Sweet potatoes; Ipomoea batatas; found in all major sweet potato production regions in the U.S. and Japan.
Streptomyces soil rot or pox is a widespread disease of sweet potato in North Carolina that results in a reduction in plant growth and severe yield losses. It is caused by the soilborne filamentous bacterium, Steptomyces ipomoea, and is able to persist in soil for many years in the absence of sweet potatoes. Fibrous roots and storage roots are infected in the field and symptom development varies based on the timing of root infection [...]. Disease is favored by dry soils with pH above 5.2.
If you are cultivating your own, this is what that article recommends under 'Management':
Disease development is favored at high pH. Have field soil tested regularly and select fields with pH 5.2 or lower. Develop long-range crop rotation (3-4 years) in infested fields. Treat soil with a broad spectrum soil fumigant containing chloropicrin 2-4 weeks prior to planting.
A lot of people cook them with peel still on and peel it with their hands when cooked (Careful, hot!).
You can peel them before you cook them, and then it's just a question of taste I'd say.
Personally, I would peel off the white as well, though I'm not sure about this.
I used to cook plenty of purple sweet potatoes when I was living in africa:) Sounds like you maybe cooked them too long...
They can be extremely tasty, sweet with a "perfumy" hint if you are lucky. They are best boiled with the skin on like you did and peeled afterwards (as the taste is preserved better that way). As for cooking time it really depends on the size (which means the potatoes are likely done at different times) but you can always feel when they are done by pressing on the potato with a fork - it should give slightly/be soft. I think it usually took me about 30min but I didn't use a clock. It may shorten the cooking time slightly to make holes in the potatoes with a fork (this also adds to the "creaminess").
I guess they do sometimes get a softer, more "creamy" consistency than your usual potato when well done. However the taste is richer and in my opinion far superior to orange sweet potato. If you prefer a firmer potato, just let your potatoes stay on the firm side (in my experience, though, the taste is enhanced by letting them go soft). But boiled ones would anyways mainly be for puddings as it gets rather sweet.
For panfrying I would suggest avoiding boiling the sweet pots first. Simply peel/wedge them raw, fry them as chips until done and serve with a spicy/hot dip - yum.
Best Answer
Using a Google image search for 'sweet potato disease', it looks like Streptomyces ipomoea:
If you are cultivating your own, this is what that article recommends under 'Management':