I have a young japanese maple bonsai (main trunk is 1 finger thick) that I've had for the last year and a half. The first year, it had a huge outgrowth of leaves in the spring, and I was very happy.

This year, while the branches have several leaf buds, they have not opened, even though it is late spring and very warm and humid. One leaf bud opened, but the leaf never fully emerged/unfolded. Several of the leaf buds have cracked and are black on the inside.

However, the trunk and branches are visibly bright green, even without a scratch test, so I am confident the tree is (overall) alive.

As the plant was in potting soil, I became concerned that root rot was the issue. I have since repotted the tree in draining, large-granule bonsai substrate (pumice, lava rock, calcined clay, and pine bark) that I mixed about 80/20 with soil, and also threw in some blood meal (extra nitrogen and keeps the squirrels out). I've been very careful about watering since, only giving more water when the substrate is very dry (once since I've repotted)

It's only been a couple weeks since I repotted, so I understand that I may just need to wait longer. But I am starting to see die-back on the branch tips.

Most of the online plant diagnoses I can find online use leaf characteristics to identify the issue, but I don't that to go off of. Some sites have mentioned verticillum wilt?

Any advice would be welcome. Thanks!

Sounds like your repotting is to blame. Some things to think about next time:

There is a limit to how much roots one can prune without damaging the overall health of the tree.

It is utterly important to keep the roots well wet during and after repotting. Overwatering is impossible on a newly repotted maple. Underwatering will kill fast as roots are not up to speed.

After repotting the tree should be kept in shade. Just to put less pressure on the newly damaged roots.

Never fertilize a newly repotted tree. Wait at least a month. Fertilizer can and will burn the last few feeder roots.

Hey Josef,

How have you kept the tree over winter? Could it be that the roots were wet and froze deeply?
Considering you repotted in an attempt to deal with the tree not leafing out, I very much doubt that is the problem.

Hello,

When I repotted, I did not remove any roots. I removed the soil, from them, which I know could have damaged the finer root system, but no drastic cuts were made.
Since repotting, it has been on an outdoor patio, in a location that remains fairly shaded all day to prevent sun stress.

The plant spent the winter indoors next to a large window (I didn't have a patio at that apartment), so I don't think cold stress could be a factor (also, I live in Texas, US. We get the rare cold snap, but are usually very warm).

I have not used any liquid fertilizer. I did add some slow-release granules to the new substrate.

Thanks,

Joseph

Ahh, good input. Rare coldsnap, that is like usda zone 10? Japanese maples are listed as hardy in zone 5-8. It does not mean that it is impossible, just a bigger challenge. You really need to find someone locally that are growing them successfully. Only one thing I am absolutely sure about is that they should not be taken indoors when one get a few cold spells per year. They endure a lot more cold than that.

Yeah I'm in USA zone 9-10.

True, it probably would have been fine with the temperatures I had, it was just indoors because my old apartment didn't have an outdoor space to keep it.
Either way, I'm confident that there wasn't any cold damage to explain the current symptoms.

Update, there's one branch that's covered with leaf buds. I pinched open one of the buds on the branch tip, which was plump but had a black exterior. the inside was still green!
I'm taking this as a very good sign. I'm guessing the tree has just been in various stresses for a while (bad watering and now repotting) and I just have to be patient?

Patience is often the.best way. Good plan.

Update:
The tree is looking real bad. No buds have opened, and black die back is quickly spreading down one of the main limbs.

Based on the symptoms, I have reason to believe that the issue is Pseudomonas syringae infection. I'm going to prune, seal, and treat with Zerotol and just hope that I can save the main stem.

Any recommendations for dealing with this bacteria?

nope.
But based on your story I am not sure it is likely. But without pictures it is hard to tell

Sorry this took so long to upload. Here's the pictures of the tree.
I know it looks terrible, this has been about 3-4 months of buds not opening and slow tip die-off. Since it's reached the main branch on the right, it's been advancing faster.

As you can see, though, the trunk at the bottom is still quite green. The scarring is because this tree is actually grafted. I've seen lots of healthy growth from that side in the past, so it's not a problem.

I'm doing my best to monitor water in the soil and not overwater. The new substrate (as of 3 weeks ago) has enough soil that it is retaining water, but is much better draining than it was. As I've said, I added some blood meal and slow release fertilizer in the mix, but I have not used any liquid fertilizer.

But I'm at a loss for what else to do apart from sterilize my tools (isopropyl alcohol) cut off all the blackened parts, seal with cut paste, and then try to treat with bactericide.
I've seen some forums talk highly about Phyton27, but my order of that is going to take a few days to arrive.

It's looking bad, but this tree has a lot of sentimental value, so I'm not going to give up until I've really done everything I can.