Late blight survives on infected tubers, including discarded tubers and cull piles. These provide the source of early season infection. If the average of the maximum and minimum temperatures for five consecutive days is less than 78F and if more than 1.2 inches of rainfall has occurred in the past 10 days, the weather favors late blight. The disease is sporadic in occurrence. Hot dry weather can halt the development of the late blight fungus.
Rain can carry the late blight spores down through cracks in the soil to infect the tubers.
To control late blight, make certain that only healthy tubers are planted and that no cull potatoes are allowed to overwinter in the garden. If late blight appears and weather favors further development, spray with a fungicide. Chlorothalonil or Bordeaux mixture are both effective. Copper fungicides may provide the best protection in weather favoring severe late blight. Hill up around the base of vines, then kill the vines with recommended vine killing chemicals or by simply chopping off the vines at the soil line at least two weeks before harvest.