Plants wilt and die slowly when their roots become compromised. There are several causes of this:

1) Root strangulation. This is best prevented by proper root pruning when planting. If the plant is not too far gone, it might be rescued by digging and removing the soil. Then cutting any circling roots that may be strangling other roots. The roots need to be opened up. On larger plants, some of the top must be removed to compensate for the weak state of the roots. Any time the roots are exposed, they must be kept moistened. Roots that dry out will die.

2) Phytophthora Root Rot or wilt. This root rot is the major killer of rhododendrons and azaleas. It develops when roots are growing in wet conditions. The rot is more prevalent in warm summer conditions. Plants infected with crown rot caused by the oomycete, or water mold, Phytophthora have roots which become clogged with brown oomycete, or water mold, internally. The roots get blocked and the plant wilts and dies. There is not much of any cure for crown rot. Some varieties of rhododendrons are vulnerable (Chionoides, Catawbiense Album, Nova Zembla) and some are resistant (Roseum Elegans, Scintillation, PJM). Sphagnum moss and bark dust combined with good drainage seem to prevent crown rot, but do not cure it.

3) Drought. It may have simply dried out. If you plant rhododendrons or azaleas in late spring, it is very important to give them some extra water while they are growing new roots. Never let the soil completely dry out' it's best to keep the soil evenly moist. Too much water or poorly drained soil might be another explanation of sudden rhododendron or azalea death. Rhododendrons and azaleas have very fine, fibrous roots that are easily damaged by waterlogging, even for short periods of time.

4) Voles. Voles, also known as meadow mice, may have chewed on the bark and roots near the crown of the plant. Sometimes they chew all the way around the trunk and kill the inner bark, resulting in death of the whole plant. Keep mulch away from the trunk to discourage voles.

5) Bark Split. The bark may also split when there are wide fluctuations in temperature in the winter. Rhododendrons and azaleas may begin to come out of dormancy if late winter weather is warm; if a cold snap follows, bark injury is likely, especially in sunny, exposed sites.

6) Wilt and slow death can also be caused by juglone poisoning from walnuts.

Plants wilt and die suddenly is usually caused by roots which are girdled by larvae of the Black Vine Weevil, Otiorhynchus sulcatus, Strawberry Root Weevil, Otiorhynchus ovatus, or Twobanded Japanese Weevil, Callirhopalus bifasciatus. Adult weevils feed on the leaves at night except the Twobanded Japanese Weevil which feed during the day. Specimens may be collected during the day or at night for identification depending upon the weevil. The major damage is caused by weevil larvae which girdle the roots and kill the plant. Larvacidal drenches may be used to kill them but are of limited effectiveness. Foliar sprays are very effective at controlling adult weevils when leaf notching starts. Foliar sprays are very effective at controlling adult weevils when leaf notching starts. Foliar sprays must be repeated until no adults emerge. Twobanded Japanese weevils are apparently resistant to carbaryl (Sevin), diazinon, and malathion. Orthene gives good control when applied as a foliar spray and drench. Since weevils can't fly and spend part of each day in the soil and part of each day feeding, you can paint the trunks with Tanglefoot to stop them, but make sure no branches are touching the ground.

To answer your question, I think the plant is terminally ill, but would still hold out hope a little longer. You seem to understand the basics very well.

Here is a link that might be useful: Rhododendron and Azalea Problems

OP said "put in an inch of pebbles at the bottom, to increase drainage beneath the roots,"

Contrary to popular opinion, it doesn't work that way. Instead, it impedes drainage.

Looking at your image, it's a good bet that it has root rot. What did the roots look like when you dug it up?

Thanks very much, Rhodyman, for a really comprehensive and help reply!

From the possibilities listed, I think either Phytophthora Root Rot, Voles, or possibly the slugs provide the answer.

A lot of moisture passes through that particular section of ground when it rains owing to the nearby drainpipe from the greenhouse roof. And I can't do anything to stop the rain, when it rains. So replacing the ill rhody with a Phytophthora-resistant one would be worth the try.

We have a great many voles around here, and moles too. As I related, when I dug the root ball up it looked as if there had been absolutely no new root growth in the two whole years that it has been in position. Perhaps voles or moles have been nibbling at the new growth? But then, why wouldn't they have partaken earlier on in the two years, and why this particular bush in the whole line, and not one of the other ten rhodies.

So, back to the rot. But then there were the 20 or so white slug eggs I found in the hole, as well as at least one adult slug. We have thousands of slugs in this neck of the woods. "Keel Slugs" they call them round here. They're small and they live in the earth, and devour anything they fancy, roots first and then stuff above ground level. But then, why haven't they caused similar harm to the other ten rhodies. So back to the rot possibility.

Summer, a warmish temperature, and a great deal of rain over a two-week period. From what you say, this could have caused the rot.

The only other possibility is that the plant may have reacted badly to too much iron supplement from me? The earth here is acidic but only just, so once a month during the growing season I mix a couple of desertspoonfuls of iron sulphate with ten litres of water and pour it round the roots of each plant. Again, though, doing this hasn't adversely affected any of the other ten.

So I think I may soon begin to hunt out a Phytophthora-resistant rhody, one of those you mentioned, and see how it fares in exactly the same spot.

A great pity all the same, after having nurtured the stricken one for two years.

Many thanks for your very helpful reply.

Jean001, to answer your question about the state of the rootball, I found to my surprise that it looked exactly as it did two years ago when I took it out of its plastic pot from the nursery. This was a bit of a shock because since the plant has been in the ground it has increased all round by about eight inches over the course of the last two years - all apparently without any development of the roots!

After digging the hole out a bit and putting pebbles in, I also pushed a thin metal peg into the root ball about ten or so times all around it in order to open it up, as advised by a number of sites online. However, this hasn't led to any sign of new life in the past week.

Am interested why placing pebbles under the rootball has not assisted with drainage in very wet conditions. Can you tell me more, please?

Thanks.