There are several environmental factors which provide the ideal conditions for the fungal disease Potato blight, 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.