Might be a sign that they are ready for harvesting.

Do a firkle i.e. clear some soil away from the base of the plant and have a look at the size of the tubers.

If you feel they are of an edible size lift them and get them in the pot!

If they are not then cover them up again and have another look as the yellowing increases!

I don't think it is a disease, had the spot been black then I might have had another opinion.

I just love early potatoes!

Could be a nutrient deficiency as potatoes are greedy feeders. Did you use any fertiliser when planting?