Negative prognosis model

From a specific date (50% germination), daily risk values are accumulated based on weather data (temperature, relative humidity and precipitation). The risk is an accumulated value of how the weather affects late blight germination/infection, sporulation and growth. Alle processes are corrected for inhibition due to drying. After the accumulated risk has reached a certain limit (150), there will be an infection risk if there is one day with high infection risk. This model is together with time of row closure, used to decide the timing of first spray against potato late blight.

First spray

There is normally no need for spraying against potato late blight before row closure. Long lasting, favourable weather conditions in areas with early production, in combination with high levels of primary inoculum, can make it necessary to spray earlier. If the accumulated risk based on the negative prognosis model reaches 150 before row closure, first spray should be applied at the next infection period. In years with long periods with wet soil early in the season can also lead to early risk of infection.

When accumulated risk < 150, warning symbol is green

When accumulated risk > 150, warning symbol is yellow if the daily risk is below 7 and red if it is above 7

When a late blight warning has been issued, cultivar suseptibility, selection of fungicide and time of application should be considered before spraying. Risk of getting an infection is highest if warnings are issued two days ore more in a row, and when late blight has been observed in the area. ‘

Fungicides are most effective when used as protectants against late blight. Infection normally occurs during the morning, which means that the spray should be applied the day before if not covered by the last spray. One application normally protects the crop for 7 to 14 days, depending on the fungicide, dose and growing conditions. 

Testing and validation

This model has been tested in several countries, and is the basis for late blight warnings in the USA (NegFry)

References

Ullrich J. & H. Schrödter 1966. Das Problem der Vorhersage des Auftretens der Kartoffel krautfäule (Phytophthora infestans) und die Möglichkeit seiner Lösung durch eine “Negativprognose”. Nachrichtenblatt Deutsch. Pflanzenschutzdienst (Braunschweig) 18, 33-40

When accumulated risk < 120 the warning status is green (no risk), and changes to yellow (possible risk) from 120 to 150. When accumulated risk reaches 150, the warning status changes to red (danger) and stays red until 200, when the model is terminated, and warning status turns grey.

The Negative prognosis model is primarily a tool to determine the time of first spray. Daily and accumulated risk will still be shown in the background, but without any warnings. For support on when to implement measures against potato late blight, the Nærstad model should be used.

TODO

This is technical info for model and web developers

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