That's too close a shot for me to believe is a cactus. Could you please take a more general shot of the plant and the fungus?

How did that happen? Yes, it looks like a fungus, and it looks like it needs to get planted. Cut off the fungused part, look for darkened areas. If you have a dark area, cut above that, cut until the internal flesh is one consistent color. Let the plant sit in the air drying, making a callous.

Interesting. Looks like you had the corky lower part of the stem buried in your potting mix. Normally I'd avoid doing that but your mix is so chunky I'm not sure if doing that mattered. The part that looks bad on the cactus is a little above the mix and not below the mix surface. Anyway, the roots look great. This isn't an ideal time for rooting cuttings, going in to winter, but the alternative is not doing anything about the problem until spring and I'm not sure you can do that and have the plant live. You could try cutting above and below the bad looking part- cutting back until all the tissue inside looks healthy.

You might get two healthy parts from doing that. The top wouldn't have any roots, you'd let it callus as Rob mentioned, and eventually root it. The bottom part, if the tissues look healthy, could be repotted at a higher level in your mix. The bottom part might grow pups or a new branch eventually. Seems like with roots looking that good there should be healthy tissue below the bad spot, too.

Interesting Poly I don't see any roots.

Roots circled in yellow. Easier to see if I click on the picture for a closer look.

The really bright part in the circle seems to be a bit of direct sunlight.

yes i have been using this gritty mix for quite sometime because of the humidity in my country. also the corking has been around since i got it.. i have chopped it off because im staying in a tropical climate, hopefully the roots will still grow. i applied rooting powder too as advice by the seller who sold me this plant.

In the past we could add some location or climate info to our profile and show that info next to our name in our posts. However, looks like that currently isn't showing next to our names. Or, at least I'm not seeing it on my end. Or, maybe you didn't add that info to your profile. Anyway, I guessed incorrectly that you are in north America and going in to winter at this time of the year like most of us that post here. However, you're in a tropical climate. Just now I had a peek at your post history and read you're in Singapore. I haven't grown cacti in a tropical climate but I wouldn't be surprised if your Astrophytum will continue growing as long as the conditions are right and that might be all year for you. Or, I guess your cacti might grow less during the wet season because there is less sunshine. There isn't anything for scale in your pics but your cactus looks like it might be bigger than most of the Astrophytum I've seen.

im not sure about that because im still pretty new to this platform. yes singapore. the weather here is pretty much the same whole year round. um yes this is quite a big one i would say.. probably about 8cm or so.

I missed this post:(.

I’ve been under the impression that Astrophytums are troublesome to regrow roots from a cutting. The roots looked healthy in the before pic, was there brown rotten flesh when you cut? That woody base is fairly normal for an Astrophytum. The big gash isn’t, but from the looks it seems like the cactus healed itself which they sometimes can do. Oh well too late now its been cut. Luckily your Astrophytum is older and has some size to it so there’s enough reserves for it to go for awhile without roots, but if months and months have gone by and no roots there’s the option of grafting. I read somewhere it took two years for someone‘s Astrophytum to grow roots from a cutting. There’s tons of info about grafting cactus out there. Goodluck;) and keep us updated because I’m curious if roots will grow and how long it’ll take.