why are my potato plants dying

Hi wonder if anyone can help me with identifying what's wrong with my potato plants? Since my potatoes sprouted about 4 weeks ago they have looked healthy and strong. In the last couple of days they have developed light green/grey patches starting from the stem which then become brown patches. One or two  of the plants have now started to shrive and die. I can't see any bugs on the leaves. I didn't think it was blight because of this light discolouration. I'd be so grateful for ideas on what it is and if there is anything I can do before I lose a lot of potatoes!