Looks like you never give the tree magnesium (Epsom Salts MGSO4). The leaves are showing a major magnesium deficiency. The symptom of a magnesium deficiency in citrus are leaves with a green delta shape in the lower center portion of the leaf with the tips and sides of the leaves yellow. At the stage the leaves are in now, even when you apply magnesium to the tree, it will take a considerable time for the present leaves to recover, and maybe never will. However any new foliage should develop normally. Dissolve one table spoon of Epsom Salts in a gallon of warm water (Epsom Salts does not dissolve well in cold water), then apply to the root zone. Epsom Salts can also be applied as a foliar spray. Could take more liquid for application to very large containers. Secondly how well does the container medium drain? The best to this tree.

Thanks for that bit of information. The container has holes in the bottom and does drain well when it's over watered. Do magnesium deficiency symptoms show up over about a 2 week period? This tree looked great about 2 weeks ago. Also, if I use the Epsom salts and the issue is actually something other than a magnesium deficiency, will the Epsom salts do any additional damage?

Epsom salts should commonly be added to a citrus tree's nutrition regimen. Most growers apply Epsom salts two or three times a year, as it is the major trace mineral needed by the tree. Magnesium is the central element of the chlorophyll molecule (the green matter of the tree's leaves). Looking at the tree's leaves, your tree is greatly deficient in magnesium. Secondarily, a fast decline can also indicate a tree with root damage, which can be caused by various reasons, but commonly caused by insufficient drainage, resulting in a medium retaining too much water, thus not enough root zone oxygen..

Thanks so much for your help! I'll give it a dose of Epsom salt tonight along w/ my other citrus trees. Hopefully the tree will turn around. Maybe I'll put a few more holes in the bottom of the insert for some more drainage while I'm at it, too.

With trees that have been growing in a medium for a length of time, it is not always the amount of holes in a container that limits the drainage, but rather many times it is the compaction of the medium that causes the retention of water, and thus the elimination of root zone oxygen.