Not so sure that is scab. Scab is usually small roundish orangy -brown marks that tends to look like the end of a cork.
Often caused by dry light loamy soil, its only sin deep and once the tuber is peeled okay to eat.
Too much manure in the soil, this drying out over the summer can also cause scab like marks. Basically not enough water being the main cause of scab.
There are many spuds that are considered to be scab resistant. Tuckers seed catalogue lists many of them. Worthwhile sending for...
hope that helps.