TBM long form, what is happening??

[wamilton]

My TBM cactus has been getting brown spot growing near the bottom:




It looks like rot, but the area is not soft like normal cactus rot. Also you can see in the bottom photo that the part under the top layer of rocks actually looks fine and corked over.

Can anyone help explain what is going on? I have several other cacti in the same area and they are totally fine...

Also it has these dark spots over the last few weeks:


I heard they are humidity spots but the area is pretty low humidity, 90-100 degrees F, 30% RH

[Tom in Tucson]

It appears that some pathogen has progressed up the intercalary meristem. In another post with a cactus having a suspected infection, I suggested some routine exploratory surgery could aid in the diagnosis, and, that's my advice in your case as well FWIW.

[wamilton]

how do you recommend I do the surgery? it sounds like the whole thing is infected, so is there even any point in saving it?

[Steve Johnson]

The black and nearly-black patches indicate necrotic tissue. If that's the case and necrosis sets in, the plant can't be saved.

[Tom in Tucson]

wamilton:

"how do you recommend I do the surgery? it sounds like the whole thing is infected, so is there even any point in saving it?"

The simplest way is to use a scalpel or very sharp knife, with some ethyl alcohol.

Sterilize the cutting tool, then cut about a half inch above the wounds. If you see necrotic tissue, clean the cutting tool again and cut further up the stem. Once you're past the infection, you can callous the cuttings, and use the typical procedure for rooting what's left.