I have had this cactus since last Summer. It was doing fine. It is kept in a window with plenty of sunlight (gets the sun from about 3pm til sunset). I water it only when the soil is dry. It has suddenly turned pale green and is brown / black at the bottom above the soil line. I wondered if it was rot, so I have taken it out of its pot to inspect it. However, I have stuck a cocktail stick down into the soil and it is dry. It is not wet around the areas of discolouration at the base. It is not mushy and soft either. I have other cacti in the same window as this one and they are fine. So if it not rotting then what is happening? I don't know what is wrong or what I need to do about it. Any help would be greatly appreciated! Thank you
Stems turn darker and harder with age, eventually turning into wood. Several of the pictures indicate that this is what is could be going on, but the stems should not have "turned pale green". The latter could be sun damage, which happens more easily if the roots are damaged or dead. There are several types of "rot", and some of them, caused by fungi, slowly hollow out the stem, allowing the plant to remain upright longer than diseases that liquefy the tissue do. If this is the cause, the discolouration will spread upwards moderately fast, over days to weeks. Black spots will often develop on the epidermis, but his type of rot can be sometimes be difficult to spot until the plant suddenly collapses. I use a felt-tipped pen to mark the border of the discolouration, it makes it easier to see if it is spreading, and how fast. A closer look at the roots could help, but unfortunately they are encased in peat, which is difficult to remove.
Many thanks for such a detailed reply! Is the 'turning into wood' the phenomenon known as 'corking'? I have removed it from its pot, as I said to inspect the base for wetness, and the roots all look white, and not the brown you'd expect to see if the roots were rotting. If it is rot, affecting the stems rather than starting at the roots, how will I know this and, more importantly, what can I do now to try to stop it from killing the plant? Thank you for your help!
Yes. Difficult unless you A) cut off the stem and look for discolourations, or B) remove all of the soil and inspect the thicker central root. Note that dead roots are not necessarily brown and mushy, they are often nearly white, wrinkled, hollow and are easy to break off. Otherwise you must wait and see if the symptoms get worse. Nothing, unless you find a healthy part and are able to root it.
Thank you so much for all of your help and advice - I really appreciate it!!! Hmmmm I will have a think whether I want to wait a bit more to see what happens, otherwise I may try removing the soil as you have suggested. Whoever thought that keeping cacti was easy?!
