Q.Kaffir (aka Thai) lime tree
I have a Thai lime tree in a pot. So far as I know, it is not grafted, but about 6 inches above the soil line a big branch has come out that has totally different leaves (rounded tips, single leaf, very large and glossy) and very large citrus fruit forming, much larger than the little Kaffir limes on the main tree. The branch is definitely not grafted onto the trunk. Do you think this is a fluke or is it somehow a sucker that has appeared out of the trunk of the Kaffir tree? Thank you, Wanda Armstrong
It would be very common for the Thai Lime tree to be grafted onto a root stock, and I do believe that is what your seeing. A growth from the root stock. There is no way to really know what was used as the root stock unless the grower you purchased from would have a record of the root stock that was used.
From my research Trifoliate orangne is a common root stock, from which the fruit is not edible, but it certainly could be something else.
I would remove the branch, as it can zap the energy from your tree.
I have included a link to Thai Lime Tree care.