Common scab is not to be confused with powdery scab.
Powdery scabs tend to emerge as discrete eruptions rather than one large scab.
Plus powdery scab is generally prevalent in wet weather whereas common scab is caused by dryish conditions.
As the name suggests it is quite common on potatoes, but it can also be found on beetroot, radish, swede and turnip.
Affected potatoes are fit to eat, and the infection has very little affect on storability.
These infections are caused by soil organisms called an actinomycete.
Potatoes grown on light sandy soil that is deficient in organic matter and is relatively dry, are most susceptible to the infection.
To control it, it is important to plant unblemished seed potatoes, and to practice crop rotation.
Purchasing resistant varieties will also help.
The condition tends to be worse on alkaline soils and under dry conditions.
Add organic matter to the soil to improve water retention and water during dry weather.
There are no chemical controls available to the amateur gardener.