Following for answers

Is your tree indoors or outdoors and if so what zone? I've had branch dieback on indoor trees and I believe it to be root rot related or root damage during hasty repotting. I also had branch dieback on a Thomasville that was outdoors in winter and smaller branches (thicker branches were fine)most likely had cell damage from cold.

I’m zone 7b so they are indoors in winter. They didn’t do well over the winter this year and lost a lot of leaves. Did repot it right when I transitioned from winter to spring and when they were brought outside after winter. Have had to do some major pruning on some mature branches which died back. Now has quite a few flowers which have budded and a flush of new growth coming from several remaining main branches and near the main trunk. But a few additional branches have died back recently as well. So it’s kind of odd with a combination of healthy looking new growth but alongside the loss of some pretty sizeable and mature branches.

I will note several of the old leaves displayed some yellow vein chlororsis as well.

Thank you for that. So less likely to be any kind of nutritional deficiency? On a related note, I have some confusion about nitrogen requirement for potted citrus. I’ve seen a number of charts that describe the number of pounds of nitrogen needed for small, medium and large citrus. For example, some charts say about 1/2 lbs actual nitrogen for small citrus trees. But does this only apply for in ground citrus? I’ve seen some charts which say the same rules apply for potted citrus. But some granular organic fertilizers will day only to add 2-3 tablespoons of the fertilizer to a potted citrus. But with organic fertilizers in the 4-5% nitrogen range that seems woefully below the number to get to 1/2 lbs of actual nitrogen over the course of a year. Anyone know which method is correct in terms of calculating nitrogen requirement for potted citrus? By the way, I do supplement with water soluble fertilizer as well since I know soil microbe life is more limited in potted plants.

To clarify my confusion regarding nitrogen requirements, here is a chart discussing pounds of nitrogen per year. https://acis.cals.arizona.edu/docs/default-source/agricultural-ipm-documents/publications/2015/az1671-2015.pdf?sfvrsn=4cecfc9a_4 As you see, it mentions small citrus trees needing 0.25-0.50 actual pounds of nitrogen. In the double asterisk below the chart, it says this chart should also be applied for container citrus. Now, having said that, several organic granular citrus fertilizers indicate an amount which would be much less. For example, Espoma Citrus Tone says for potted plants to do 1 tsp per 4 inches of pot. So for a 20 inch pot, for example, that would only be 5 tsp of fertilizer. The actual amount of nitrogen in that quantity would be WAY below what is recommended on that Arizona citrus chart. Based on that chart, for a nitrogen content of 5% in Espoma Citrus Tone, you would need to use 2-3 lbs of fertilizer each of the 3 main feeding times during the growing season.

I know many folks use purely water soluble DynaGro and the like (which I do some water soluble fertilizer intermittently as well). But for anyone that does granular organic fertilizer in potted citrus, how have you been handling the nitrogen requirement? While the 5 tsp amount for a 20 inch pot seems quite low, it also seems like quite a lot to do 2-3 lbs each feeding during the growing season. I would imagine the 2-3 lbs amount would also result in quite a bit of salt accumulation in a potted plant.

Twig die back is common to all citrus cultivars. A citrus tree is a unit, and the die back in some cases is the tree's method of keeping the balance between the scion and the root system. There are of course other reasons, that are to the care one gives to the tree. If for example, insufficient, water.

I’ve attached some photos of what some older leaves look like before falling off. Also have one showing where there is a flush of new growth as well. So just wondering what people’s thoughts are. Think it is related to overwatering? It’s outside in full sun all day in a relatively small pot which has a good draining mix of pine bark with a little peat moss. Or do you think it could be another problem? Is it worth repotting or would that be too much shock? As I mentioned, there is a flush of new growth, particularly close to the lower branches and trunk and there is a decent amount of flowering as well.

Some twig dieback continues to happen in a progressive way almost like it’s marching back from distal to more proximal. Down near the trunk there is quite a bit of new/flush growth which looks healthy. But concerned about this progressive march down from distal to more proximal branches. Any thoughts?

https://www.yara.us/crop-nutrition/citrus/managing-limb-twig-dieback/

You sound exactly like me before buying a Bluelab pulse meter. I use to over-fertilize the life out of my plants. This use to happen to me all the time. I imported a lot of citrus fertilizer and was fertilizing to many times per year with too many different products. I thought the same that the pot should require similar amounts to a ground plant. Your EC is probably through the roof. The Bluelab meters are not cheap but the money they save in the long run if you kill a lot of plants pays back quickly. The new growth looks very healthy so I don't think you have a mineral deficiency. I keep my EC below 2. When I used the meter for the first time many of my plants were close to 5 EC which is why I was having lots of issues.

Potted citrus need a lot less fertilizer than an in ground tree because of the small root ball. Always estimate on the far conservative side. Most prepackaged potting soils have more nutrients already available than people realize.