How many ribs does the stock (bottom part) have? If it has 3, then it's probably Hylocereus, which eventually develops a woody "stick" inside the the scion (top part) that makes it difficult to separate and root. I'd wait until it gets a bit warmer this spring, then cut off the scion at the base (but above the union with the stock) and let it dry for a week or so, then root it on perlite or turface. Don't try to pull or twist it off, as you said it might get ugly!

Hi hanzrobo :) hi cmc :) Thanks for the replies! I initially severed on the hylo side, but you could see a lot of hylo in the sperical top cacti. I had to slice into the 'rebutia' to remove the triangular hylo. Not too happy about that part, but as you say, the season is on my side. Seems like you can stick just about anything in a bit of soil at this time and voila! New plant. I should dip in rooting powder right away? I'm not too sure about the order of things. Previously I've dried out my piece/cutting and then dipped in the hormone, but perhaps I've been doing things wrong...mind you with the fungicide that makes sense... The top is the healthiest most solid little green being inside . I've not sliced a cacti open before and used a very sharp Henckels knife - it went like a hot knife through butter it was that easy. The inside was just so...clean, green, and solid. I'll try to get some pics up...hubbie's in the market for a camera.

You may have seen this site, and this may even be where you got your info. I wish I could help you as O. compressa grows wild all around me, but we're half a world away... Anyhow, check this site out if you haven't seen it.. Bill in SC USA Here is a link that might be useful: O. compressa grafting link

The top is a moon cactus(don't remember Latin name ) the rootstock roted. The root stock a cardon had top stem rot hopefully the graft will set and new life for both plants!