Bumps on my potatoes - dirt or disease?

I sorted through my potatoes last week before putting them in the root cellar.  Many of the Russets and Yukon Golds had dirt colored bumps on them.  The red potatoes and purple fingerlings had no bumps.  Our soil is sandy loam, leaning towards the sandy.  We had this last year but I assumed it was dirt.  This time I noticed that some tubers had the bumps, some didn't.  None had just a few bumps so that got me thinking that it might be a disease.  While harvesting I didn't notice if one plant had bumps and another didn't.

The bumps are dirt colored when dry.  When I wash the taters the bumps are more black.  I can pick them off when they're dry but I think it takes some skin off with it.  I washed a "good" and "bad" potato (picture).  After sitting wet for a while I just tried to rub off the "dirt" and it didn't all come off.  Some is still stuck in the texture of the potato.

Anyone know what this is?

Thanks!

Hey Mike I think you have black scurf.

The following is taken from Cornell University:

This fungus is a common soil inhabitant and tuber colonizer, most easily identified by the small black sclerotia (black scurf stage) on the tuber surface. Sclerotia range in size from as small as a pinhead to as large as half a pea and are best described as dirt that won't wash off, but can be removed with a fingernail. Rhizoctonia also causes a russeting or checking of the tuber surface, which is common in heavier soils. These symptoms can be confused with those of common scab. The sclerotia remain inactive in storage, but contribute to poor appearance and reduced marketability. Tubers used for seed should be treated with a fungicide, for sclerotia can be a source of inoculum for sprout "burn" and stem cankers on early emerging plants. Delayed harvest increases sclerotia size and number.

Link to Cornell's page of all things potato disease

Hope this helps!

Thanks James, that looks like it!  So I looks like I should save the spuds that don't have the scurf for my seed potatoes.  Thanks!

You're welcome! Glad to help!