Hybrid fruit tree

I bought four fruit trees on a bargain offer. The plum tree is a dual in some way.
The graft is at ground. The split in the trunk is 9"" up. One half is an old looking twig that is showing plum blossom.
The other half is much stronger, but I don't think its a plum!
There is no sign of a graft at the trunk split.
The odd branch also has spiky short twigs which occur regularly at inch intervals, but the pattern repeats as three spikes- most fruit trees would alternate. No blossom at all- just leaves, this year and last.
The leaves are light green, ovoid, fairly shiny and very gently serrated on the edge, a bit like an ash. Presently about 15-20mm long. Bottom line- should I dig this tree up and start again? Or prune off the much bigger dubious bit and hope the twig blossoms?

 

Hi Colin, can you add a picture of the trunk? It will make things easier to see what's going on.

 

Sounds like blackthorn to me (Prunus spinosus, sloe). That always seems to have thorns sticking out every which way.
It's related to plum (also prunus) so could be the rootstock, or maybe some sort of reversion. Or maybe whoever grafted it likes sloe gin

 

I have images of the two trunks and detail of the "faulty" half attached below. You can see half in full blossom. The other half has the spikes I mentioned clearly visible. Went for a walk today, saw dozens of trees, shrubs and hedges and nothing like this!

 

I think the graft was higher, at the point where the flowering variety starts. It looks as if the rootstock produced a shoot which was allowed to grow and is now beginning to take over. You probably don't want this, as rootstock varieties won't produce nice fruit. You can see the wide variety of rootstocks used, here:
Rootstocks for Plum trees

 

PS, the most common one is St. Julian, which is derived from Prunus insititia which includes damsons..

 

Definitely the rootstock trying to take over. Cut it off and keep an eye open for new shoots around the wound. Rub them out when they're young and soft.

 

This was why I was confused. There is a definite "knobbly" point that looked like the graft, about 9 inches below the point where the growth splits into 2. There is no evidence of any graft at this point, and to have 9inches of rootstock with no roots from it seemed very unusual. So I planted with that knobbly point just above ground level.
A possible leader has been pruned from the flowering part. The rogue part is pretty straight.
I think it's coming off! As you suggest.

 

Forum would not allow direct reply to post above with Rootstocks link. I looked at this and a sloe derived rootstock seems the best possibility, bearing in mind the rogue growth spikes and the fact it has not flowered.

 

Read the label! I cut off the rogue branch. On the back of the label, I found a picture that says the top is Prunus domestica Opal and the rootstock is Prunus Hybride St Julien A.
So the mystery deepened. The main graft is definitely at ground level- several small knobs, much bigger than the trunk. The trunk bark has had several side shoots removed, but looks similar to the desired branch and nothing like the rogue, which is fairly smooth with lines parallel to the growth direstion.
Pictures of Prunus Julien A show a typical plum, no spikes. So where this odd branch arrived from and what it is looks like remaining a mystery.
Hey ho for the next battle against woolly aphid. I find the first line of attack here is to stop the ants from farming the stuff. Follow the ants up the tree to find the aphids....Kill off the nest...rub out the aphids....

 

Getting rid of the top growth from the rootstock will enable the named hybrid to grow more strongly and it should be better able to withstand aphid attacks.

I find that not using insecticides and feeding bluetits near to my fruit trees and roses etc attracts them to the area and they harvest the aphids for their nestlings and bring the fledglings to help themselves. I’ve not had to do anything about aphids for years.

 

When working in a workshop on an apple orchard they usually planted new trees each year.
I used to chat with the farm manager quite a lot.
Anyway, he told me one day that a massive bunch of bare root trees that were piled up in the yard were double grafted, root stock then a different section finally the variety on higher up.

Trouble is I can't remember the reasoning behind it but it must have a reason.

Could that have any bearing as to why the graft appears so high up.

 

Some root stocks are incompatible with the scion material so a second variety, compatible with both, is grafted in-between. Apparently some pears are not compatible with the usual quince rootstocks.

 

Thanks Nigel I knew there was a reason, I was thinking along the lines of some varieties don't make good strong stems but your explanation sounds good.

 

Yes and yes, I think. When I truncated my rogue branch, I looked very carefully, and there is evidence of a graft at the join point. So my root is Julien, the trunk is "don't know" and a little bit of plum is at the top. The rogue grew out of the trunk part, but still succeeded in having bark that was very dissimilar. The trunk bark also manages to look like the bark of the plum- age maybe?
I'll see what happens next- I hope the remainder will sprout some new branches, but anything from the trunk will be rubbed out.