Hard to say for sure considering winter dormancy is coming up. I'm thinking to let the soil dry out if you haven't already. Perhaps spray with Neem oil to eliminate any potential bugs and it at least has some topical antifungal properties. If you have a heat mat I would consider using it to warm up the root ball too.

Without seeing the soil the symptoms are typical to being too wet (too much water or too little drainage). Lots of assumptions in that last sentence but it is a safe bet to stop watering for a month or more.

Is your cutting rooted? at this time of season, I will check the root system. I will do bag rooting so I do not have to guess.

http://www.socalplumeriasociety.com/rooting-cuttings/bag-rooting-methods

the_first_kms2Yes I reduced watering to once a week the past two weeks, and the soil is almost dry, temperature started to cool just few days ago.

Kawagoe(Home of SF 49ers Santa Clara CA) No it's not a cutting, it's a full potted plant I purchased last year, and it overwintered very well, new leaves started to show on April, but they never grew up, and now no leaves and this fungus showed up.

I will let the soil dry and see.

What about the cut, is it clean? And is there any problem with the other branch?

And if I want to repot it, when is the best time to do that? and how?

Thanks for your help!

As far as the cut goes...it might be clean enough but should have a watchful eye on it and the other branch. It has a chance of black tipping further as we get into winter. Generally speaking No leaves = no need for water. 1x per week may be too much right now so I am glad to read you will let it dry out further. A warmed rootball will also help the plant naturally wall off the damage areas.

I would not repot this one until Early to middle spring unless the soil is the suspected culprit. In my Zone (8a/b) it can be somewhere around early March. I would also search for additional information about fast draining soil mixes from other forum members which may be in your same or similar climate. Sometimes easier is better so if all else fails use a garden soil amended with perlite or use a cactus/citrus mix amended the same way.

the_first_kms2 thanks for your notes!

That's exactly why I asked about repotting, because I've bought a fast draining soil and I would like to see it happy in it, like my other plants which I've repotted in this soil and look very happy with it.

Spring in our climate also starts nearly mid March, I hope it will survive until then.

As for the root ball, I will cover the pot with a wool cloth, like I did last year.

Wish me luck!