Have you ever been scrubbing a potato and there are clumps of dirt that just won't scrub off? Well, I have news for you: that's not actually dirt. That's a fungus, called Rhizoctonia solani. And since it's growing into the potato, no amount of scrubbing will get it off—you're going to have to peel that potato to get rid of it. But if you don't peel the potato, you don't need to worry. The Rhizoctonia solani won't harm you.
Rhizoctonia solani is found in soils all over the world. In addition to the black spots on potato tubers, it can also cause root rot, web blight, damping off on a variety of different plants, as well as wire stem of crucifers [1] and crater disease of grains.[2] It can be distinguished from other fungi under the microscope because it doesn't form spores, it has sections [3], and branches at right angles (yellow arrows), which no other genus of fungus does.
Notes:
[1] Crucifers is a group of vegetables which includes broccoli, cabbage, cauliflower, Brussels sprouts, kohlrabi, bok choi, collard greens, canola, rutabaga, turnip, radish, horseradish, wasabi, mustard, cress, and watercress.
[2] See http://en.wikipedia.org/wiki/Rhizoctonia solani and http://www.apsnet.org/publications/phytopathology/1998/April/Pages/88_4_366.aspx.
[3] Actually, all fungi have sections. A related group of organisms, called oomycetes, don't have sections. Oomycetes are the organisms most often responsible for damping off. But when most people see oomycetes growing in their potting soil, they'll just call it a fungus. It's not important enough to correct them.
Image attributions:
Potatoes with Rhizoctonia solani spots is my own.
Rhizoctonia hyphae under the microscope is modified from http://commons.wikimedia.org/wiki/File:Rhizoctonia_hyphae_160X.png, which I donated to Wikimedia Commons.
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