Potato blight spreads by spores from old, dumped potatoes to new plants in mid-summer. The foliage quickly rots with brown spots appearing on the leaves. The fungus needs moist conditions and it is worst in a wet year and in areas of high rainfall. The spores spread rapidly from plant to plant, sometimes causing all the foliage and stems to rot.
Apart from bringing growth to a premature end and reducing yield, the blight spores are washed into the soil to affect the tubers, which later develop a bronze rot, and become unusable. Some varieties, such as ‘Rooster’ have a reasonable level of resistance but will still suffer damage.
Spraying is essential in most years, except for early crops. Spray with Dithane, Bordeaux Mixture, or Liquid Copper. Old potatoes should be burned or buried deeply, not just dumped in a corner where they will grow and send up spores.