Helping Large Cactus Recover from Significant Mite Attack

Trouble shoot problems you are having with your cactus.
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Johnson7893
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Joined: Fri May 13, 2022 6:20 pm

Helping Large Cactus Recover from Significant Mite Attack

Post by Johnson7893 »

Hello everyone! I’m writing in here pretty sheepishly, as I’m afraid I have a mite situation – which I recently learned, thanks to this board – that I let get completely out of hand, and I wanted to see if anyone had any pointers or tips for helping the cactus recover.
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This is a photo from when I first got the cactus, for comparison's sake. I believe it's a fairly old Golden Torch cactus. As you can see from the image I posted, it started out pretty healthy – although, you can see some light grey areas on the top right barrels that I now understand is probably mite damage, so I'm guessing it was infected from the beginning. The light grey areas grew very gradually over time, and at first I thought it was a matter of the cactus not getting enough light. I installed some full spectrum LCD bulbs in the ceiling outlets to get it some more light, and that seemed to make it a bit happier. But, out of concern that it still wasn’t getting full enough light, I was watering very sparingly to avoid root rot – and stopped altogether in the winter months.

The branches started getting paler (which I perhaps foolishly chalked up to just the winter, lower light conditions, and lack of water), and because they got paler, it actually masked the spreading mite damage. So, when I went to finally re-water them about 3 weeks ago, they immediately got some color back in unaffected areas, and the significant extent of the damage became clearer.
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As you can see, the gray has turned to more of a rust-ish color, and now covers about ~33% of green areas, if not a bit more. After doing some serious digging, I concluded it was mite damage, as there was a good number of webs and the damage matched images I found on the web. I immediately sprayed down the cactus with diluted isopropyl alcohol (at night, in low light). Two days later, I followed up with neem oil (again, at night and in low light).

I'm now on a regimen of spraying alcohol and neem oil once each per week, and spraying the whole plant down with a water mister each day I don't do a treatment. I have not ever actually seen any mites--I ordered a magnifying glass after I had already done a number treatments and haven't been able to spot any since it arrived, but am continuing to monitor.

With all of that context out of the way, here are my questions:

- Do you think it's possible for this cactus to make a full recovery (aside from the rust, which I understand is probably permanent scarring?) I'm concerned that the rust now covers enough of the cactus that it will inhibit too much photosynthesis for the cactus to survive. That said, FWIW, I cut off one of the etiolated tips this morning, and the inside of the cactus looked completely health and decently juicy.

- As you can see, the cactus as it stands now is significantly less plump than it was when I first got it -- likely exacerbated by the lack of watering in the winter and the mites sucking out all of the juices. When I first realized about 3 weeks ago how shrunken the barrels were relative to when I first got it (again, it happened gradually, so I didn't even realize it), I immediately gave it a big water soak, but it didn't seem to do much to plump it back up. I let the water get completely dry again, which only took about 7 days, and then did another big soak -- again, it plumped up a bit, but not much, and the soil dried up pretty quickly again. I'm obviously very concerned about overwatering -- should I continue soaking it once a week, only when the soil is completely dry, until it plumps back up? Or is that too often and unhelpful to the plant?

-If it were to make a recovery, how long would you guess it would take for the plant to return to anywhere near normal, at least in terms of plumpness? I'm guessing we're on the order of months, if not seasons, but was wondering what the experts think.

- A couple of the etiolated tips on the cactus have developed small black spots on them. They are tiny, but it looked to me like it might be rot. As I said above, I've already cut off a few of the tips, and the cactus looks really health inside (and the black spots have never returned). That said, I found a couple of the spots on my healthiest growth, which I'm really loathe to trim. So, wanted to check if there's a chance these spots are just cosmetic, or if I need to lop it off just to be safe. I've included images of these spots below.
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Those are my big questions, but I am open to any other helpful tips folks might have, or issues they might see. I also recently bought a much more powerful grow light, so I'm hopeful the cactus is now getting enough light (my light meter seems to think so, at least!).

I'm embarrassed and bummed I let the damage get this far, but I frankly didn't even know spider mites were a thing! An important lesson learned, I suppose :)

Thanks in advance for your help, everyone!
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greenknight
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Location: SW Washington State zone 8b

Re: Helping Large Cactus Recover from Significant Mite Attack

Post by greenknight »

I think you're right on the ID - Echinopsis spachiana (syn Trichocereus spachianus). Nothing special you can do - just give it good growing conditions, it will continue to grow. It could do just fine - though the scarring is permanent. yes.

Water thoroughly when dry during the growing season, that's just normal care. Don't over-water, always wait until it's dry. Don't know if those scarred stems will ever plump up like before - transpiration, which is evaporation from the plant's stomata (pores), is the engine that drives water uptake, and the stomata on the worst scarred areas are probably too damaged to function.

I would just monitor those spots for now, if they don't spread you don't need to do anything. You could just carve out the affected areas back to healthy tissue, instead of cutting off the whole tip - this would make more scars, but you wouldn't lose as much growth.

The way this cactus could fully recover would be if the damaged stems were replaced by new ones. These grow and offset freely, you could develop a whole new top in a few years by gradually pruning out the damaged stems to stimulate more offsets. For instance, where you removed an etiolated tip, you could just cut that stem back to a stump (best to cut at an angle). Later, when you remove ones that have healthy, growing tips, you could make cuttings from those for backup plants. I know this would be painful, but I think it would be best in the long run. Easier to light it if it's shorter, too.
Spence :mrgreen:
Johnson7893
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Re: Helping Large Cactus Recover from Significant Mite Attack

Post by Johnson7893 »

Thank you so, so much! Super helpful. I'll try to post an update in a few months on progress in case folks are curious.
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anttisepp
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Re: Helping Large Cactus Recover from Significant Mite Attack

Post by anttisepp »

It's not a good idea to hold cactus in the corner of the room, though under artificial light.
I imagine it could be perfect for Howea or Monstera but not for a succulent. These guys never forgive lack of direct sun.
IMHO
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ohugal
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Re: Helping Large Cactus Recover from Significant Mite Attack

Post by ohugal »

I think greenknight gave a good answer. I concur with anttisepp, it needs more light. Are you able to put it outside? I don’t have any experience with growing lights, but make sure it gets enough of it. Length as well as intensity. There are applications for smartphones with which you can measure light intensity. I think mites also occur in dry, stagnant air, so a good air flow is important. Corners usually don’t provide this. Please be carefull with repeated applications of neem oil. In high concentrations it puts an oily film on the plants surface.
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greenknight
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Re: Helping Large Cactus Recover from Significant Mite Attack

Post by greenknight »

If you could get it closer to the window it would help. Moving it outdoors for the summer would be great, but it's awful big to be moving around much.

Neem oil once a week should be fine.
Spence :mrgreen:
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