I got these rooted bridgesii about 2 months ago and they have been potted since then in 1:1 sand to cactus soil. They've been developing these black spots up top, which quickly callus to tan. I dug one of the spots up and it was only superficial..it didn't go deep into the cactus at all. My first guess was some sort of parasite but maybe it's just shock from environment change.

id say too much water.

Are you positive they were rooted? The reason I ask is I have had that on un-rooted cuttings but never rooted ones. Regardless it should be fine. The lighter part is usually caused from no roots to bring in moisture so the cactus starts to pull away from the skin in the sun.

Could be over water like FE mentioned but from the looks of the soil I don't think so.

Maybe a virus, but one that's killed by sunlight. These are hardy plants..perhaps it will take care of itself leaving it outdoors in the summer/fall.

...another idea...if it is a virus, it could have been picked up from the soil or compost used in the pot. Try repotting in a newly sterilized cactus-potting mix (plenty of online directions for this...you put the soil outdoors with a plastic sheet over it for a few days and let it "cook".
Most allergists know that house plants are the worst thing for allergy sufferers ... all sorts of stuff gets in there and takes off.

I'm not experienced with this, but I do know that watering plants with water that contains flouride causes the ends to die.

SWIM had a similar problem (black spots forming) before and it had to do with the soil as it went away after a proper soil mix was used. In your case it is likely the soil (and the root damage which results). These cacti (esp brig) need a very loose highly draining soil. Sand is good but the particles are small and they can still pack down making drainage difficult. SWIY should def add some perlite in. This wil greatly increase the drain-ability. Also leave the soil loose do not pack it down.

Change the soil to a mix containing dirt and perlite (40:60 at minimium). Even better you can keep some sand as well say (40 perlite 20 sand 50 dirt). Sand is good for root formation b/c it is soft but perlite is best for drainage. The problem should go away although the black spots won't go away they should turn light brown with time. After a few weeks no new major spots should form This was SWIMs experience.

Also make sure there are holes in the bottom of the pot for proper drainage.

It could also be over watering if so try and water less, only water when the soil is bone dry. But def change the soil. If the soil mix is correct you can't really overwater because the water will just drain away. When you water it shouldn't sit on the top of the soil but should rather seep right in. If it sits it is a sign that the soil is packed too hard.


Another possibility is sun burn. However it does not look like sub burn because it is only in a few select spots. Sun burn typically occurs at the apical meristem (the top tip) as this is where the new growth is and it is the most sensitive. Although the spots are at the tip they are isolated in cases of sun burn the entire radius would appear damaged and furthermore it is not typically black but a brownish rust appearance.

Good Luck

You're right about the brig being more susceptible to soil conditions.. have some torch and pedros and they've been fine in similar pots. I'll go buy some perlite today.. thanks all for the suggestions!
peace

Little update. I repotted them with 4:1:1 soil-perlite-pumice. They seem to be doing well.. you can see the fresh rot free growth! Is there anyway to to keep the perlite from "floating" to the top of the pot?