Q. For several years my saguaro has been turning brown slowly but the process has speeded up this year. I have two others, one of which I had to transplant after it grew too much for its original location, but neither of them has problems and seem to be completely healthy. I really don’t believe I am overwatering it, but it is a possibility
Q. The last time I have seen this kind of browning on the surface of a saguaro is in a landscape where they were wrapping the Saguaro each winter in an attempt to protect it from winter cold damage. They used burlap.
It caused a lot of surface disease problems on the cactus because of a lack of air circulation. They finally stopped doing that but the damage was already done and the surface of the cactus had a lot of brown damaged areas.
I hope you are not doing that. If you are, stop it. Let it breathe and take your chances. I know they are expensive but wrapping them causes more damage than good.
Make sure that water is applied to the surface of the soil surrounding out to a distance of six or 8 feet. Do not water too often. Water deeply in these areas about every 2 to 3 weeks. I would rather see it at three weeks than two weeks and actually three weeks is really too often.
These plants store water in their fat bodies and can live on this water for a long time. When water is present they absorb it like a sponge, very quickly and that fat body swells up and expands like an accordion. That's why it's ribbed. Don't wrap it in the winter time and water it deeply and not very often.