My guess is this has been caused by guttation. Although I have never seen it on sweet potato, I've never grown it as a houseplant, but I have seen this on other houseplants. Some, when they are over watered, secrete the excess liquid from their stomata ( respirational pores on the upper sides of leaves). When and as this excess liquid dries, it leaves behind little crystals resembling sugar. You can just wash that off, it's soluable. Perhaps this is what is happening to your plant. I also suspect too low a light level, causing it to be very leggy.

Thanks for your thoughts. I'm not yet convinced this is caused by guttation as I have not witnessed the moisture build up...and I inspect this plant often. I also noticed that with other guttation pictures, the moisture builds up only around the edges of the leaf. That is not the case here. Any other comments out there?

Does it wash off and melt like sugar? Do the leaves show damage after you wash it off?

Sweet potatoes are among the vegetables high in calcium oxalates, which can be exuded through plant pores (stomata) under certain conditions. It will form sand like crystals when it dries. The mechanism is not exactly like guttation in which a function of hydathodes exuding excess internal water.

I've never seen this in sweet potatoes, but I've never grown them (inside or outside) .

I will wash the leaves and see what happens. I don't think the substance is damaging the surface, but the leaves are curling. I'm not sure if this is related.

how to get rid of it?

===>>>> ummmmmmm ... go buy new sweet potato .. and start a new one???

keep that in mind.. if you decide you need to start spending money on curing this ...

no use spending say.. 5 bucks.. to cure a fitty cent potato ... lol ... yam.. whatever ....

ken

ps: all my houseplants.. in z5 MI .. are outside for summer.... makes me wonder about your indoor .. outdoor plant.. in heavy a/c .... anywhere near a cold air stream????? ... just shot gunning the thought process here ....

Hello! Mine has done the exact same thing! I was trying to search online but can't really find much about it. I also have my plant indoors. And not all leaves have the sugar like crystals and the amount varies.

As has been said, oxalate crystals. Not damaging. Occurs with certain environmental conditions.

See https://as-botanicalstudies.springeropen.com/articles/10.1186/1999-3110-55-32

The things on the stems appear to be root initials. So I suspect high humidity.

Mine is doing the same thing but I live in Colorado and its not humid here at all. It seems more like its pushing out a chemical than it is water. They aren't hard crystals, almost waxy. Its with many other house plants and the sweet potato is the only one doing this. Also not on all leaves, but the leaves it is on it stays.

Small blister–like bumps on the leaves of ivy geraniums, sweet potato vines, tomatoes and other plants growing indoors or in greenhouses are symptoms of oedema (edema) and intumesence. Fortunately, these are physiological problems and not caused by disease or insects.

Basically, the plant forms these growths in response to the environment. The two terms are often used interchangeably, and the cause is not well known. Many researchers believe the plant absorbs more moisture faster than it loses it through transpiration. Others are finding it related to light quality.

These conditions cause the blister-like bumps that eventually turn brown, dry and corky. The problem often corrects itself once the plants move into the garden and receive the preferred growing conditions.

In the meantime, continue to water and fertilize as needed.

A bit more information: Oedema can be mistaken for mite and thrip feeding. Hold a piece of white paper under the damaged leaves and look for tiny specs, the mites, moving across the paper. Use insecticidal soap or Summit® Year-Round Spray Oil to control the mites.

It started out all over this leaf as white sugar like substance, didn't wash off, and now turned black on this leaf

Next time, bury half of your organic sweet potatoe in a pot of dirt. A sunny window and do not over water or it will rot. Cut the slips off as they grow and put in a glass of water until 2" or more roots appear. Then plant in the garden.