Hi there, I've been trying to get to the bottom of this... A couple of my (indoor) christmas cacti have developed this same issue: -rusty, rough spots, mostly on exterior leaves -often accompanying fine black particles that feel gritty and rub off when touched -leaf drop -apparent desiccation My oldest plant is the worst affected. All are in a northwest facing window in ample, but filtered light. Repotting two of the plants recently seems to have exacerbated the problem. My original theories were spider mites (but little to no webbing is apparent) or thrips (the black particles are round and don't seem to move). So now I'm at a loss... then I found impatiens necrotic spot virus online; possibly that? I would love to save the plants, especially the oldest and largest, which is the worst affected. Most recently, I mixed a small quantity of granular pesticide into the top layer of potting soil and washed them thoroughly with insecticidal soap. I cut the oldest (largest) plant back quite a bit to remove as much of the diseased tissue as I could. That's what you see here. Please let me know if anyone there has seen this before, what it might be, and how to treat my plants to remedy it. Thanks so much! Amy in Ypsilanti (an almost-Master Gardener--took and loved the course, but didn't complete the volunteer hours for health reasons)