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I live in the UK and have a Trichocereus bridgesii/Echinopsis lageniformis "monstrose" cactus that I bought from a garden centre 3 years ago. Every year in early summer several new stems grow from sections on the existing stems near to the spines, starting as a bright green colour and growing throughout the summer until they slow down and turn a darker green around September.

Unfortunately I've always found that near the end of this growing period the surface of some sections of the new growth appears to "dissolve" - looking almost as if it has been burnt off by acid - revealing bright green flesh underneath which quickly forms into a white crust. Luckily once the growing season finishes this whole process stops too. Here's a picture:

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I'm not sure whether this is a pest or a fungus or otherwise something to do with the plant itself. It spends most of the year inside and sheltered from the damp cold UK weather with minimal watering during the autumn/winter months and every 2 weeks or so in spring with regular feeding. In the summer I put it outside when it's warm and sunny, and I wonder if it could be getting infected when I do this, but I've never seen any small insects on the cactus. Also the repeated occurence of this white crust every year - as well as the fact it stops spreading when the plant stops growing in autumn - makes me suspect that pests might not be the problem. But I'd like to be able to stop it happening this year because it looks very unsightly and I'm quite fond of this cactus!

If anyone recognises this disease or has experience with this species of cactus and can give me any tips on how to prevent it returning this coming year I'd be really grateful. Thank you!

  • I would guess sunburn, dead tissue from exposure to sun after a period of low light, but then why would there be callousing t the bases too... – J. Musser Feb 21 '16 at 18:18
  • @J.Musser that's a good point! I didn't think that cacti could get sunburn - especially this species which said "full sun" on the little note that came with it - but after your comment I noticed one side of my plant is more damaged than the other, and it normally doesn't get rotated when it's outside. I looked this up and it seems like this one might actually be quite susceptible to burning e.g. see bit.ly/1L3CRr0 so you're probably right – Alex Saad Feb 21 '16 at 18:36
  • Many cacti get the 'brown scalies' because of cold. Might the perception that it does it only when actively be growing really be that it only happens when it is outdoors? So, maybe bringing it inside overnight is the thing to do. – Jim Young Feb 21 '16 at 22:42
  • @JimYoung I tend to bring it in overnight even in the summer months but very occasionally leave it outside. So it could be the cold although I think it's probably unlikely. That said, it's good advice to keep it inside overnight even in the summer, and also good to know that whatever it is it doesn't look like an obvious infestation. – Alex Saad Feb 22 '16 at 0:05
  • @AlexSaad Most desert cacti only get sunburned if they are kept out of direct sun for a while (long enough to adjust) and then moving it back to full sun. I've done it, and the damage looks about the same as you have. But mine was all on the top/side facing the sun, not really at the bases – J. Musser Feb 22 '16 at 0:16
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What you're seeing there is quite normal callous formation - it's a bit like a scab when you've cut yourself. The cause of the corky callous layers is damage - either from insects, or sudden chilling, physical injury and sometimes, underwatering in summer.

I'm no cacti expert, but it's possible, from your description of some of the new growth 'dissolving' towards the end of the growth period, that it's being caught by colder temperatures, particularly if its outdoors. New growth would be particularly vulnerable to changes in temperature, and that might explain why those areas die back. Once they die back, callous formation is both inevitable and healthy, although, of course, unattractive.

It may also be that you are overfeeding, causing excessive growth which will be particularly weak and susceptible to both sunburn and a drop in temperature - this particular cactus is not a fast grower. As you're in the UK, I'd be inclined to keep it indoors all the time, but somewhere with good airflow, bright daylight and a little winter sun, to avoid both the possibility of anything 'grazing' on the plant, or fluctuations in temperature damaging the plant.

Some info in the link below which might helpful - you will note it talks about this plant being pretty cold hardy, but that's only once its acclimatized, it won't like sudden changes in temperature

http://www.cactus-art.biz/schede/TRICHOCEREUS/Trichocereus_bridgesii/Trichocereus_bridgesii_mostruosa_B/Trichocereus_inemis_mostruosus_inemis_B_penis_cactus.htm

Lastly, cactus can get scale infestation - these look like raised little beigy brown discs or bumps, so you might want to check that isn't a problem by inspecting the smaller areas of damage to make sure there's not an active infestation currently.

  • Thank you for all your tips as well as the information about sudden temperature changes - I didn't know that the rate of change of temperature was also an important thing for plants! I will also keep an eye out for scale infestation, although the different comments/answers on this post have convinced me whatever is causing this is likely to be to do with "environmental" factors like temperature/water/light rather than disease. – Alex Saad Feb 22 '16 at 18:23

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