Is this a sunburn or something else?

[Armen Tsirunyan]

Hi all,

So 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. :

DSC_0575.JPG (45.74 KiB) Viewed 520 times

Is this sunburn or something else? Is it treatable?

[Steve Johnson]

Definitely looks like sunburn, although I'd consider it to be scorching because the skin is permanently damaged. Since you have to winter your cacti indoors, it's best if you keep them away from windows, otherwise sun exposure through the glass can lead to the problem you're seeing here. Looks like you have a fairly young plant, so unless the growing point got damaged, healthy new growth over time will make the scarring less noticeable as it gets bigger. If the growing point dies (called "going blind" in cactus terminology), your Mamm will start growing new heads around the dead point. I can show you a similar example of this behavior, when my Coryphantha retusa went blind after an attack by armored scale. Here it is with 2 new heads at the end of the growing season last year:



This year -- growth of the heads on June 28 and September 20:



For collectors who need to winter their cacti indoors, gradually acclimating them to full sun when they come outside in springtime is required. Just something for you to keep in mind if you're not already aware of the issue.