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I've put my cacti for wintering into an unheated attic, by a window where the sun falls after 2 P.M and till dusk. Since it's negative temperature outside (so it's already winter in a sense) I didn't expect the sun to be strong enough for a burn (especially considering the windows are dusty). But there you go, a couple of days ago I noticed these yellow spots on my Mammillaria sp.

Is this sunburn or something else? Is it treatable?

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It's really hard to tell if this is sunburn. It is damage, especially the yellow/brown parts. The darker areas may be just a temporary color change (reddish - purplish), or rot (blackish). It's hard to tell from a photo). I have this happen when I move cacti from one environment to another. So if you had these outside, but it was rather cold, and you moved them inside, and it was warm, then they could get sunburned, even though it would seem that there is rather low light conditions. Maybe this plant was in a shadier environment before? Or it could have gotten 'frosted' if you left it outside till recently. Though doesn't quite look right for this as it seems to all be on one side - as if from a light source like winter sun. But frosted cacti look darker green at first, then slowly darken and die, so it could be a delayed reaction. Many Mammillaria are not very tolerant of cold (though some are extremely tolerant).

Unfortunately there is nothing to do but wait. This is permanent damage (at least the yellow/brown part) and will not reverse. But the cactus can survive, and it's scars can sometimes add 'character' to the plant.

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  • I hardly ever have this problem. But a friend does. He takes his cactus in and out, from warm bright places, right out into the sun, then back in, then to dark cold places, all over the place. He doesn't listen to me. He complains about this and blames it on the heat and sun outside. I put mine in once (usually), in a cold but bright room, and out when it's still pretty cold, but it eventually is extremely hot against a brick wall. I only have issues when I leave them out too long, or bring them out in cold temps too early (they are much less hardy when going back out again). – Eric Deloak Dec 1 '15 at 19:22
  • It looks like cold damage, but it's probably only skin deep. I'd keep an eye on it and treat anything if it comes up, but it should dry up and scar nicely. None of the areoles are damaged, so it will still be able to grow out just like any other cactus. – J. Musser Dec 3 '15 at 3:57
  • Nearly all my cactus that exhibited this recovered. Some I like better for it. I did leave two out in temps where it never got above freezing (and I think the low was 9F), that were hardy to 15F. I brought them in and they looked fine. Then they turned dark green, then oozed, then blackened, then started to rot. They were completely dead. Not a good thing. They were huge columnar beauties too. Learned a tough lesson. – Eric Deloak Dec 3 '15 at 16:56

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