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Forty-eight hours during which the minimum temperature is 10°C and the relative humidity is 75% or more. These periods are the basis of the system used in England to forecast an outbreak of late blight of potatoes (Phytophthora infestans). Sporulation of the potato blight fungus and subsequent invasion of potato leaves is favoured by moist weather. By examining meteorological data A. Beaumont worked out that blight could be expected ten days after a 'Beaumont period'. Careful monitoring of local meteorological data enables the advisory services to record Beaumont periods and give farmers accurate advice on when to spray their potato crops.
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