Did you spray with viegar solution while in sunlight? Powdery mildew I have seen was mostly in bigger blotches, more on surface of leaves. That doesn't mean it isn't. Another possibility could be oedema/edema. I never sprayed what I thought was pm on jades, just took off the leaves with it (it wasn't spread on whole plant). Are your plants in humid environment? God air circulation helps. White spots are ofen salts deposits, from too much fertilizer. I really can't tell from your photos.

Temps in 20-ties C are quite good, warm enough. Nights down to 10*C or just below are OK too, they actually help with flower development (unless too warm during days).

Repotting is usually recomended during active-growth period, but it is always better to repot plants that are in poor soil, that may cause more problems than repotting.

Pruning extensively doesn't hurt plant. Prune anything that needs to be pruned, and use clippings to root into new plants. Plant usually responds to pruning by branching (as long as plant is healthy). If weather is cool, make sure plants do not sit in wet soil for too long.

I have pruned jades down to bare trunk out of seson, here is one that was pruned in very last week of December (in cold Canada :), and it didn't have any problem growing new leaves and branches...here is old photo (see dates for new growth):

Rina, thank you so much for your response! I really appreciate it and had been feeling quite lost!

To answer your questions:

I did spray with a vinegar+baking soda solution when it was a sunny day and they dried quite quickly. Some of the remaining solution dried on the leaves which wiped away pretty easily.

We haven't had much humidity, but there were a couple of humid days in the summer (though I would not describe this as a humid climate).

The plants are outside on the back patio, so I think they do get good air circulation (though sometimes too much wind).

Thanks for sharing your pictures of the aggressive pruning; they give me confidence...especially as you treated yours in Canadian winter!

So, at the moment, as I understand it, I will wait until we are back from holiday next week and then I will soak/de-clump the bad soil from the roots of the remaining 6 plants (and maybe leave to dry out a couple of days if they need any cutting?) and then repot in nicer, newer soil. And, at the same time, I can aggressively prune the jade (sunset jade?) that looks ill but in an unknown way so it doesn't spread. And I shouldn't worry about the temperature for now. Do I have that right?

Thank you so much for your help. I feel like the plants really need to be repotted and given some nutrients and it seems like waiting any longer would make it much worse; I just needed a little reassurance that I wasn't going to make them more vulnerable right now.

I've also attached a couple more pictures of the affected jade. The black spots didn't seem like oedema to me becuase they weren't coarky and always started on the edges of leaves, leaving lesions, and because it seemed to be spreading. But I may have been misunderstanding. I had assumed they couldn't be getting too much water because of the water just running off the bad soil but maybe that made it more water retaining?

And one of the whole plant:

I kinda skimmed so I am sorry if I missed some information. First, your jades look totally fine in general so I am not totally sure whats going on. Whats your water quality like? Mineral heavy waters can leech minerals into the plants when you water and it kinda comes out in the pores, that could be going on with the white stuff. For me in central Texas calcium does it. The brown I would say could be from the vinegar + baking soda + sunny day... basically you burned the plant. It's fine, just leaves scars like you see.

As far as your soil compaction goes, you need to add a bunch more "drainage" to the mixture. That soil looks like 100% peat or clay... shivers. You can pick up a bag of perlite or pumice and do something like a 50/50 ratio. There are a million posts around on soil if you want to read a bunch.

you don't need to worry about "nutrients" right now. They are really okay for quite sometime.

I also agree with Gabby that your plants look good, at least from what I can see in photos. I can see couple of dry leaves in pic 1, which seems to me normal. Another leaf in same pic seems to be broken, with broken edge just dried up. If spraying with anything, it is best to do out of sunlight. Some ingredients may burn leaves in sunlight, even if not otherwise harmful to plant.

Soil does look very 'dense', it could be too water retaining. If it contains too much peat, it may have become hydrophobic and it actually repels water. If to retaining, it may be staying too wet for too long, and roots may be suffering. Do as Gabby suggested - mix in lots of sifted perite.

And do not worry about nutrients...succulents grow in very lean substartes in their habitat. Definitely do not use 'rich' soil thinking it may provide nutrients. If really concerned, it is much better to fertilize, lightly, with balanced fertilizer. I don't, and grow mine in gravely mix, made of grit, perlite and little turface. I have few small pots with bit of soil in the mix, but there is likely only about 10% of soil - added because I was running out of mix I usually use :)

Whe repotting, get rid of old soil from the root ball. Trim off any damaged roots. The do not need to be drying if they have roots; if you have to do lots of root pruning, maybe an hour or two are enough to callus those cuts. If you need to wash off the old soil, letting plant air-dry for few hours should be enough (set it on something absorbent to help). If pruning off branches, the cuts should callus. Few hours or overnight is usually enough. But jade can live without water for a long time (weeks, even months) - they may shrivell, but still live. So do not worry much about how long. Cuttings, without roots, should be stuck into fast draining mix and do not water for a while. No roots - no water...they should start growing roots quite readily, and that is time to start watering.

Thank you both so much for your help and apologies for disappearing. I got quite ill after posting about this and had to leave them all be for a little longer (which I was nervous about!). But I have since repotted all the plants into a better mix (and split some into multiple pots). All seem happier, but the particular jade in question has seemed so much happier than before. (As a note, I was most concerned that some of the leaves seemed blackened around the edges and the blackness seemed to be spreading.) I think you were right that some leaves got burned when I sprayed them, thinking I was helping instead.

I gave it a very good (aggressive) prune after your encouragement that it could handle it even in the cold and got rid of any questionable leaves before repotting it in a much faster, better draining mix as you both suggested. A million times better! After a couple of weeks of the proper "soak and let dry" watering, it looks like a whole new plant--and has ALREADY shown new growth!

I've just moved house though and was so nervous to move all of these freshly repotted and pruned plants, but I think they are okay! I really appreciated your encouragement and I feel a bit more confident about it all now.

Update picture:

New patio/potted garden area:

Thank you again!

Thanks so much and all the best to you as well.