Naerstad Model

The model is based on several years of experiments where trap plants have been placed next to a potato field infected with late blight. Infections on these plants have been correlated with weather conditions the day of exposure, and this is the data the model is based on. The model predicts if there are favourable conditions for spore production and the following spread, survival and infection of these spores. The model produces an infection risk, and a value of 2.5 corresponds to one spot of late blight on each trap plant exposed. This value of 2.5 is the threshold where the model issues a warning.

Data

The model uses hourly values of the following parameters as input:

  • Air temperature
  • RH
  • Precipitation
  • Wind speed (2 m height)
  • Wind speed (10 m height)
  • Net radiation
  • Short wave radiation
  • Leaf wetness
  • VPD (0.61078*EKSP(17.269* TM /(TM +237.3)), k)P

Spore production, dissemination and survival

The VPD needs to be under 220 Pa (RH>approximately 90%) for at least 87 degree hours to initiate spore production. Number of spores increase with duration of the period with high humidity. A VPD between 220 and 520 Pa reduce the spore production, and a VPD higher than 520 will stop it completely.

Spores are discharged into the air when the RH decreases, and free water on the leaf surface will inhibit spore release. Rain will wash the spores off, and sunlight will kill a large number of the released spores.

Risk of late blight infection

The spores need free water to start growing, either from rain or dew. The leaves need to stay wet for at least 40 degree hours for the spores to have time to both start growing and infect. The amount of spores able to infect increases with the duration of the wet period.

Risk of late blight infection is calculated as follows:

Risk = (TSWH(t) /40)*VRS(t) *IR(t) TSWH(t) = temperature sum for wet hours

VRS(t) = viable released spores IR(t) = Infection risk (=1 when TSWH(t)>=40)

Interpretation of the model output

Risk < 1 = no warning Risk > 1 and < 2,5 = possible warning coming up if conditions persists Risk > 2,5 = warning issued

When a warning is issued, any control measure should be considered based on cultivar resistance, chyoice of fungicide and time of last application. Risk of infection is highest after several consecutive days with warnings, and especially if late blight has been observed in the area.

Eksempel på varsel

Risk < 1 = no warning Risk > 1 and < 2,5 = possible warning coming up if conditions persists Risk > 2,5 = warning issued


When a warning is issued, any control measure should be considered based on cultivar resistance, chyoice of fungicide and time of last application. Risk of infection is highest after several consecutive days with warnings, and especially if late blight has been observed in the area.

The following input data are required:
observations - array of weather observations. An observation consists of:
timeMeasured - timestamp for observation
elementMeasurementTypeId - Code for parameter
logIntervalId: 1 for hour value
value: the observed value
Four different weather parameters should be provided:
* TM - Mean temperature (Deg. Celcius)
* RR - Aggregated rainfall (Millimeters)
* Q0 - Global radiation (Wh/m2)
* BT - Leaf wetness
They should all be hourly values, they should all be complete hourly series in the exact same period.
Minimum period length is 4 days (TODO: Verify!)
The following input data are optional:
calculationStart - the time when calculation should start
calculationEnd - the time whe calculation should end

This is technical info for model and web developers

{
	"loginInfo":{
		"username":"example",
		"password":"example"
	},
	"modelId":"NAERSTADMO",
	"configParameters":{
		"calculationStart":"2012-08-20T00:00:00+02:00",
		"calculationEnd":"2012-08-30T00:00:00+02:00",
		"observations":[
		{
				"timeMeasured": "2012-08-20T00:00:00+02:00",
				"elementMeasurementTypeId":"TM",
				"logIntervalId":1,
				"value":13.77
		},
		{
				"timeMeasured": "2012-08-20T00:00:00+02:00",
				"elementMeasurementTypeId":"RR",
				"logIntervalId":1,
				"value":0
		},
		{
				"timeMeasured": "2012-08-20T00:00:00+02:00",
				"elementMeasurementTypeId":"Q0",
				"logIntervalId":1,
				"value":0
		},
		{
				"timeMeasured": "2012-08-20T00:00:00+02:00",
				"elementMeasurementTypeId":"BT",
				"logIntervalId":1,
				"value":60
		},
		{
				"timeMeasured": "2012-08-20T00:00:00+02:00",
				"elementMeasurementTypeId":"UM",
				"logIntervalId":1,
				"value":60
		}
		]
	}
}