Black spot - rot?

[Srasra]

Hi friends!
I’m fairly new to cacti and am looking for some advice.
I bought this friendo two weeks ago, had him repotted, and just this weekend noticed the base of one is turning black. I scooped away the dirt and it doesn’t go down far. The left side is unaffected. The black goes all around, but thus far is not softer than the green part.

The soil is fairly dry and I have not watered it at all as i was giving it some time after being repotted. The nursery did not water it either and the potting mix is dry.

Is this rot? If so, can I save him?

[Steve Johnson]

First of all, see this:

https://www.cactiguide.com/forum/viewtopic.php?t=43819

The black area is dead tissue known as necrosis. Once it starts, it won't stop, and unfortunately your cactus won't survive since the necrosis started either in or below the base. I should also mention that your cactus is a desert species, and the potting soil needs to be leaned out with some sort of mineral gravel for free drainage and proper aeration of the roots. Pumice gravel is excellent, and you can also go with calcined clay granules (AKA Leca balls). My default recommendation is a mix of 50% soil and 50% mineral gravel. Try again with another cactus, and be sure to clean the roots as thoroughly as possible before you transplant. Avoid potting soils from a local nursery, and use this instead:

https://www.amazon.com/Rosy-Soil-Cactus ... hdGY&psc=1

[Srasra]

Thank you; sad but I expected as much. Is the other portion still salvageable? Showing no black spots. I plan on getting a new pot and the potting soil if you think I can split the two.

[Steve Johnson]

Thank you; sad but I expected as much. Is the other portion still salvageable? Showing no black spots. I plan on getting a new pot and the potting soil if you think I can split the two.

Nice thing about the forum is that experienced growers can help beginners go through the learning process, so you're coming to the right place. We do have a few important things to unpack here...

Your large columnar appears to be a San Pedro cactus (Trichocereus pachanoi), very common and easy to grow. Given the amount of plant material you have to work with, the necrosis isn't bad yet. Sterilize a sharp knife with alcohol and cut the stems above the necrotic tissue. If the exposed tissues are nice and green, give them a liberal dusting of sulfur powder. But if you see even a little bit of black stuff, continue to cut until you see nothing but healthy green tissue. Then apply the sulfur powder -- here's a before-and-after example of how to approach the process:

Echinocereus_pectinatus_'Coahuila'02062015_06.JPG (95.14 KiB) Viewed 2140 times

Keep the cut stems upside down while the exposed tissues dry out and form a callus. 3 or 4 weeks should do it. In the meantime, shake off the excess sulfur powder once the tissues are reasonably dry. You can use that pot as a rooting pot, although you'll need to dump out the soil, then clean and sterilize the inside of the pot with alcohol or bleach (I think bleach will be better).

The stems will have to start growing a root system before your San Pedros are able to take up water, so don't even think about watering yet. When the cut surfaces are well-callused, here's the next step:

• Fill your rooting pot halfway up with a 50% Rosy Soil/50% mineral gravel mix. That's your substrate.

• Fill the pot the rest of the way up with potting sand, which you'll find on Amazon here:
  
  https://www.amazon.com/Original-Ingredi ... r=8-4&th=1

• Dig the stems most of the way (but not all the way) down into the sand. Stabilize them with a plant stake if required.

Mist the stems every 4 weeks or so -- believe it or not, this helps in the rooting process. Our Australian horticulturalist-in-residence (MikeInOz) has a lot of experience with rooting cacti, and he gives his cuttings a year before he tips them out of his rooting box. Following his advice, you'll have to therefore be patient. Believe it or not, your San Pedros won't die of dehydration, so wait until you take them out of their rooting pot in spring 2025, clean out the sand, and repot with the 50% Rosy Soil/50% mineral gravel mix. At that point, you should be looking at nice healthy root systems, and the San Pedros will be ready to accept deep watering in spring and summer.

Don't know how many other cacti you have (I see 2 in your last photo), but you should be growing all of them with the 50% Rosy Soil/50% mineral gravel mix I'm recommending. Straight potting soil is fine for leafy nonxeric plants (African violets, ferns, etc.), not fine for xeric plants like cacti and succulents because it suffocates their roots. If you can't find pumice or calcined clay granules at your local nursery, Amazon gives you a nice selection to choose from:

https://www.amazon.com/s?k=pumice+for+p ... -doa-p_5_6

https://www.amazon.com/s?k=calcined+cla ... -doa-p_1_8

Speaking of...

Cacti need plenty of light, and growers who keep their cacti indoors as houseplants could seriously use better lighting. Not enough natural light in the house (the usual artificial lighting won't cut it either), so we can solve that problem with full-spectrum LED grow lights:

https://www.amazon.com/s?k=grow+lights+ ... -doa-p_1_5

Cacti also need good ventilation. Problem solved in the spring and summer because you can keep the windows open. For the fall and winter, it wouldn't be a bad idea if you place a fan near your cacti to keep the air moving.

[Srasra]

Thank you so much- this was incredibly helpful! Guess I have a weekend (year?) project ahead of me. I’m glad I have a chance at saving him!

[Steve Johnson]

Thank you so much- this was incredibly helpful! Guess I have a weekend (year?) project ahead of me. I’m glad I have a chance at saving him!

My pleasure!

Assuming that your other cacti are in the same potting soil that got the San Pedro in trouble, now would be a good time to take them out of their pots. Gently remove the soil and inspect the roots. Do they look healthy? If so, excellent. Do you see a bunch of white fluffy stuff in the roots? If you do, those are root mealybugs -- you'll need to knock the infestation down ASAP and before you repot. Soak the roots in a 2% soap-warm water solution for about an hour. Plain dish soap is fine, insecticidal soap even better. Let the roots dry out completely.

Before you repot, you'll need to get the roots of your cacti as squeaky-clean as possible. To do that, take a container of fresh water and swish them around to loosen the soil and float it out of the roots. Use a blunt object like a q-tip handle, knitting needle, etc. and gently "tease" more soil out of the roots. If they still look dirty, throw out the water and refill the container with fresh water, then do the swishing around and teasing again. When you're satisfied that the roots are clean, let them dry out, then repot in the 50% Rosy Soil-50% mineral gravel mix I recommended. Let your cacti settle into their pots for a few weeks (maybe a month if you're repotting before the end of March), then start watering them deeply. Water every 2 weeks in spring and summer.

As I said in my last post, your San Pedro cuttings won't be ready for watering until they grow root systems. I realize that waiting for months might be a test of patience, but there's something you can do in summer (late July or August) -- gently (and I mean gently) pull up on the stems. If they feel like they might come right out, gently seat them back in the potting sand. But if you get some resistance, you have roots, and it's possible that they may already be growing down into the substrate. At that point it would be tempting to remove the stems from their rooting pot, but best if you continue to be patient and repot them next year.

If you need any additional help, let me know.

[Srasra]

Update! I was able to salvage almost 2’ of the necrotizing cactus and the cutting is currently chilling upside down. The other San Pedro has been successfully removed, roots cleaned, and will be repotted once its roots are sufficiently dry. No mealybugs and the roots look good! Thanks again for all your assistance, I look forward to repotting tomorrow and seeing how my new friend does with his new soil mix.

[Steve Johnson]

Update! I was able to salvage almost 2’ of the necrotizing cactus and the cutting is currently chilling upside down. The other San Pedro has been successfully removed, roots cleaned, and will be repotted once its roots are sufficiently dry. No mealybugs and the roots look good! Thanks again for all your assistance, I look forward to repotting tomorrow and seeing how my new friend does with his new soil mix.

Glad to help!

While we're here, I wanted to find out if you're fertilizing your cacti (you should be). If you are, please let me know about the fert you're using. Its label provides a guaranteed analysis, and here's an example of the information we're looking for:

FloraMicro-FloraBloom.jpg (83.55 KiB) Viewed 1986 times

The balance of N, P, and K is important, the "rule of thumb" being -- P lower than N, K higher than both. Once I see what you have, I'll do an evaluation.