  4
 52-59 (6-15°C)  0  0  1  2  3  4  4
 59-80 (15-26°C)  0  1  2  3  4  4  4
 above 80 (more than 26°C) No severity values accumulated
  • Average temperature during period of relative humidity (RH) of 90% or greater

Late blight is first expected to appear no earlier than within 1-2 weeks after 18 SV have accumulated starting at the time of first emergence of green tissue from the source of late blight inoculum. The source of inoculum could be plants growing from infected tubers in a cull pile, volunteers growing from infected tubers that survived the winter, or infected seed tubers. The first green tissue is most likely to be emerging from any potato cull piles in your area, so it's best to use that date.

  • Irrigation* can create late blight favorable conditions in a field that a weather monitor will not be taking into account. Irrigation that starts when the leaves are still wet from dew in the morning, or continues after dew has fallen at night will extend the wetting period for that day.

Page is refered to http://www.ipm.ucdavis.edu/DISEASE/DATABASE/potatolateblight.html

Model developer  and literature

Krause, R. A., Massie, L. B., and Hyre, R. A. 1975. BLITECAST, a computerized forecast of potato late blight. Plant Disease Reporter 59: 95-98. MacKenzie, D. R. 1981. Scheduling fungicide applications for potato late blight. Plant Disease 65: 394-399. MacKenzie, D. R. 1984. Blitecast in retrospect a look at what we learned. FAO Plant Protection Bulletin 32:45-49.