Lack of feed

when did you plant them, have they flowered?

Thanks for the help. I plants them in special potato grow bags in early March. I emptied one bag yesterday and had about 9 lbs of potatoes from the three plants, and they tasted lovely for dinner today. Now the second bag is turning yellow and there are black spots on the leaves ( my guess it might be potato blight but I'm not sure as its about 35 years since i've had a vegetable garden)