Leaf dropping is a sign of stress in jade plants. A primary culprit in the dual problem of leaf spotting and subsequent leaf loss is under-watering. Though jades are succulents, they need regular water when they're actively growing in spring, summer and early fall. Well-watered plants have firm, plump leaves, while thirsty jade leaves are somewhat soft and limp when touched. During the winter, the plants rest and you should only water when the potting medium is dry to the touch.
Sometimes, during the jade plant's active growth phase, the plant may seem incessantly thirsty, requiring water more than once a day and showing spotted or dropped leaves. The jade probably needs to be repotted. Choose a pot with drainage holes that is larger than the original container by at least 2 inches in diameter. Place a 1-inch layer of drainage material, like coarse gravel, in the bottom, add a layer of potting mix and center the jade's root ball in the new container. Fill in around the sides with fresh potting mixture, tamping down to firm it. Water thoroughly.
If the plant is pot-bound and showing signs of stress, but cannot be transplanted to a larger container, try pruning the roots and crown. Start by trimming the main branches of the plant back by about one-third, using sharp clippers and disinfecting the blades with rubbing alcohol between cuts. Carefully remove the plant from its container and use a garden knife to trim the root ball evenly on all sides, making it about one-third smaller. Wash out the original container and repot the newly reduced jade plant, filling in the empty pot space with fresh potting mix.
When remedying the effect of leaf spotting and loss by rehydrating the jade plant, do not overdo it. The soil should be moist but not wet and the plant should not be allowed to sit in a water-filled saucer. Jade plants develop root rot in overly wet soil and the trunks and branches become soft and mushy, and the plant will likely die. Cuttings from jade plants that have been pruned can be inserted into small pots containing a equal mix of moist potting mixture and sand or perlite. Place in indirect light. The cuttings should root readily.