United Kingdom 
Pieris with scale and spider mite ... we have a rather large Pieris that was healthy and stunning for many many years but it's been looking progressively ill for a few years now. A gardener has told us it has probably got spider mite and also some scale and recommended spraying it with chemicals. We really don't want to use chemicals on our garden though.
I have a few related questions ...
Does anyone know of a natural but effective way we could save this plant? I've read of white vinegar and dish soap mixtures but don't want to waste my time and energy if that's a hopeless track!
The foliage has got quite sparse from the damage the pests are doing. Would it be likely to bush out again if we are able to get rid of the little pests, or would it be better to remove the plants and start again? The main stem is more like a mini trunk now being so old, it's very woody and bark like. (I'm so sad because it looked like the most beautiful colourful mini evergreen tree before!)
Would it be possible to plant a new Pieris plant in the same spot without it getting infected? If so, how long of a time gap would we need to leave before replanting? Are we talking days, weeks, months, seasons or is that area just dead to another Pieris now?
Many Thanks in advance
7 Aug, 2019
Answers
A heavy spider mite infestation outdoors in the UK is fairly unlikely too. The wet & low Winter temperatures would kill them off & break their lifecycle.
Have you checked your soil pH? A thick mulch with ericaceous compost might help
7 Aug, 2019
Thanks guys. Perhaps I didn't word it very well... It definitely has scale -that I can see for myself. And there has been some sooty mould on there too so there is a honeydew problem. The gardener first said it's spider mite, but I pointed out the scale. He said main problem is spider mite with some scale. He pointed out where you could see the mottling of the leaves where he said they're eating away at them.
Thank you for the RHS article link. I've had a look, I've not seen any of the lace bugs on the plant, but the leaves do look similar to those that are shown in that article. But they do have additional whitish mottling where it has apparently been nibbled on by spidermite(according to gardener), and you can definitely also see the scale 'lumps' underneath that won't budge.
Any thoughts on a vinegar/dishsoap solution being of any success?
7 Aug, 2019
The sooty mould is associated with the scale infestation - get rid of the scale and that will disappear, though if its bad, you can wash it off after clearing the scale. I, too, very much doubt its spider mite - in the UK, that's rarely a problem on outdoor plants.
Vinegar/dish soap is unlikely to do much to clear the scale, and guess what, they're chemicals too, and certainly not natural in a garden environment. There are organic spray treatments such as pyrethrum based products you can use which are much less damaging to the environment and other insects, see link below. The trick is to spray at the right time, targeting the newly hatched nymphs and not so much the adult scale. If you can see scale attached to the woody parts, use a cloth lightly moistened with meths and rub that over the wood to get them off - but its not suitable to use it on the foliage.
https://www.rhs.org.uk/advice/profile?PID=224
7 Aug, 2019
Are you sure its not Pieris Lacebug (see link below) - a gardener saying its 'probably' got something isn't really a diagnosis, especially if he can't point you to a visible scale infestation, which it should be if its there. You don't describe the symptoms, but read the link below and see if it fits with what you've seen
https://www.rhs.org.uk/advice/profile?pid=663
If it is Pieris lacebug, then you'll get the same trouble with a new one - best to treat the plant you've got.
7 Aug, 2019