Cacti come in all sorts of colors, but black should not be one of them. If you’ve noticed parts of your cactus or even the whole plant is black, surely this is not good. What can you do for your cactus at this point?
Why does cactus turn black and what can you do? Cacti turn black due to fungal diseases, including bacterial necrosis, crown rot, and phyllosticta pad spotting. To save your indoor plant at this point, you should remove the affected areas and try to prevent the spread of infection to the rest of your cactus as well as other nearby houseplants.
What is phyllosticta pad spotting? Is it always possible to save a blackened cactus? I’m going to answer those questions and more in this article. If you own a cactus or plan on adding a cactus to your houseplant family at some point, you’ll want to read this entire article!
Fungal and Bacterial Diseases That Cause a Cactus to Blacken
A cactus with brown or black spots or even one that’s entirely discolored is the plant’s cry for help. While internally, the damage may have been going on for a while, you’re just now seeing it externally.
There exists a few fungal and bacterial diseases that are most likely responsible for a blackened cactus. While I touched on them in the intro, I’ll discuss each one in more depth now.
If you grow other houseplants besides the cactus variety in your indoor garden, then you’ll never have to worry about them having bacterial necrosis.
That’s because bacterial necrosis strikes cacti only, and only certain species of cactus at that. These include the organ pipe cactus, barrel cactus, prickly pear, the cholla, and the saguaro cactus.
The Erwinia bacteria, which earned its name after Erwin Frink Smith (a well-known plant pathologist), is what triggers bacterial necrosis in cacti. All it takes is any branch and trunk wounds in your cactus and it’s possible for bacterial necrosis to enter.
The cactus then develops necrotic pockets, or areas of plant tissue that have died. These pockets allow the disease to travel much more easily due to the weakened areas throughout the cactus.
Some species of cactus, including the saguaro cactus, may not have these exposed areas from bacterial necrosis forever. These spots soon become patchy and cork-like.
This is the cactus’ attempt at self-healing, but the corky areas still have bacteria within them that furthers the spread of this disease.
These cork areas progress even further, eventually turning black. By this point, any healthy tissue is now dead and completely rotting away. The tissue can even reach the point where it cracks and releases a liquid in a dark brown hue.
While bacterial necrosis is treatable, the sooner you catch this disease, the better. At its earliest stages, your plant’s survival rate is around 80 percent.
Do keep in mind that that number does steadily decrease the longer the disease goes on.
The next disease that can affect your cactus, and turn it black, is crown rot. Unlike bacterial necrosis, crown rot is not a cacti-only disease.
Everything from your shrubs and indoor trees to vegetable plants can be impacted by crown rot, so if you’re not already well-informed about it, now is a good time to learn.
Crown rot, also known as crown rot disease, occurs when a fungus born within the soil wreaks havoc on your houseplant. If you use soil that’s too heavy for your plant or you overwater or water too often, the fungus develops. What’s so deadly about crown rot is that once the fungus is in your soil, it’s almost impossible to remove from the soil.
This fungal disease starts off subtly, as you’ll only see symptoms at your cactus’ soil line. This will look like dry rotting. Eventually, the affected areas of your cactus turn tan, then dark brown or black. By this point, some of the tissue has died, so some serious damage has been done.
The younger your cactus is, the higher its risk of death from crown rot, especially once it gets to the advanced stages where the plant is now black. Even adult plants may not survive crown rot, but it depends. Like with bacterial necrosis, it’s possible for your cactus (and other houseplants) to begin leaking a brownish sap if the infection progresses past a certain point. Then it may be too late for any remediation.
Phyllosticta Pad Spotting
Yucca plants, cacti, aloe, agave, orchids, and more are all susceptible to developing phyllosticta pad or leaf spotting.
The disease has two fungi types that like to affect houseplants. These are ascospores and conidia.
Ascospores arrive in the air and travel via the wind, where they can pass from one houseplant to another. Even if your plant never goes outdoors, if you open the window on a nice day, that could be all it takes for ascospores to get in and begin their germination.
Conidia transports itself via water. When you water your houseplants or open the window to let the rain do it, the conidia can get to unaffected houseplants and pass along phyllosticta pad spotting to them as well.
Unlike the other diseases we’ve discussed, whole parts of your cactus won’t look black with phyllosticta pad spotting. Instead, the plant develops spots that are sometimes purple if not black. These are lesions, and unchecked, they will become bigger.
Their shape changes, too. While they were circular spots before, now they’re streaky and look more like diamonds.
It can be upwards of six weeks after the infection begins for your cactus to visibly display symptoms. By that point, the spots could have spread to a whole cactus arm and eventually the entire plant.
Can Freezing Temperatures Change a Cactus’ Color for the Worst?
Besides fungal and bacterial disease, to prevent cactus blackening, another thing you must look out for is the plant’s temperature.
It’s no secret that cacti prefer warm environments; They are desert succulents, after all. Even though they’re adapted to indoor and outdoor growing, cacti do not do well when the weather turns too cold, especially temperatures that dip near freezing or below.
Within a few hours of consistent cold, your cactus can be injured by the very low temps. These injuries manifest as black spots which look wet at first.
As they become drier, they may turn crispy or brittle and often fall or break off from the plant. This often occurs due to tissue damage from the freezing cold.
Unlike the fungi or bacteria that can infiltrate your cactus, freezing temperature injuries are usually not that serious. Your cactus plant can restore its tissue, but it will take time, so be patient.
Once that happens, the black spots should disappear. Just make sure that going forward, you don’t leave your cactus outdoors when it’s cold.
Even if you grow your plant indoors exclusively, keep all windows closed in the winter. While your home as well as away, one way to avoid this is to leave your thermostat set to a range of 70 to 80 degrees Fahrenheit.
How to Treat a Cactus with Black Spots
Let’s say your cactus has been afflicted with either a fungal disease or an injury from freezing temperatures. Either way, it’s not looking so good.
If you see large black spots, especially covering whole swathes of your plant, then its disease is already quite advanced. While that might mean saving your houseplant is likely going to take time and be difficult, you can still do all you can to keep your cactus alive.
Here are some steps I recommend you take.
Isolate the Cactus
First thing’s first; you should move your cactus. Whether your indoor garden consists of only two or three other plants or dozens, fungal and bacterial diseases can easily spread to other unsuspecting plants.
Remember, with phyllosticta pad spotting, the disease can transfer via the wind or water, so it’s not worth risking your other healthy plants.
Take your infected cactus and put it far away from the rest of your indoor garden until its condition begins turning around.
Remove Damaged Arms
Since the cactus’ disease will keep going and going without intervention, you have to stop it in its tracks. This means getting rid of the infected bits of cactus.
With pruning shears or even a gardening knife, begin to cut where the cactus is black. You want to triple-check that you sterilize your pruning tool both before and after you’re done with the cutting.
Unclean tools will easily spread bacteria or fungus to other houseplants, possibly killing them.
Cut in a layer-like way so you can get far into the cactus arms. Fungal diseases can reach surprisingly deep, so keep cutting until no more rotten parts remain. Remember that rot and infection will affect not only the outside of the plant, but internally as well.
Your cactus may look brown inside when you cut it open, and this is to be expected.
You must keep cutting until you see no more brownness/blackness left, either on the inside or outside of the cactus. Only then can you confidently say the plant is in the best possible condition it can be in right now.
Yes, this may mean there’s not much left of your cactus, but what remains has a much higher chance of survival. By leaving even any rotting bits on your plant, your cactus might not make it.
Change the Soil
If you recall from earlier in this article, diseases like crown rot start and remain in the soil. You could treat a case of crown rot by cutting away at your cactus, but if you leave the soil intact, the plant is likely to get crown rot again.
Even if you don’t think your cactus is suffering from a case of crown rot, it’s still not a bad idea to replace their soil & consider adding additional nutrients to the new potting soil or potting mix after the development of a fungal or bacterial disease.
Cut Again if Necessary
You’re finished for now, but that doesn’t necessarily mean your duties are done. You want to watch your cactus carefully over the days and weeks to come. It’s possible that even though you thought you cut away all the black and rotted areas that you potentially missed a few.
Unfortunately, if that’s the case, then what is left of your cactus will probably begin rotting out again.
If that’s the case, then prune yet a second time. Please remember to disinfect your shears both before and after cutting to limit the spread of plant diseases just like you did last time.
If your cactus did not rot after a few days or weeks, then the parts where you cut before should begin to harden and form a callus.
Then, the cactus should begin regrowing from the cut spot.
Should you find yourself really unhappy with your barely-there cactus, you can always graft various cacti species together into one type of super-cactus. Just be sure that all the cacti your grafting together are healthy to avoid spreading rot or diseases to the other arms and parts.
Why is my cactus turning gray?
You have a different issue with your cactus than it becoming black or brown. Instead, it’s gone gray. For some cacti, this gray color has a metallic sheen, almost like a silver, and for others, it doesn’t.
Either way, it can be very confusing if your cactus didn’t look like this when you first brought it home.
It could be that your cactus has matured, as some species of cactus are supposed to be gray. Since a gray cactus isn’t a sign of a fungal disease or rot like a brown or black one is, just keep taking care of it and appreciate its unique color!
Also, there’s the possibility you could have mistaken your cactus for another plant species.
Many times people think they’re buying or adopting a new cacti when in fact the plant is really a Euphorbia or something like it. Euphorbia is a genus of flowering plants that are also commonly called spurge.
While Euphorbia can look a lot like some cacti, it’s important to point out that a Euphorbia is not a cactus.