The first possibility is that you have allowed it to become completely dry at some point. Is that a plastic or terracotta pot? Terracotta can dry out very rapidly in sunny conditions. The plant should re-grow.

Otherwise, this is almost certainly a root problem. There are no airborne diseases which attack citrus in this way. Spider mites are a possibility, but the leaves don't look like it. I would guess the roots or stem are rotting. Adding layers of soil is never a good idea with citrus because the plant gradually sinks and the stem then becomes liable to rot at or below soil level. Also, it appears your watering device is right by the trunk - again not a good idea.

I would carefully remove the plant from the pot and inspect the roots. Are the roots white or cream coloured, and spreading throughout the container?

Or is there a lot of excess and damp compost with dead roots and some turning black and breaking off? If so, completely wash off all soil, trim off damaged roots. Then re-pot into a smaller pot with a really free draining mixture. Lots of grit and orchid-bark if you can find it. Raise the tree so the first roots are just at the surface. Keep just damp and water with a dilute soluble balanced feed. With luck, it may recover!

(Photos of the base of the trunk with all loose soil brushed away would be interesting.)

Thanks very much for the feedback. I've tried to dig back all the ground around the stem of the tree and leave as much of the upper root system exposed as possible, but from the sounds of your hypothesis this indeed won't be enough. Therefore I'll definitely take your advise and try going back to square one with the roots in mind: de-pot, sanitize any rotting or ill roots, and try re-potting with a lot of non-absorbing material around the root dirt pack. Will stick with terracotta unless informed otherwise.

I've noticed that even though the H20 it gets (I've turned the auto-spigot off and water manually until this gets solved) drains straight through, the actual ground stays pretty remarkably wet. I'm thinking this may be the potting mixture my wife tends to throw on "to give it ground," which may not be the greatest quality -- and worse still, per your warning, just helps the root system to sink deeper into trouble. Ie. there's no mulch or leaves in there, but there is a lot of very water-retaining dirt!

I'll give your suggestion a go ASAP and report back in a few weeks. Good news is, everything above ground level (apart from dying foliage) is solid and without any rot, mildew or peeling. So I'm feeling confident your analysis and remedy will do the trick!

Thanks ever so much once again, and I'll let you know if the Parisian "citronnier" bounces back.

I agree with Citrange, your tree's problem is most definitely inside the container. You must pull the tree and examine the roots to find out what is the problem. If you have never repotted the tree you will most likely find that one of the problems is that the tree is root bound. - Silica

Many thanks to Silica and Limeinqueens for the tips. Am stopping by the gardening center later today to see what they have in the way of mix to diversify/dry out the ground composition, and carry out the prescription Citrange suggests.

As an initial step, however, I removed some of the surface dirt last night and tried to inspect the root system. Was rather surprised -- and chagrined -- to discover that far from being large and nearing root-bound status, it was limited to a relatively small, compact fist of larger (yarn-sized) roots bunched upon themselves; basically a knot of large-diameter spaghetti rather than the mess of angel hair you'd also suspect. Which I'm guessing is the problem. Not sure how one favors the growth of the smaller feeder roots, but I'm hoping a healthier ground environment may do that.

Individual citrus roots are thicker than on most potted plants. They do not produce very fine roots. However, your 'small, compact fist' of roots shows that the environment was not conducive to good root growth. Lack of oxygen due to overwatering is the usual reason. When re-potting do not use a pot much bigger than the current root ball.

The amount of water is critical to the success of growing potted citrus in cool climates like in Paris or here in England. Too dry can be equally bad, but - if you observe your plant closely - you can usually see when the leaves are just starting to go a bit limp. A quick watering will restore it. Too wet takes longer, but, after the plant grows very little for some time, it is often fatal.

Note: USA 'dirt' = UK 'soil' or 'compost'. Here 'dirt' is something dirty and unpleasant!

Another very real cause is that in your attempted grafting you used infected budwood or infected tools that have introduced a virus or bacteria into your tree. Also, when you prune back dead/dying wood, you must prune back to just above the last green bud; if you leave some of the dead/brown limb, it will continue to die back.

Thank you (once again) to Citrange2 and Johnmerr. I've gone ahead and done the re-potting per instructions and have loaded up, in a smaller pot, with orchid bark. I got all the old ground (not dirt!) out of the root system as possible, and saw that clump looked fairly healthy; not apparent signs of rotting, no odor of mildew. Given that, it may well be Johnmerr is right about my grafting attempt. Doubt the budwood was bad (again, another tree here I tried grafting branches from the same donor tree to is doing fine) but may well have erred in cutting materials. It may well be -- as Charlie Brown said of his Christmas tree -- "I've killed it."

In any case, in addition to the re-potting I've also now fairly radically cut back all limbs to the last green bud, well beyond the dying wood.

I'll now wait and see how it reacts for a couple of weeks, watering in a careful manner to see how it goes (it's hot enough here now -- in the mid- to upper 20s C every day -- that after a few initial days in the dryer soil, I'd be more wary of aridity than drowning it.)

Thanks again to one and all for all the kind input, and I will keep you posted.