Question

Blue Tipped Cactus   QUESTION: I've had the cactus for over 10 years and it's been growing nicely. It has been in the same place & pot for a couple of years.

In the last week it has started to turn blue at the tip of 1 of its 2 shoots (shown in the picture).

Do you have any idea what this might be?

Thanks :)

ANSWER: I'm not sure why it is getting blue tips.  Cacti are dormant in the winter and shouldn't be watered from Oct. to March.  Is this a new issue?  If so it could be caused by too much water.  If the tip is firm I would just stop water and occasionally mist it ( about twice a month), keep it in a cool place.  The blue may be the beginning of rot and if that is the case the blue tips will shrivel.  I'd remove them. If you are really concerned about rot I'd look at the roots to see if they are healthy and repot it in very well drained soil. I'd also remove the affected shoots. This is best done in the spring when growth becomes active again.  It depends.  I would first stop watering,keep it in a cool spot and see if that stops progression.  If it doesn't, repot and remove the shoots. Then I'd repot in the Spring remembering that cacti like to be crowded and most don't have very deep roots. Most cactus problems are caused by too much water.  They don't need much during their growth period and none while dormant.

To further confuse this, the blue may just be a natural variation, tho after 10 years this seems unlikely.

Hope this helps.

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Full Plant   QUESTION: Hi,

Thanks for your answer. The blue area is softer than the healthier part of the plant but still reasonably firm. It looks like it may be turning black.
I did water it a few weeks ago so it could be that. I've attached another photo showing the full plant, assuming it starts to shrivel and rot do I need to cut the whole shoot off, or is it possible to just cut the rotten part of it off?

Thanks again for your help.

Answer
If it is soft  and turning black I'd cut it off.  You can cut about an inch below the rotting part and dust the the remainder with a fungicide powder or rooting powder just to prevent disease in that part.  You could also cut that whole new shoot off, cut the rot off the top, let it callus and start a new plant.  You may want to wait till Spring for that.  I think the rot is from too much water so keep the mother plant totally dry till March and in a cool place (here in Massachusetts I keep them in an unheated bedroom and occasionally mist and then gradually start
watering. I'd also repot it then.  A good rule of thumb is repot every two years in well drained soil.  
Your pot is too big for the cactus. It should be an inch or two around the cactus, which I can see would be difficult with a tall one like yours, but you can stake it till it settles down.
Every cactus person has their own potting medium and we all think ours is the best.  Very frustrating.  I've been using a soiless mix and am not too fussy.  About 1/2 peat moss- the light stuff that comes in a huge amount, 1/3 perlite and 1/3 sand -  the builders kind that you sift the big particles out of (or if you don't want to buy a big thing of sand you can use 1/2 perlite) and then add some dried blood and bonemeal and some crushed dolomite  and crushed charcoal, but loosely crushed.  Moisten what you are going to use and then don't water the plant for a week or so.  Then, if it is warm and sunny, drench it and don't water again till completely dry. The important part about the mix is that it drains well but maintains a bit of moisture - thus the peat moss. There is also packaged cactus mix. I don't like it, but might be easier for one plant. If you put supplements in the mix I suggested it won't need added fertilizer.The packed mix also has fetilizer in it. Let me know how you do.