I have had both heat-burned and cold-damaged plants exhibit this symptom. The tissue would get damaged, and as it heals, it forms a scar which is often raised, bumpy, and unsightly.
If those patches continue to spread, and your plants are not subject to extreme temperatures, then it may be another issue altogether. Otherwise, those patches are just benign, though unsightly, scars.
Last edited by Orchidreamer; 03-09-2013 at 03:54 AM..