Not enough water, insufficient organic matter in the soil. Good news is you can still eat your potatoes, just peel them before cooking!

As MG says, you need to make your soil more acidic. The addition of compost will help. You could also line the trenches or planting holes with grass clippings to achieve this. You could also look for the varieties which have some resistance to scab.

Just a thought did you lime the area prior to planting?

PS - if you did this could be the trouble.