Large cactus has black spots - Please help

[canny1234]

Hi

I have the below cactus, now sure what it is called. I've had it for 3 months and it has grown about 20 inches in the main bit, and 10 to 15 inches on the other stems. It is indoors, has direct sunlight 8am - 11am and very bright indirect light until end of day. The room temperature is normal. I water it every 2 to 3 weeks. Haven't changed the soil or pot since I purchased it.

4 weeks ago I realised a black spot covering the top end of one of the new born stems. This spot hasn't grown large since but other spots have appeared in other stems.

I am attaching all the photos below. Is this normal? Doesn't feel normal, is it a disease, if so, what should I do ?

Many thanks for your advice

[jmoney]

That is not good. Looks like rot. Is it soft? If soft, it is rot. The black portions will have to be cut out.

[canny1234]

Thanks Jmoney.
The sections that went black first feel are now dried up, and the black bits break off like crisps.
The freshly black ones are soft inside. Does that mean rot? Is that because of too much water? Would cutting them off stop the rotting for the plant?

[jmoney]

Black and soft indicates rot. Typically rot at the base of the plant is caused by overwatering/soil with insufficient drainage. I'm not sure what would cause rot at the top of the plant. Maybe fungus? Another member would have to chime in on that aspect.

As far as getting rid of the rot so it doesn't spread. Yes, cutting off/out the rotted areas would stop the rot IN THAT IMMEDIATE AREA. But it will only stop it if you cut out/off all of the bad areas. This is done by cutting off/into the plant until there is no more black/brown coloration of the flesh and it looks like you have reached green, healthy tissue.

Photos 2-5 I would only cut out the small, localized areas where the rib is rotted.

Photo 6 Looks as if that entire top section (2"?) would have to be chopped off.

As for preventing the rot in other areas, that would require pinpointing what is causing the rot, which unfortunately I am unable to determine. I hope that you are able to save your cactus.

Please let us know if you need clarification on any of this and I hope that someone else is able to tell you why it may have rotted in those areas to begin with.

[canny1234]

Thank you I will cut off these sections as suggested! And hope that this will save the plant. The black is almost always happening at the new grown tissue. Could this be a fungus? If so, is there anything I can do to stop it from spreading? Please let me know if anyone has any suggestions!
Thanks

[jmoney]

Again fungus was just a complete guess on my part. It could be something entirely different causing this rot. Just remember to only water when soil is dry and make sure that the pot & soil that it is in is draining adequately. Soil should not remain very wet or that will cause rotting issues. But it sounds like you water it pretty sparingly already according to your initial post.

[teo]

Also, it's not a cactus but a Euphorbia.

[jmoney]

Also, it's not a cactus but a Euphorbia.

I thought it might be a Euphorbia but I couldn't really tell from the photos so I didn't want to mislead.

[george76904]

Just a note about euphoria. I would strongly advise wearing gloves to cut with (like disposable nitrile) because the SAP of euphorbs is pretty nasty stuff. It's very irritating.

[canny1234]

Thank you! Is it ok to leave it alone after cutting or do I need to put any sort of powder over the cuts ?

[Robb]

Thank you! Is it ok to leave it alone after cutting or do I need to put any sort of powder over the cuts ?

You can dust the cut surfaces very lightly with sulfur, but personally I would just leave the plant to callus over naturally.