Here is a little update about my attempts of growing Morus Nigra in my zone 7a.
It seems I managed to kill a graftet Nigra "Jerusalem" I got as a gift. The tree was growing in a container and out of a sudden all its leaves became yellow and pale, plus it stopped growing while my other nigras are still actively growing. I did suspect some root issue since the container was too small for the size of the tree and I already had its repotting on schedule for the winter. I saw the tree was in trouble and decided to repot immediately, probably a mistake. The tree shed all its leaves and after some weeks without progress I did some scratch testing at the rootstock (alba). The cambium of the rootstock was already brown and dead. I did some emergency grafting to some alba seedlings I had around but I probably was too late. The buds did show some green still but appeared quite dry already. I don't know if the yellowing of the leaves was caused by a dying rootstock or a late grafting incompatibility. The tree did not grow much this year, plus it did show the same symptoms in late summer last year. I suspected issues with my fertilizing but wasn't able to work it out. Tried different things without success (calzium, magnesium, iron, nitrogen)....
Some time ago I did a report about my ugly grafts of scions I gathered from two big old trees. Those grafts are doing exceptionally well. I am quite surprised to see aprox 2 feet of new growth from the upper graft. It is still growing.
The lower graft did put on only 6-8 inches but thats because of apical dominance/shade from the upper graft.
The tree is in ground.
We are having a very wet summer. Still there are no signs of fungal or other problems yet.
I do have another purchased Morus Nigra "Sham Dudu". It is grafted high on an alba rootstock. The grafting point is not very well attached to the stock. I am going to bud it lower onto the same stock. It is in a container and doesn't seem to have the same trouble as my (ex) "Jerusalem". It is growing very well too, aprox 1'4''. Again I didn't suspect it to grow as much, cause everywhere I read they are very slow growers and the tree is growing in a container.
Right now I try to root some green cuttings of Morus alba/rubra "Wellington" and a Morus Nigra. I would really like to have them on their own roots.
I built a fog chamber and am using bottom heat. It seems to work with "Wellington" but doesn't with the Nigra. It is still too early to throw in the towel but the green cutting of Nigra didn't develop callus yet while the cuttings of "Wellington" did. Both show no roots but I bet the cuttings of Wellington will root in the end. The leaves still look healthy, while the leaves of the Nigra cutting are looking weaker every day. The experiment is going for 6 weeks now. I already potted up some rooted fig, plum and apricot cuttings I did start at the same time as kind of my control group.
If they don't root I will try more mature cuttings. But I don't have enough material to do larger testings yet.
To build the fog chamber I used a fish tank and combined it with a fogger regularly used for reptile houses. That idea I did lend from @Alcedo. I would really like to thank him for bringing this up. But sadly thats not possible since he passed away.