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Hello everybody! Thanks for checking in! I have several types of Hylocereus here. I live in Mallorca island, Spain. I know it is not the best climate for them, but they did well the first 3 years or so. Now they got this thing and it is spreading (see pictures). Do you know what it is, and most importantly, what can I do about it?


Help much appreciated!

To me this looks like sunburn damage. The piece that is horizontally placed is affected, while those stems that are more upright do not show this condition. I also noticed, that the same kind of spots found on the cactus stem of image 1 are also present on the concrete near it. Those do look like algal spots to me and do show up on plants at times.

Harald

Yes, the orange spots look very much like the same algae that's growing on the wall next to it.

The orange stuff on the wall is a lychen that live quite specifically on rocks. Its is entirely possible the algi that live in the lychen have some how infected the cacti but it seems unlikely to me. I think the orange spots on the cactus look fungal. You may wish to cuts the infected stem off. Whats the soil its going in like? I would think the soil and roots are the first thing to check its the most likely point there will be problems.

While I do not believe, that the lichen is actually killing your cactus, except for perhaps occupying space on it, which does decrease photosynthesis by the plant, I want to point out, that a lichen is actually a symbiotic relationship between an alga and a fungus. The "host plant" is only used as a substrate and does not supply nutrients to the lichen.
Of course, you could go ahead and cut a piece of the stem off, in order to check on its health. Then you should also be able to see, if there is an infectious agent or if the discoloration is only superficial.

Harald

Welcome gentlemen. I appreciate so much your comments. I can see how similar the spots look to the lichen from the walls now that I look at the pictures, but a close naked eye inspection shows they are certainly not lichens. The plant tissue is damaged underneath the spots and they aren't incrustations.

The sunburn was a very good observation, I didn't notice there are more spots at the exposed areas. But at the same time, this happened during the winter when the sun is weaker. They did well the past two summers.

Hylocereus undatus did not present any spots, only the Hylocereus megalanthus did. So whatever problem they are having, undatus seems resistant to it so far...

If there's any kind of fungal infection. pruning off the infected part is the only solution - fungicides are preventive, not a cure.

If this were sunburn injury, the lower stem surface would most likely still be green. Fungi, on the other hand, will spread onto parts of the plant that are still healthy. They will also over time produce either a mycelium, that is visible on the surface and may be of various colors or you should notice small bumps, that are generally grey to blackish. In case of a rust there may be pustules which do have yellow, orange, reddish brown, brown, or black areas in them. Those would be the spores of the fungus.
Of course, discolorations and die-back also occur, because of frost damage or insect/mite feeding. But the former would be noticeable all along the stem, while the latter most likely would not be on the upper stem surface.
Without being able to see a piece of the affected or infected stem surface close-up, it will just be a "wild guessing game" to arrive at any diagnosis and the chance of being wrong is very great.
If you have had this plant for more than one year, think about what you may have changed as far as its environment is concerned.
That might give you a clue about what could have brought upon the condition it is showing now.

Harald

I've finally removed the infected parts... The rot seemed to not stop. I poured some copper sulfate but the rot kept spreading down the plant. The safest way to go is probably remove the whole plant, but I wanna try this half way measure first. Thanks everyone for the insights. All the best!