What to Do to a Saguaro with a Black Spot on the Trunk

The reader sent this picture to me after following the directions found in the Arizona publication below. He told me the outside had a black spot on the surface of the "trunk". I sent him a link to the publication and this is what he did. Just make sure you use a sanitized knife.

Q. I’m growing a saguaro cactus in my yard and it’s developed a black spot on its surface. Do I need to be concerned?

A. Yes, you should be concerned. As you know, this is not saguaro country, so our location poses some problems when growing saguaro here. At planting time, make sure the soil drains well and amend the backfill around the roots with a small amount of soil amendment. Be very careful not to damage the plant in any way when planting.

I use the publication provided by the University of Arizona on this topic and it can be found here.

            First, let’s find out if there is a problem or not related to that black spot. Take a sharp, sanitized knife and cut the black spot out of the saguaro. Look at the “flesh” inside the cactus and under the black spot you removed. If the “flesh” is green and clean, there is no problem. Leave it alone and let it heal. It will.

This was a picture sent to me by a reader several years ago and I don't remember the specific question. But looking at the brown discoloration on the trunk I would be thinking about cold damage or someone trying to wrap the saguaro during the winter to protect it.This is not the black spot that I'm talking about. I don't have a picture of that.

            But if the “flesh” under the black spot is black or brown and oozing like it’s rotting, then this could be a problem called Bacterial Necrosis of Saguaro. If this infected black spot is low on the saguaro, there may be nothing you can do except wish it well and hope for the best. But it will probably die.

            If this disease is caught early and all the infected “flesh” is removed with a sharp, sanitized knife, there is a good chance it will heal and recover.

            This disease spreads easily from infected plants by insects landing on open wounds or being careless during planting. So be very careful not to damage the plant in any way during planting.

Please make sure that any knife you use is sanitized before cutting into living, plant "flesh".