Pieris need an acid soil. They cannot take up iron if the soil no londer acid. Your plant may need an urgent feed of ericaceous fertiliser, if you have not already been doing that, as it has been in a planter for so long, the soil around it has run out of benefit to this plant. So it might be a good idea to put it in a decent pot on its own, with ericaceous compost, if not too far gone to rescue it. Feed it first.

Thanks Dorjac, i like the sound of that. I was thinking of re-freshing the compost but wasn't sure if it had been infected with a soil based disease. I have an identical plant in an identical planter which is showing no signs of the same trouble. Will this eventually go the same way?

Yes Tiger it will...

I'm not convinced its the compost causing the trouble - any chance of a photograph of the sick plant?

Sorry I did not mean to imply that this was a soil based disease - the problem is having the plant in a pot which has not been fed for years.

Maybe an application of sequestrene before it turns its toes up completely? It sounds awfully rapid for chlorosis, but it's worth a try.

Just been reading the comments so far. Been giving it some thought, are there any problems it could have caught from water? The reason i ask this is because it is sited right next to a water feature. It does get some splashes on it's leaves. I had a " Forest Flame " actually in the border very near as well. This suddenly showed the same symptoms so i moved it a year ago and it's now recovering! Could there be any thing in the water? As i said previously i have an identical plant sited some distance away that has no such problems.

I will send photo's later if this will help.

I had a lovely Azalia once in a large pot of ericaceous compost. It grew large over time and flowered very vigorously one year. Then it suddenly went yellow and dropped its leaves. You mention that your plant had many flowers then went down. I suppose a great deal of soil resource is used up suddenly. My Azalia never recovered.

Thanks for the input Dorjac, i have just posted some photo's in the hope that someone may have had this problem and hopefully an answer!

I have had a look at your new question and photos, Tiger - and commented beneath it.