I soak my kumquat trees in a bucket of water to hydrate all dry soil areas. Ilet the tree drip dry and then pull air through the root ball to pull out the excess water and aerate the soil.

You should be watering your trees until the water runs out from the bottom...It's a big no no to water just enough, they call that watering in sips, especially when watering with tap water, fertilizer and even worst organic ones Undissolved salts collect in the mix, especially if it's that fine like a sponge or filter and destroy the once healthy fibrous roots needed to do the bulwark of your trees needs.....You could do that using rain water...

As Susanne said and she gave some good advice, it is root damage and I would also flush my mix out with lots of fresh water at least once every two weeks.....Any leaf issue is usually attributed to root damage first then other factors.

Your edge roots in a clay pot may look totally fine, since they are the closest to the edge of the pot where oxygen and air is available since clay is permeable, but in the middle of the roots furthest away from the edge of the clay pot, that is a different story,,

Why don't you buy barbecue wooden sticks and or wooden dowels and poke it through the root zone and wait until it comes out dry before watering and then water thoroughly until it runs out from the bottom carrying all the salts away with it?

Why don't you buy a coupe of artificial lights if it's getting less than 6 hours of direct sun a day and use them?

I would stop fertilizing until you use a mix that is much porous than that. I would suggest making a much more porous mix for the next spring so that you won't have to worry about these issues...

MIke

Why do you say fert burn? I only see one brown tip. Also, it is organic fert being used, and since there is not enough microbial action in a container, then little/no fert uptake is going on. But maybe the buildup of salts is an issue?

Yes there is one brown tip and some other that starts to get the typical first yellowing. Then all the leafs that already came off to me looked burned.

I'm with Susanne...By the description of the watering practices, I would not trust that very fine mix fertilizer above to not hold too many salts.

Let me ask the poster..Do you ever 'flush' that mix out?

If not too many salts are sitting in that mix as we speak, and that is just one cause...

I don't know about you all, but do you notice sort of like a white powdery film on the top of that mix?

Hello everyone,

Thanks so much for all the very useful comments! This has been very educational.

Even though it was not my first guess, now I also think that
the cause might be root rot due to too much fertilizer. And indeed I do not tend to flush the soil. I
just did it, with lots of fresh water, as was suggested. The soil is super
compact and the water took forever to go through. I do not remember it being so
packed when I water, it must have gone worse in the past few weeks due to the
fact that I did not add enough water, fearing it would be too much with the
drop in the temperature.

Good news are that it is a plastic container, even though it
looks like a terracotta one. And I already added some artificial lighting, incandescent
at the moment but will try to get some led lights soon. I am still wondering if
I should re-pot, risking upsetting the fruits, but hopefully saving the roots
in the long run and giving them a better and more porous soil. I am hoping for
the best.

Yes, I would repot now into a lighter fast draining soil. The tree will be stressed and might loose more or even all of its fruit but you definately safe the tree. It will reward you with lots of flowers and fruits later. If you get it out of the pot, make sure there are enough draining holes in the bottom. If you can get a plant raiser between pot and saucer that would be ideal.

What does your 'organic fertilizer' fertilizer look like?

Many organic fertilizers are like pellets that actually make the soil mix sticky or hard...

Mike

I have gotten advice to remove all the remaining fruit to relieve
the roots of extra work so they can replenish, and also, to get it away from the
terrace into the room where there’s more warming and less light in order for
the plant to rest and not use its resources. I am not sure if that is what it
needs, to me it always seems that a little sunlight its getting is not enough
for a kumquat. Mike, to answer your question, I really think that the organic
fertilizer is just fine, it is the lack of watering through that eventually
lead to the problem. The package says that it is 2.7% nitrogen, 1.3% phosphorus
and 5.9% kaliumoxide

Who gave you that advice?

I can tell you that warming up your tree and providing even less light is a recipe for disaster..

Also bare rooting a weak tree at this time of the year is the not the best thing to do.

I can tell that the 'organic fertilizer ' has done nothing for your tree..I would consider another method my friend..

By the way, some of those roots look nice and white. Were they all? Did you pull some dead ones off.? It seems I see no fibrous roots, but that is just me. But at least you have some whiter ones. Did you have to pull any dead roots off? How long did you expose those roots to the air? How did you fill in the new soil? What kind did you use? Where does this tree reside now? Under lights? In a sunny window?

Mike)

I don't see ANY root burn, as was suggested.

Your tree is re-potted. Go ahead and eat the fruit It is time to sit back and let it do its thing. It will probably pull through next spring

Oh, I see only now that a part of my previous comment is missing.

Yes the advice sounded weird to me too, I got it from a worker at the plant store. The tree is still at its old place, at the terrace which provides most sun my apartment can muster, and is regularly exposed to fresh air. I do bring it in in the evening under a light, which is best I can do for now to increase the light exposure.

I was also very happy to see that the roots are not damaged, there was nothing to be removed. That is why I found it so strange that more and more leaves are falling off and the tree is almost bare now. I bought a soil mix for Mediterranean plants, like olives, citruses, palms, oleanders (or at least that is what it says on the package). I did not keep the roots out of the ground for too long, but it did take some time to remove them unhurt from the previous one as the soil was really tightly squeezed around them.

I am sad though to hear that the fertilizer did not work...what would be other suggestions?

Thank you all for posting!