What are these lesions?

[stuka]

I don't know the history of this cactus. Neighboring cacti appear to be healthy, but this one is covered with lesions. Any ideas on what they are?

IMG_0031_x1024.jpg (54.29 KiB) Viewed 390 times

IMG_0032_x1024.jpg (57.71 KiB) Viewed 390 times

[hegar]

Cacti that are grown outdoors are impacted by all kinds of environmental things including hail, frost, drought, excessive water content in soil, insects, diseases, and sunburn, just to mention the most common problem causing factors. I would keep an eye on the affected plant stems and monitor them regularly to see, if anything is changing. Are the spots enlarging or multiplying? If not, they probably are just cosmetic blemishes. If the neighboring stems are all healthy looking and have been exposed to the same environment and/or if these lesions were not present when you planted the cactus, then I would be extra watchful and most likely destroy the infected/affected cactus stem(s), because then there is most likely a pathogen involved, which could possibly spread to your healthy plants near the diseased one. You could take a close look - preferably with a magnifier - and check the lesions for the presence of fungal mycelium, which would look like fine strands of string and sometimes causes a "mat" to form or perhaps tiny specks, mostly grey to blackish that are present on the lesion surface. Those would be fungal fruit bodies and they do contain fungal spores. Bacteria do not have this feature. You could also go ahead and excise a good-sized lesion or two with a sterile cutting tool like a knife dipped into rubbing alcohol or clorox bleach. Then you will be able to see, if the tissue underneath is discolored, and if this is the case, how far down the discoloration extends. If there is a discoloration, you will have to cut the part of that does show it. Sometimes the whole stem will have to go.
In short, if the spots do not enlarge, multiply, etc. and the appearance of the plant does not change over time there is nothing to worry about. However, if things go from bad to worse you will have to take action.

Harald