What Causes the Leaves to Turn Brown and Dry on a Rubber Tree?
Rubber tree sits in a very well lit room with large windows. Leaves are browning in center and spreading out towards edges of the leaf. On underside of a few leaves are clear looking bumps in small clusters, almost glassy in appearance. I told the owner the leaves look like they have been burnt. Perhaps too much sunlight!
Browning in the center out to the edge is rather unusual - possibly a bacterial or viral pathogen? Really need more info, such as how long has the owner had the plant, how large is it, how many leaves are affected, are they old or young leaves, what is the fertilizer regimen, how many leaves has it lost? For sake of argument, I will suppose plant has been in place for at least a year, so we can assume neither over or under watering. A good thing to try is to leach the soil (run alot of water through it, about 5 times the volume of the container), thus rinsing out accumulated salts, in case there is a salt build up. Need more info to make a real stab at answering the question.
As for the bumps on undersides of leaves, can you scrape these off with your finger? If so, they would be scale, which you can treat for. If not, they would probably indicate some sort of infectious agent. Best thing would be to take some leaves to a county extension office, or maybe a college or university botany lab.
Don't think it's too much light, they will take very high light. Can't imagine what the glassy looking bumps could be. Usually ficus leaves start browning at the edges and spreads to the center. If the lower or older leaves are the only ones affected, it might be the age of the leaf.. Most ficus will "throw" the older leaves as the stem grows in diameter.