I have a very old German Prune Plum tree with a rotten trunk (see attached photo). I have added some structural support to keep it from falling over, but this will only last temporarily. I would love some suggestions on how to seal up the rotten hole in the trunk to prevent further rot and possibly add some structural support. What filler material should I use? If attempts to save this tree will be fruitless (pun intended), should I not waste my time? Thank you.
I’m not seeing the photo either … and am not the guy to ask about how to post one 
Concrete was used years ago, for large trunk cavities. I’ve drilled bolts with washers and nuts through limbs & trunks ... and saw wires employed, too. I’d scrape out the decayed wood, removing nesting material for grubs, and allow it to stay as dry as possible.
Also, as John suggested, you can graft a piece (scion) of it to another European prune tree, or to a rootstock grown for the same.
The HOS has an annual Spring event where rootstock can be purchased and custom grafts are done. If you cut & stored some wood from this season’s growth, around January or Feb., brought it to the event, had our experts recommend a compatible rootstock, for a nominal fee the grafters could make you several ‘new trees.’
Somewhat lazier ..I’d often purchase (in your case) a European Prune tree from a nursery, remove & graft 3 of say 4 limbs with your variety, then you’d have a much larger tree, with some pollination from the ‘base variety,’ too. Once an HOS grafter, I’d suggest then have folks like yourself show up with the nursery tree … then graft over it’s limbs on-the-spot ..you’d be charged per graft, or not 
I agree with everything John and Viron have said.
It's not too hard to start plums from dormant hardwood cuttings. They have a natural tendency to grow roots from the stems, if kept moist. The way I have done it is cut dormant sticks, about 1/4 inch diameter and about 1 foot long. I make a couple of cuts vertically in what will be the lower end, to expose the greenish cambium. Cambium is sort of the "stem cell" (pun also intended 
). I like to use rooting hormone then, per label instructions. I don't know if that is necessary. Then I put the sticks into the ground, about half their length, is a spot that will be sheltered, moist, shaded - such as behind a shed. If winter will freeze them, I cover with leaves. If I have voles (I do), I cover with hardware cloth. Then as long as they stay moist, I leave them alone. I check them in the Spring. If they have grown roots, then you're in business.
There have been times that I wanted to use plum prunings as row markers in my garden. They often take root and grow. I also used some much bigger sticks, more like 3/4 inch diameter, 3 feet long, to hold fencing in place. They grew. Not always, but it's interesting what can happen.
Grafting is also fun like John and Viron said, and can give you a fresh start on a new rootstock.
What is a German Plum? Is it like an Italian plum? Or like the Grench ones?
Idyllwild
simplepress
jafar
Marsha H
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