If you were here in the U.S., I'd bet a dollar to a doughnut that its a bacteria called 'Streptomyces scabies', better known as scab.
An authoritative site says "Scab is soil-borne and can live a long time in soil without a host to feed on. It is the causative agent on all the root crops with warts, scabs, or brown sunken depressions on the skin. The bacterium doesn't hurt the inside or change the flavor of the root crop but you shouldn't store root crops with scab because other organisms can enter through those scabby spots.
Don't use lime or wood ashes in soil where you plant root crops. Also, if you are using manure make sure that it is composted completely. Plant your root crop area with a legume companion for a couple of years in a row. Also, scab doesn't like soils with high pH. The best thing is to establish a raised bed area with good compost for the root crops and use the old beds for other vegetables."
Perhaps you had beets or potatoes planted the same soil last year...