I'm a bit worried about some of my seville orange seedlings. New growth usually dies back and new leaves are often malformed. Is this some kind of disease?

I once got these seedlings as a gift from someone of another forum. This is the better looking of the two seedlings. I have put the other one in-ground in the back yard since we have a very mild winter...

Anyone?

Thanks for posting the pictures. I've got some of the same thing on some of my stuff, both in ground and potted. I thought it was because my plant wasn't happy in the winter time. I've had some die but I'm not sure it was from whatever this is. I hate to say it looks like a fungus or virus to me but that really means I don't know. Kind of like at the Dr. when they don't know what's wrong ! Tom

Whenever I hear "die-back", my default assumption is root rot. The soil mix in the pot you show looks like it might retain too much moisture (although of course it is hard to tell from a picture). Does it drain freely and dry out between waterings?

Same here.
Root rot.

In fact, the soil the seedlings were originally planted in held too much water. I replaced it with the soil in the picture which consists of (almost) pure cocopeat, which dries much faster.

I don't think it is root rot, as only the very new growth was affected, while the balance of the tree seems to be quite healthy. There is certainly more than one reason for twig die back, but one of the most common for twig die back and leaf drop in container trees is when the top gets too large for the capability of the root system. One other point. New tender growth of both leaves and stems, transpire water through their surface much faster than the older mature growth. Therefore new growth requires a constant supply of moisture to replace the moisture lost through transpiration. If your tree has had periods drought stress, the top new growth would be the first and hardest damage. - Millet

As far as I can remember from changing the soil, the roots are quite healthy. I had a severe root rot problem with my bergamot (on PT rootstock) a while back and replaced the soil of that plant with the soil I'm now also using for the seville. I expected the bergamot to die but it is actually recovering nicely now!

When the seville starts new growth, this new growth is fine for the first 2..3 leaves and then starts to weaken, with irregularly shaped pale yellow-green leaves. I usually remove those parts of the new growth.

I also have a seville that I bought in a nursery, which is actually a seedling of about 5 years old. This plant grows in the same soil as the others and is doing great.

I have to say that I always have issues with potted seedlings... I had similar problems with sweet oranges, grapefruit, pummelo and PT. Usually when a seedling doesn't grow as expected I simply throw it on the compost pile. I do want to keep the sevilles though...

I noticed that the PT seedlings do much better after putting them in-ground in the back yard. Since we are experiencing a very mild winter I did the same with one of the seville seedlings. I'm curious to know what will happen!

Your last answer suggests a probable answer. It may be the water. It looks like the growth on blueberry plants when the soil pH is too high. New growth starts out fine, and then turns brown and dies. City water if it is soft will have a high pH. It could also be a build up of salts, also from the water.

pH of tap water is indeed quite high here. But when the plants are outside they usually get enough rain water that I don't need to use tap water...

Anyway, I really should start using rain water!

My first thought was the same as HardyVermont, that being high soil soluble salt damage, as the damage does look very similar to soluble salt damage. However, all the other leaves look healthy. If indeed it was salt damage, one would think that at least some of the tips and margins of the remaining leaves on the tree would also show some signs of burning. - Millet

I don't bring any answer, but I draw your attention to the fact that the die-back seems to begin by bark splitting.
This could be a symptom.

I noticed that too, and this worries me a little...