Recently noticed these pale green patches appearing on some 2 year old San Pedros. They are growing under glass in a conservatory in the UK. Been growing these type of cacti for years in the same conditions and never had this problem until a few months ago. It seems to be spreading now, which is worrying. No sign of bugs. I'll attach a picture. Any ideas?

Even behind glass blocking most of UV, have they been getting a lot of sun light? Some say higher nitrogen nutrients can cause this when the growing season picks up. I've seen the same and some spots even becoming somewhat recessed; I'd still call it sun burn.

The conservatory faces east, so the plants only get direct sun in the morning, and being in th JK, it's not that intense. Ive grown lots of different trichos in the same location over several years and this is the first time Ive seen the pale blotches. I did use a seaweed extract fertilizer on them, but have used it before without issue.

Starbob, did the ones you saw recover?

Haven't been able to get a proper look at the photo yet, but it does look a lot like sunburn. One of my specimens got burnt from winter sunlight for just a couple of hours so don't underestimate what those few hours of morning sun might do. Important question WRT this is whether the blotches are just on the sunward side, or are they on the shaded side too? If the latter, best check for spider mites.

DFZ, the blotches are unfortunately on all sides, and are spreading to all nearby cacti, including some much more mature specimens. No sign of any insects, unless there are microscopic cactus parasites(?). Ive browsed cacti forums, but not come across anything similar. Thinking I should get the affected ones out of the greenhouse before I lose 7 years worth of cultivation!

I can upload more pics if that'd be helpful.

addtl pics can be helpful certainly. Are the splotches recessed/sunken at all? Is the spotting isolated to younger growth like top growth from the past year or so? You could try a calcium magnesium supplement as a cure-all kinda thing. Brevipalpus mites for example are microscopic and did a lot of damage to my cactus in the past, but the damage looks quite different IME. Worth taking a look with a jeweler's loop though.

Some more pics.

With a proper look at the new pictures it seems to me like maybe a virus or some nutrient deficiency. My technical expertise here is a bit limited though.

It may also be worth checking for pests on the roots. Look at the most badly affected specimen first.

Have you looked for tiny critters under high magnification?

I have similar pale blotches. I remember having them in the past and nothing bad coming out of it. My cacti keep growing, some have those blotches, some don't.

On Reddit you'll find some posts where people suggest such blotches could be TMV, but there is no consensus and apparently even if it is TMV, it doesn't affect cacti beyond their looks. And it doesn't spread by simply growing a cactus next to infected one, or via pests.

Also check out this video: https://www.youtube.com/watch?v=IyrF4WMHIPA