Help! I recently moved my 30+ year old cactus outdoors for the summer season. It had been inside all winter with very limited light. I moved it outdoors (high temps 85 F Low Temps 45 F) into the sunlight for about 4-5 hours of the day. I accidently gave it a little too much water about a month ago and havent watered it since. Since then it has slowly turned purple and now more black from the bottom up. The new growths from last year are shrivled discolored. Is this cactus dying due to being overwatered once? Is this a bad sunburn? What can I do to save this cactus (Repot in dry soil)? Does a black cactus mean it is dead or will it turn green again? It does not seem to be really soft and mushy. I have posted a few pictures of the cactus along with a couple smaller ones that were treated the same and are doing well. What kind of cactus is this? Thanks.
It is a Cereus, I have seen several names for this species, peruvianus and hildmannianus are the common ones. This is not sunburn. There are cacti with a natural black color, but this is not one of them. I am pretty sure it is dying, especially the black marks on bottom of the offsets (picture 8) indicate disease. You can cut off a healthy piece, dry and root it (there are plenty of detailed descriptions of this procedure both on this forum and on the rest of the Internet), just be sure that you cut an inch or higher above the blackened area (and if the cut is discolored, cut off a bit more), internally these infections extend further than can be seen on the skin. Not sure it is worth it though, there seem to be few thick, healthy pieces left, maybe the two in the last picture will do. You might need a saw, the hardened parts on this species are too hard for a knife.
Thanks so much for the help and advice. You said you thought it may have some disease. Can you expand on possibly what type of disease would have caused this condition? Could this disease be treated? Would overwatering and rot cause this condition. I wonder if the cactus could be saved rather than cutting off the green shoots and starting over?
has it gotten very chilly at night since you put it outside?? such weird weather all around...we had a frost warning three weeks ago and it had been consistantly in the 70's during the day and 50's at night for the couple weeks before that. you're a bit more north than i am, so cold weather might be what caused the problem. would only take 8hours, or so, of 35-40 degrees to do serious damage to a cactus that can't deal with anything lower than 50's and even less time if it was actually at the freezing mark. such a shame, too, whatever caused the problem - such a beautiful plant!! i'd definitely take cuttings of the healthy parts so that you still have something. i'd also unpot and take a good look at the roots and whatever part of the base of the trunk is under the soil level...you may find that the bottommost part is actually undamaged and that the roots are good. if that's the case, you can cut off whatever above is bad and then repot it. the base will continue to grow and you'll still have the original plant (kind of) - on top of whatever cuttings you can get from the upper parts/branches.
Not much, it is either a bacterial of fungal disorder, but I cannot tell which type it is in your case. Bacterial rot ususally results in the interior of the plant turning to a foul-smelling mush. Not a chance, I'm afraid. I have collected cacti for many years and read a lot about treatment of diseases, but never seen anyone that has managed to cure them. In some cases the infection stops spreading by itself, but it it too late to hope for that in this case. Besides, the base and the roots are almost certainly gone (this kind of infections usually start in the roots), so the plant will die even if the disease itself miracously disappeared. Yes, overwatering + too compact compost is the usual cause. Personallly I prefer to have a very porous compost with low water-holding capacity and water often.
