Most likely a watering issue at this point. The ones in the larger pots look better to me. Tell us more about the culture. Indoors? Outdoors? Sun? Shade? Temp? Humidity? What is the potting mix like? How often do you water and how do you know when to?

They are indoors, at the south window, temp around 22°c-25°c (71-77°F)
Potting mix is a standard soil with perlite.
I water them when they look dry. Now they are all wet because I watered them recently.
I have some others that are in the bigger pots with the same problem...
Thanks for your answer :)

Looks like manganese deficiency. Fertilize with Foliage Pro 9-3-6.

you are stuck with this problem wen ever you grow kumquat trees from seed. If you don't mind the extra cost I would suggest foliage pro 9-3-6 It has much the sane micros that 24-8-16 Miracle Gro. You can deliver enough micros with FP with out blasting your plant with the high N-P-K of the MG. Use rainwater for its lower PH and or add a tablespoon of vinegar per gallon water. That will work well.

What Variety are they. They are growing much faster than mine did.

6b Steve

I don't know if I can get foliage pro here in Belgium.. I only have the 16-7-15 fertilizer that I use.
I use water that I collect from a spring in my neighbourhood and I already add a tablespoon of vinegar :)

These are the oval kumquat (fortunella margarita)

Those are the second hardest citrus trees to grow. I have the Meiwa round Kumquat. The Meiwa is just a little bit harder to grow. I will pray for both of us

6b Steve

jelina, manganese deficiencies are very common when a citrus tree produces a new flush. Most of the time as the leaf matures the symptoms disappear with out doing anything. Foliage Pro is a good fertilizer for container plants, but most any fertilizer with a formula containing a higher level of nitrogen and potassium ALONG with the trace elements should be just fine. Check the trace elements listed on your fertilizer and see if it contains manganese. Your trees will be OK.

Thanks Steve and Silica,

I hope it is true that this is normal ;-)

I just checked my fertilizer and it is high in N and K and it does contain the trace elements.

I hope they will be allright soon! Thanks for your help!

My kumquat seedlings have the same issue, still growing well. Perhaps a bit more fertilizer and a bit of iron and magnesium sulphate will make mine look better..

Edit typo: seedling, not seedlings. Have just one, can't afford to lose it!

how long does it take for a kumquat seedling to get to fruiting age?

Kumquat trees will fruit after the tree has put on a genetically predetermined amount of leaves up from the roots. My seed grown tangerine trees loose their thorns at leaf number 110-115 leaves fro the base. It has not flowered. My 2 fukushu kumquat trees from seed are at Leaf count 55-60 with no signs of flowering. I don't know what the count is.

6b Steve

I have several kumquat seedlings and a few are showing the same deficiency. I used foliage pro but likely not enough. I do think it is interesting that not all show it, since they were all treated about the same as far as FP dosage.

Jontte Kinnu a " bit of iron and magnesium sulphate: will not solve a manganese deficiency. People sure are overly concerned about a manganese deficiency during, or right after a tree's growth flush. One can see how common this type of deficiency is as just on this thread 3 or 4 people all said their tree shows the deficiency while their tree is in a grow spurt. If one is overly concerned they can always apply a foliar spray of manganese sulfate or manganese nitrate to quickly cure the "problem".

Silica: Well, no, it doesnt cure the manganese deficiency. I dont know what I was thinking when typing that, but I meant it will do some good for the plant. Thanks for correcting anyway :)

I've had same symptoms with other tropicals, but it has got over with time, so I don't worry about it. I was about to order some foliage spray to give a boost of trace elements, but forgot it when other plants recovered. Maybe I will this time, I want my plants to look no less than perfect ;)

Great explaination, thank you.

Pff, my kumquat decided it's had enough. On the morning I found it droopy and two lowest leaves were all crispy. That was clearly a watering issue(overwatering), no root rot were to be found but the area where the trunk changes to roots, were really soft.

So this one ended up with citrus damp off? Or?

I'll just start from the beginning.