Common Scab is caused by soil-borne Streptomyces bacteria that infect developing potato tubers. The bacteria can persist in the soil for years and cause rough, corky lesions as the tubers grow.

Soil conditions make a big difference. 

Common scab is favoured by neutral to alkaline soil, while potatoes naturally prefer somewhat acidic conditions. The disease becomes more common as soil pH rises, particularly above about 5.2–5.5. Adding lime before growing potatoes can therefore increase the risk of scab. Dry soil while young tubers are forming also strongly favours the disease.

How to Fix It
Check your soil pH: Avoid unnecessarily raising the pH of soil where potatoes will be grown.
Don't lime before potatoes: If your soil needs lime for other crops, apply it elsewhere in the rotation rather than immediately before potatoes.
Keep soil evenly moist: Consistent moisture during early tuber formation helps reduce scab. Avoid letting the soil repeatedly dry out during this critical period.
Plant resistant varieties: Potato varieties differ considerably in their susceptibility to common scab.
Rotate crops: Avoid repeatedly growing potatoes in the same location. Rotation can help reduce disease pressure, although it will not eliminate Streptomyces from the soil.
Use certified seed potatoes: Start with healthy certified seed rather than planting scabby potatoes from a previous crop.