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 favorable 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.
The model uses hourly values of the following parameters as input:
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 number of spores able to infect increases with the duration of the wet period.
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)
Risk < 1 = no warning Risk > 1 and < 2,5 = possible warning coming up if conditions persist Risk > 2,5 = warning issued
When a warning is issued, any control measure should be considered based on cultivar resistance, choice 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.