Posted: 23/07/2013 at 18:03
The first year I grew potatoes I had this and was terrified it was blight. It turned out to be a magnesium defficiency. The area of the leaf between the veins was turning yellow with some black spots. Magnesium is essential in the production of chlorophyl hence the loss of green colour if there is a defficiency.
The quick solution was to add 4 tbspn of epsom salts to a litre of water and spray onto the leaves once per week (don't do this during the heat of the day or you'll scorch the leaves). If the leaves start to return to their usual green colour then you have a magnesium defficiency and need to treat the soil too.
The soil could be too acidic to allow the potatoes to uptake the magnesium already in your soil or you could just not have enough. You would need to do a soil test to be absolutely sure.
With blight the spots usually have a yellowing "halo" around them and the leaves tend to curl and wilt. If it is blight you need to cut all of the foliage off and burn or bin it. Leave the potatoes in the ground for few days so that any spores on the ground will die off. Lift them and do not store them. Blight fungus only lives on living plant material - potatoes, tomatoes, deadly night shade etc.
Blight usually overwinters on those tiny potatoes people often leave in the ground. This is why I now grow all of my potatoes in pots. It's much easier to remove all of them from the soil.