Potato Late blight evaluated by the calculation of "SMITH Periods"

Sensors needed: Air temperature, relative Humidity

Biological Basis of the model : Phytophtora infestans can grow if temperature is lower than 10°C. But sporulation will be nearly nothing at this temperatures. Therefore it needs a moist period with temperatures higher than 10°C to get a reasonable sporulation. Infection of Phytophtora infestans needs free moisture. In longer periods of high relative humidity free moisture either by rain or by dew is very much probable. Definition - What is a Smith Period? Two consecutive days with minimum temperature of 10 °C and 10 hours of relative humidity higher than 90% on the first day and 11 hours of relative humidity higher than 90% on the second day is a Smith Period. If the criteria for the fist day is fulfilled and the second day reaches 10 hours of relative humidity higher than 90% this indicates that 90% of the Smith period or Near Smith. In the graph you see reaching a Smith Period (in sum 21 hours in two consecutive days) on the 16th of May. There have been favourable conditions on the 15th of May (minimum temperature of 10 °C and 10 hours of relative humidity higher than 90%)  and on the 16th of May (11 hours of relative humidity higher than 90% ) and so a Smith Period was calcualted (Calculation once a day at 23:00.



Smith periods or near Smith periods are pointing out periods where the climate is very favourable for the disease. The model points out periods with a very high risk of this disease. Experience: This is an empirical model showing very good results in UK where it is used as a negative prognosis too. As long it is to cold for 2 moist days with temperature always higher than 10°C no spray is needed. This model is only valid where temperature increase during spring is very steady (Ocean Climate).


Smith, L. P. 1956. Potato blight forecasting by 90% humidity criteria. Plant Pathology 5:83-87 (Basic model). Hims, M. J., M. C. Taylor, R. F. Leach, N. J. Bradshaw, and N.V. Hardwick, 1995. Field testing of blight risk prediction models by remote data collection using cellphone analogue networks, p. 220-225 In: Phytophthora infestans 150: European Association for Potato Research (EAPR)-Pathology Section Conference, held in Trinity College, Dublin, Ireland, September 1995 to mark the one hundred and fiftieth anniversary of the first record of potato blight in Ireland and the subsequent famine. L. J. Dowley, et al. (Eds). Boole Press, Ltd. Dublin. pp. 220-225.