Possibly rotl. Take them out of the pot, and dry off the roots, trim it back and destroy the leaves, clean out the soil, wash the roots off with Clorox 50% water. Put the whole mess into an empty pot and place in the shade.Trim those leave off down to the trunk. If the trunk is rotted trim all the way down to where it isn't. Now make sure there is plenty of air circulation. Has summer started yet where you are? (Winter isn't here yet) If so don't water until you see new growth. I did notice that you are from Australia, and live on the West coast. If you have moisture in the air, no water is needed during your cool months, they don't do well here in high humidity. Mine are all root bound and are not growing in peat moss. Many are coming into flower now, as the weather is cooling down at night for us. If you can't find any healthy wood/ leaves/ stems, trunk, you must destroy the plant. Crassula ovata 'Jade' is not known to get diseases. Josh remember Australia is like Africa, the opposite weather that we are getting. Everything is reversed including which direction the toilet water goes around the bowl. I think it could also be black mold, he needs to remove this plant from the rest of his collection. It is known to grow on the East Coast of Africa, on rocky hill sides. Drainage of the soil is very important grows in full hot sun among rocks. Norma

Rina -- thanks for asking, I only got it a few weeks ago. Yes, I pruned the heck out of it! It's starting to sprout all over now but I don't think the old leaves will ever turn up. Here's a photo where you can see my aggressive pruning job and some of the new growth. (And some of the old leaves which turn down.) I was worried to cut it so much, but it responded perfectly with new growth. I think it will have a ton of new branching soon. I have it in an east-facing window, so it gets nice light in the morning. In a few months, when I'm confident it's doing well, I may prune back the last few of the leggy, curved branches over the side of the pot. I feel really lucky to have gotten such a beautiful old plant and hope I'll have it for many years :)

Shelbyguy nailed it. Seed starting mix has very little nutrients so you don't "nuke" the seedlings (over fertilize). Although they can grow quite a bit in this mix, I prefer to transplant to regular soil as soon as the first set of true leaves are out. Be gentle at this point as they are quite delicate, but they will thank you later. Other than that your plants look happy. Thai Dragon is a hearty plant. They were one of the few plants to survive last years exceptionally cold/wet winter here. They were making fruit the entire time too, although it ripened very slowly. -HabBob

Jades will loose old leaves, they usually dry up to crisp and eventually fall off (or you can take them off). There could be few at the same time, but usually just 1 or 2. Do you have just few or may yellowing at the same time? Black spots could be oedema/edema, that is a disorder, not a disease. Those spots could be raised, like a pimple, often hard and you shouldn't try to take them off (they would have to be scraped, using bit of force, and would leave wound on the leave). Oedema could also look like irregular spots. If the black spots were oval-shaped and quite same in size, it could be scale insect but I do not think that's what you have. Powdery mildew could also have darker spots, but that is a fungal disease and often covers more of the leaf. It could also be 'fuzzy; - doesn't look like you have that either. If the soil was hardened around root ball, it is likely because it is mostly peat that became hydrophobic. Not best for the plant, since hydrophobic peat is quite difficult to re-wet. Hard as it is, it repels water and regardless how much you water, plants roots can't get any. Since you repotted it with that hard soil on, all new soil will get saturated but roots will stay dry. It is better to get rid of old, hard soil and loose some, even lots of roots. Jade will regrow roots in better potting mix quite fast. The mix should be well draining. You can make it so by adding lots of sifted or rinsed perlite (or pumice if available). Mixing it in 1:1 ratio is recommended.