large brown spots on kumquat leaves

[incubator01]

I have a very large kumquat tree since last year (spring) and after a few months it started to lose leaves, first green ones, then some started showing brown spots. Despite all this it survived the winter and since its pot was too big anyway I transplanted it into full soil (mixed with peat and pine bark) in my greenhouse since my citrus do better there.
Currently the soil is medium-dry (definitely not wet), I watered it one week ago.

But it is now losing all of its leaves, most with the brown spots.
It's a fortunella reale kumquat grafted on volkamerian rootstock.
Included are images of the leaves, I don't know what this is, overwatering would be weird since I watered sparingly during the winter. At first I thought it was Citrus Alternaria Brown Spot but those spots look different.

[brian]

I have many kumquat trees and have never seen that.  It looks sort of like sunburn but wouldn't explain why leaves are being lost all over.  My only suggestion is to dig around the roots to look for rotting, dryness, disease, insects...

[incubator01]

Can't be sunburn because it had this even in the winter, and we are now having spring sun.
When I gently (not deeply) sprayed the roots they still looked bright yellow.
I am starting to think there's a fungus at work but again, I'm unsure
I did notice more parts of branches turning brown which does indicate lower parts of the soil may still be too moist, or he's dying slowly.
A shame since this is a tree with a thick trunk that was delivered last year in a 44cm diameter pot, but at the root flare I did notice some vertical cut (damage or crack) in the the bark, maybe this introduced a fungus.

For now all I can do is wait and trim down everything that turns brown. I could always intermediate spray with an anti fungal product but since there are no more leaves I don't know if it'll work.

[brian]

I don't know if your issue is indeed fungal, nor what type, but all I can say is I have had reasonably good success with AgriFos/GardenFos against phytopthera infection, with about 2/3 of the damaged trees recovering.