Okay so last year I bought a moderately frost damaged Meyer lemon on C-35 rootstock. It mostly recovered (still somewhat sparse leaves but pretty healthy looking). I repotted it in a mixture of three parts perlite, one part pine bark fines, and one part peat moss. And I moved it indoors for the winter.

Although it dropped it's fruit and lost a few leaves it was doing pretty well until about a week ago. Suddenly it (and only it, there are several other citrus overwintering in the same general area, in sameish potting medium) started having it's leaves yellow around the veins. I responded by giving it a watering with a general purpose water soluble fertilizer figuring it need a bit more nutrients, however it quickly defoliated entirely. I wasn't too worried until I noticed that the stems of the tree were starting to die back, and those that were still a healthy green were looking withered.

I really don't know what's wrong here, with the potting medium so heavy in perlite, I've found it impossible to over water plants potting in it. So I don't think that's the root of the problem.

I've never had this level of a problem with any of my citrus before, so I'm kinda at a loss at what to do.

Imgur gallery with pics, the older damage you see is from the frost damage before I bought it.

http://imgur.com/a/NPvfz

This happened to one of mine and the cause turned out to be that the drainage holes weren't in the low areas of the pot so even though the mixture appeared to be free draining it was actually pooling a quarter inch of stagnant water on the bottom at all times and the roots rotted.

Not sure if that is your cause but worth checking.

One thing is for sure - the problem is inside the container. You need to pull the tree and find out what it is. - Millet

Plus a major scale infestation, which is apparent in the close up shoot.

Igor, your eyes must be a lot better than mine. - Millet

Brian, if the tree has scale they will look like brownish scabs on the branches about the size of a BB.

Am I the only one who sees the sticky stuff/residue along the edge of the container on the close-up shoot??

For me these two white spots are scale.

I really don't think scale is this tree's problem. - Millet

Well that was sooner then I expected, it's been raining so I had to jump on the chance to depot the tree.

There is a lot of root damage, I'd guess around 95% of the roots have rotted and have loose bark. The tap root actually dies about half way down. I was actually considering cutting it off, but that's a rather big wound for the root system. I did cut of several of the smaller dead roots off, and currently have the tree loosely potted back in it's medium, until I get some advice on what to do.

I am going to try grafting some of the healthier/less dehydrated wood right now, before the tree declines any more, because I'm not too optimistic about it's survival chances now.

Album with some pictures:
http://imgur.com/a/wmbe2

Aliette (fosetyl-Al) is (was) the only chance to save it.
Put the root one hour in an Aliette solution then re-pot in a new medium.
Pour the solution on the medium.
Put the pot in a fresh place in full shade and pray.

The prayer is optional. )

Maybe to use other rootstocks.
For example, Poncirus trifoliata is highly resistant to phytophthora.

I would like to respond at a greater length to this tree's problem, but don't have time at present. A quick comment about the tree's rootstock, which Sludge reported the rootstock to be C-35. C-35 is a good root stock and is rated as being highly tolerant to phytophthora foot and root rot. I believe ithe problem was the medium the tree was planted it. - Millet