Scale, I think. Never found an adequat recepe against it. Isolate and/or throw away the worst. Most plants will survive, but are not showy anymore. Good luck. Hard decisions.

Thank God I have never seen this on mine. I hope it all works out for you! Can you use some sort of safer Soap or Horticultural Spray and see if it halts it? How about a drench of systemic insecticide? I would try anything to see if it works. Don't just toss them. Try until you figure it out or else you will loose others before you can even figure a treatment for the future.

Red spider mite and /or sunburn......

The dried skin on stem should be sufficient proof of damage... pretty certain i could see red dots in places too

Azt--I am so sorry. I know how I'd feel if my collection was impacted like that. Good luck.

I feel so bad for you. As Nil suggests, use/get a loupe and look if you see red spider mites. They are bright red and move slowly. I think there is also a more brownish mite, but with that kind of damage, they should be easy to see. I also think it looks like red spider mite, but you have to be sure before you can treat.

If you have mites and or scale, easy to get rid of...Don't despair..

I appreciate the experienced eye. I'm leaning towards mites, as well. The entire collection is affected with this plague. Everything from my beloved, mammilaria theresea to all my hybrid echinopsis. Many are irreplaceable and rare as the sources are no longer available. My plants didn't have issues with sun in the 4 years before, then one Spring I began to see what looked like dry rot. It appeared while under the patio protected from weeks of rain in late Winter.

Thanks milevskistefan
My understanding of mite infestations is they are more active when the cactus begins growth of new green. Some mites like humid others hot and dry so still no clue what type if they are present. 30x loupe......I'll see where I can find one.....

Thanks for the help, advise and support.

Red spider mite damage. Get a systemic miticide TODAY.

Yup I'm with Spanish fly. The nite you talk the faster they destroy. To topically kill them immediately get Safers Soap spray at Lowes. Good luck

ewwmayo has a guide that has helped me in my own battle [here[(https://www.houzz.com/discussions/insecticide-pesticide-insights-dsvw-vd~3742440)

Our main spider mite in SoCal is the two-spot spider mite. So that's what I would be looking for. And they have been terrible this year due to the lack of rain. The bigger red mites we have are predators of the two-spot spider mite.

Thanks for the info, everyone. Appreciate the lead Nii13. Just ordered Floramite. Not many miticides can be found or shipped to California will be starting with Safer (thanks Mike) to blunt the new onslaught. I really hope I can save my treasures from these hateful pests.

My cactus blooms bring me so much joy every Spring. Sigh, declaring war....