Blisters on Christmas cactus leaves caused by overwatering
Q: I have a Christmas cactus in a pot on my south-facing deck in Roswell. It has yellowish spots that look like blisters and is losing leaves and branches. What can be wrong?
A: You need to lay off the lovin’. You are watering the plant too much. When roots take up water faster than it can be used by the plant water pressure builds up in the internal cells of the leaves causing them to burst, leaving dead cells that are visible as a blister, primarily on the undersides of leaves.
The damage is called oedema (which may also be spelled edema)
The solution is obvious: water the plant only when the soil surface seems dry to the touch.