could be potato blight, this is not good if it is, look on the underside to see if there is any sign of white mould.
Very hard to get rid of.

Thanks, I don't see any white mold. how should I treat it??

too early for blight. Looks more like a nutrient deficiency.
Are you growing them in bags?

Im growing them in a raised bed, what do you suggest I should add??

Are you growing them in soil or compost....what fertiliser did you add prior to planting. That may give a little clue.

I used top soil and cow manure, I've just added veg' fertilizer.

pleased to hear its not the blight, watering enough?

is that straw or bark you've got on top of the soil. It looks very dry. If it is, it could be removing nutrients and moisture from the soil.