I had not checked on my cactus for a while, possibly a month or two, due to it being winter and the cactus not needing to be watered. I originally would check it and make sure to turn the pot around in the window so each side would get equal sun. However, I obviously stopped doing this when I stopped checking it and I finally noticed some startling looking spots that have showed up.

I will attach pictures. These spots are not soft or squishy, and are actually just as hard as the rest of the cacti, if not a tad bit harder. I'm thinking it's from lack of light, due to it being on the side that was not facing the window for a good month or two. But I would like someone's opinion who has a lot of experience.

There is also lots of little webs on the cactus, I imagine they are just from spiders. I don't think there are spider mites or any other pests.

Looks like sun damage as you said if you would always turn it and when you stop it's not used to getting that much light on that side.

This same thing happened to one of my trichs!

It's an indoor cactus, no Sun...

It starts as a translucent white film, then fries the cactus...

-eg

Hmmm, if it was the inside side that got those spots though, there might be another issue.

If it is not sunburn, my guess would be pest damage. That looks like corking that can happen from bugs that suck the juice out of plants. Give it a really solid once over for pests, an maybe get a soft toothbrush and rub it down good with some warm water.

who knows at this point
pretty common from anything that can damage the skin, scale,sun, other pests etc
if its dry and hard then further damage or loss of plant is unlikely

In my case that's what I figured, it does not appear to be spreading, and the pup growing in the same pot as my damaged trich is unaffected...

I just hope it doesn't happen again, I lost a good chunk of unusable plant tissue from one of my larger indoor stalks...bummer...but the rest seem fine.

-eg

To the original poster:

Is that a Trichocereus peruvianus?

-eg

It is a bridgesii.

And I believe it was the side that was turned away from the window for a while. However, the two basal pups that have grown from it seem completely unaffected, as someone else mentioned aboved. So I find it hard to believe it is pests, otherwise the pups would be suffering too right?

So the cactus has been placed outside in the sun, same place I had it all of last growing season and it loved it there.

The weird appearance has seemed to spread to the rest of the main stalk, except for the very top couple inches that are new growth. The arms that grew from it last year also still are untouched, and seem healthy.

Other then the discoloration it seems happy and healthy, as it has been growing just fine and the new growth looks good. I'm curious as to why this is spreading though. I noticed some fine spider Web like material clinging to the spines, but I don't see any actual bugs on it. It's also not soft to the touch, it's as firm as a cactus should be. Should I treat this as a pest issue and see what happens? What would be the best bet to get rid of pests if they are present?

Thanks for the tips in advance. I want to get around to cutting and cloning, but I want to treat this issue first.

I'm still having similar issues as well...

Though it's only spreading to a damaged section of the upper stalk...

I've got to get to cutting and cloning...This weekend.

That same damaged stalk just put out two pups though! (Picture attached)

One of the pups has 6 ribs, and I want to keep it healthy, I may have to remove and root it on its own if I think that what ever this is will spread to it...

-eg

I knew it had to be bridgesii or peruvianus.

I wish I would have picked up some potted bridgesii, my next one will be bridgesii for sure, I have heard nothing but good things.

(My damaged stalk was also damaged on the side facing away from me, though it was facing a light...I'm glad I revived that stalk because it's putting out pups, but I think I'm going to re-pot it, get a clean pot, clean soil, clean up its growing space, etc...)

-eg

I'm thinking ofor doing the same with mine. I'll replant the damaged base, while also taking cutting from it to plant, and removing the small arms from last season and planting those.

I just want to know why it turned out how it did.