Sounds like your soil mix is wrong, a good homemade compost, manure and some seaweed at their start helps.
Did you feed them when first planted with a potato fertiliser or Growmore? That helps.
At this stage, add in and mix carefully a handful of fertiliser like Growmore.
But yellow leaf sounds like a magnesium deficiency. Add some homemade compost or seaweed and let the rain or watering take it down.
The will revive and keep up the healthy leaf.
