An Ongoing Discussion on Citrus Trees in NYC

Question:
The problem I encounter is with my lemon tree. I have this tree for 4 yrs and it has always been a happy tree, or at least as happy as a citrus tree can be on the 34th floor of a building in Manhattan - flowered, grew and constantly made fruit. Suddenly, approximately 2 weeks ago, the leaf's central vein and parts of the leaf started turning yellow [I have attached 2 pictures in pdf format]. I repotted the tree in soil suggested as best for cacti and citrus and administered water with Miracle Gro and extra mg sulfate added, thinking that maybe the yellowing was due to mineral deficiency. But now I have my doubts - none of the pictures I have seen on line describing nitrogen, Mn, Fe or Zn deficiency seem to result in this leaf pattern. Could you please suggest what this can be and how I can fix it? Or direct me to someone who can help me out with this?

I, Alex Feleppa, then followed up by asking a few more questions:

What time of year did you repot your tree?
I HAVE REPOTTED THE TREE 10 DAYS AGO, ABOUT 7-10 DAYS AFTER I HAVE FIRST OBSERVED THE LEAF PROBLEM. THERE ARE NO IMPROVEMENTS AS YET, AND THIS MORNING I SAW THE FIRST LEAF DROP.

The pot that your tree was repotted into, does it have adequate drainage? Do you fertilize the tree regularly? And finally, when did you add the Miracle Gro and Mg sulfate?
YES, IT IS A CERAMIC POT WITH A DRAINAGE HOLE AT THE BOTTOM.
NO, I HAVE NOT FERTILIZED THE TREE FOR ABOUT A YEAR.
I ADDED THEM ABOUT 10DAYS AGO AFTER REPOTTING THE TREE. I HAVE WATERED THE TREE TWICE SINCE, [ABOUT 4 DAYS AGO AND TODAY] BOTH TIMES ADDING A LITTLE MIRACLE GRO TO THE WATER [NO MAGNESIUM SULFATE THIS TIME].

Even though the problem has come about quickly, your tree looks rather large so it might take a little while for the tree to bounce back. Larger plants, and especially trees, often work on a schedule that is slower than our patience allows. Perhaps there was some stress in the last year that the tree is finally showing.

It sounds like you are doing all the right things. I just found this paragraph on the University of Florida's website:

"Yellow vein chlorosis may be attributed to the girdling of individual branches, tree trunks or roots due to a number of factors including water damage, Phytophthora foot rot, root rot, ant damage, or physical damage by equipment. Yellow vein chlorosis may also occur on twigs and branches due to cool weather in the fall and winter due to lack of nitrogen uptake from the soil. In leaves showing yellow vein chlorosis, the midribs and lateral veins and a narrow band of leaf tissue bordering them become yellow while the rest of the leaf remains green."

Because lemon trees can grow so quickly, my gut instinct is that it needed to be repotted. And by doing that I hope you already solved a problem. When you repotted it, if you had noticed lots of rotting, mushy roots then perhaps we might consider some of the diseases listed above. Since that didn't seem to be the case at all I think it is safe to rule out diseases as the issue.

In addition to the roots wanting more room (which you have now supplied), they also need fertilizer. A regular fertilizing with Miracle-Gro during the summer months should supply the tree with enough nitrogen and micronutrients. If you have not been doing that in previous years then the soil might have gotten rather depleted over time.

If you have lost some leaves I consider that a natural reaction to being repotted. The tree went through some stress when it was repotted, so it may lose a few leaves while it acclimates to the new pot. That is normal. The important thing now is to keep watering it and caring for it as you always have. It's when we panic and change our agenda drastically that plants can decline even more. Water it as you have been. Follow the fertilizer instructions and ease up on that in the fall. Obviously keep the tree in as much sun as possible. Lastly, keep me posted and let me know if the yellow chlorosis spreads to more leaves.