Black spots on cactus

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• Buttercupdays Posts: 4,546

  March 2021 edited March 2021

  I have some cacti, but not an expert. Here's my two pennorth anyway.
  

  It does look a bit like scale insect in the pic and you could see if one or two could be scratched off with a thumbnail, if you can do so without injury.
  

  However the colour down at the bottom of the stem looks wrong, as if it might be going mushy and I would incline to cutting the whole stem off cleanly at ground level, so you can enjoy the 3 healthy stems.

  Take care to prepare a thick wadge of newspaper before you try to handle it, cactus spines are the devil if you get one in your finger!.
  

  
  

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• clarke.brunt Posts: 215

  March 2021 edited March 2021

  Seeing as you've got 3 stems which appear to be bursting with health - look at how much growth since the last slight constriction (about half-way up) which was probably a winter pause in growth - and then you've got one tired old stem which is hardly growing at all, I reckon that's all it is - just general age and lack of vigour in that stem. You could cut the offending stem off near the bottom - the brown bits aren't going to turn green again. It doesn't look like scale insect or rot, so not doing any harm leaving it, except appearance. I wonder what changed - maybe a year ago - the top half is better grown than the bottom half.
  

  
  

  I've had similar - sometimes cold damage, but probably not in this case (and this is a fairly hardy cactus), unless it was ages ago before the healthy stems grew. I suppose I don't even know if the 4 stems are connected together, or are rooted separately - it wouldn't normally produce offsets from below ground level. It looks like Trichocereus pachanoi (which maybe we're supposed to call Echinopsis these days). The Aloe is looking good too.

  
  

  I use Trichocereus pachanoi as a grafting stock for other cacti, so I've usually got rather a lot of them a few inches tall just waiting until I want to graft something onto them. The spines are rather insignificant compared to many others species which makes matters easier.
  

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• dollparts1971 Posts: 8

  March 2021

  Thanks both for your replies. I did try scraping one of the blisters but nothing budged, so I don't think it's scale either. @clarke.brunt I did repot the cactus last year May. That's when it grew the upper part. That sorry looking part of the cactus didn't look too good back then either, tbh. I was thinking the same thing, about just removing it. Do you think its worth propagating it? Or should  I just bin it? Thanks for identifying too. I thought it was Trichocereus pachanoi but its good to have someone who knows about cacti confirm it. May I ask how often you feed your Trichocereus? Some say every two weeks in growing season and others say  just two or three times a year. I've purchased some cactus focus feed and don't want to over feed my cacti or succulents as know that's bad for them.
  
  I also want to thank you for your help regarding my acheveria plant. I have now reported it. Leaves are a little shrivelled as I had to let the roots dry out for a few days and it was out of its pot, but it seems to be doing OK. I've now put it in cactus soil mixed with 1/3 perlite. I hope it'll start to grow better now.  Have attached a pic. Im trying to propagate the leaves I had to remove. Be good if I get some new plants from them  

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• clarke.brunt Posts: 215

  March 2021

  It's up to you if you want to try propagating the Trichocereus - I like propagating almost for its own sake, sometimes more than bothering to grow the adult plants properly. You're probably not wanting to cut anything off the 3 nice stems, but could try a piece of the marked one - maybe cut at the 'neck' about a third of the way down. I often end up leaving bits of cactus just somewhere out of soil until they show signs of growing roots - prop it upright through, otherwise it will grow roots from the downward side. If you want to put in dryish soil a bit sooner, then I'd still let the cut dry for a few weeks before that.

  
  

  Fertilising (all my cacti): I don't get round to watering as often as they'd probably like in summer (too many of them) - I'd use the hosepipe, but tap-water here not good for anything that might remain in the same soil for years. I essentially use weak fertiliser every time. I fill an 80 litre dustbin from the water butts, and chuck in a few spoonfuls of some general purpose fertiliser (like Phostrogen, or Miracle Gro). Given that the recommended dilution is something like 1 spoon per gallon/4.5litre, I'm probably ending up with something like quarter-strength. Don't think of cacti as something 'very different' - they're just plants, they mostly like watering in summer (once the soil is nearly dry being the usual suggestion). And I don't water at all in winter - this is in a greenhouse going down to 5 degrees C - I stop watering by around November, and haven't yet re-started (March) this year. Last year, I put quite a few cacti outside for summer, so they can enjoy the rain and bright sun. There are a few exceptions - like 'Christmas' cacti flowering in winter, and I have quite a lot of other things - succulent pelargoniums, and some of the 'mesemb.' family which grow and flower exclusively in winter (that's their best guess of what to do in a UK climate, if they come from somewhere with a dry summer and winter rains).

  
  

  The Echeveria (if that's what we think it is - certainly something like that) is looking good, and the leaf-cuttings should grow (probably only thing that can go wrong is too much water before they've rooted).
  

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• dollparts1971 Posts: 8

  March 2021

  Hi @clarke.brunt Thanks for your reply. I feel I have nothing to lose by propagating the Trichocereus if I'm removing it anyway. I'll give it a go and see what happens. I'll leave it upright to root and then plant it. How long normally before it shows roots, in your opinion? I'd love to see your collection. Do you have any pics on this site? Or any you'd be willing to send? Happy for you to message me privately if you don't want to post on here. Hope you don't think I'm being cheeky. I don't know anyone who has a cactus or succulents

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• clarke.brunt Posts: 215

  March 2021

  Hi again @dollparts1971 Maybe I'll chop a bit off one of my Trichocereus pachanoi for comparison in rooting it - as I said, I've got rather a lot intended for use as cactus grafting stocks. I tend not to get round to whatever grafting I had planned, so eventually cut the tops of the stocks and start again, as I don't want them too tall. Any grafting I do is either to propagate rarities, or to try to save bits of something that's rotting. Seeing as it's mid-March, my cacti are still un-watered and pretty much dormant (yours maybe less-so, being in the house and warmer). Picking numbers out of the air, maybe 8 weeks to put out roots starting now, maybe 4 weeks if it was summer. The great thing about many cacti is that they don't mind the dry rootless period.

  
  

  I have a website - a bit plant-centric, but just for anything I fancy putting on there - I don't sell anything, and don't really care whether anyone sees the site or not. Here https://www.viridis.net/cactus/aloe_poly.html is one page (from which others can be found, and my email address is on all of them). That one happens to be my success story with Aloe polyphylla but there are few cacti in one of the photos as well. Also pleased with pages about growing mistletoe, and live webcams of nesting swifts.

  
  

  There a British Cactus & Succulent Society https://www.bcss.org.uk/ with branches round the country (of course not meeting at the moment due to the pandemic). Some people have marvellous collections (though my style is more 'anything interesting' and 'propagation', rather than huge pristine 'show plants' which some people go for).
  

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