There are several environmental factors which provide the ideal
conditions for the fungal disease Potato blight (Latin name – Phytophthora infestans).
These environmental factors have been quantified, and described in two different ways:
A Beaumont Period is a period of 48 consecutive hours, in at
least 46 of which the hourly readings of temperature and relative
humidity at a given place have not been less than 20C (68ºF) and
75%, respectively
A Smith Period is at least two consecutive days where minimum
temperature is 10C (50ºF) or above and on each day at least 11
hours when the relative humidity is greater than 90%
Potatoes are most vulnerable to potato blight when the leaves have met
in the row & by watching out for these particular weather periods at
this time, it is possible to forecast the onset of Potato Blight. Then,
protective fungicides may be applied to prevent the onset of the disease.
Both a Beaumont and a Smith period give relatively accurate forecasts,
but a Smith period is more commonly used. Information on Smith Period
reporting & blight forecasting can be found on the British Potato Council website.
