i have a large rhododendrum in a pot, it's leaves are turning yellow, any advice? Poor thing has been there for years but still produced fine blooms this season...
Has it ever been repotted into fresh compost, and does it get fed with anything?
It's been a long hot summer and plants in pots are entirely dependant on you to keep them adequately watered and fed. It sounds to me like it's going anaemic either from lack of food in exhausted compost or because it has had too much hard tap water and can't take up iron and magnesium.
I would suggest re-potting if you can, in the same pot if you haven't a bigger one. Remove it from the pot and scrape off some of the soil at the base, sides and top of the rootball and then place fresh ericaceous compost in the bottom of the pot, down the sides and on the top. Give it a good drink of sequestered iron diluted according to the instructions and keep it fed and watered in the future with food for ericaceous plants.