Cacti usually come in all sorts of colors, but black should definitely not be one of them. If you’ve started to notice that a part of your cactus or even the whole plant is turning black, undoubtedly this is not a good sign.
Cacti turn black mainly due to fungal diseases, which includes bacterial necrosis, crown rot, and phyllosticta pad spotting. To save the indoor cactus 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 any other nearby houseplants so that they don’t get affected.
Can Fungal and Bacterial Diseases cause cactus to blacken
A cactus with black or brown spots or for a fact even one that’s entirely discoloured is the plant’s cry for once help. The damage may have been going on for a while internally, but you will be just seeing it now externally.
Most likely, there exists a few fungal and bacterial diseases that are responsible for a blackening of cactus. Let us discuss each one of them in more depth from now.
1. Bacterial Necrosis
If you grow any other houseplants along with the cactus variety in your indoor garden, then you don’t have to worry about the plant having bacterial necrosis. That’s because bacterial necrosis strikes only cacti, and that too only certain species of cactus. They include the organ pipe cactus, barrel cactus, prickly pear, the cholla, and the saguaro cactus.
The Erwinia bacteria, which got its name after Erwin Frink Smith- a well-known plant pathologist, is what triggers bacterial necrosis in cacti. All it takes is any wound in your branch and trunk of your cactus and it’s highly possible for bacterial necrosis to attack. Then the cactus develops necrotic pockets, or areas of plant tissue that have died. These pockets allow the disease to travel through out the cactus much more easily due to the weakened areas in your cactus.
There are some species of cactus, which includes the saguaro cactus, those who may not have these exposed areas from the bacterial necrosis forever. These spots soon become patchy and cork-like. This is the attempt of self-healing by the cactus, but the corky areas will still have bacteria within them that further spreads the disease.
These cork areas progress even further, eventually turning black in colour. By this point, any healthy tissue left will die and completely rot away. These tissues can even reach a point where it cracks and releases a liquid in a dark brown hue.
Though bacterial necrosis can be treated, the sooner you catch the disease, the better it is to thrive, since in the earliest stages, the plant’s survival rate is around 80%.
2. Crown Rot
The next disease that can affect your cactus and turn it black is crown rot. It’s not like bacterial necrosis, crown rot is not a cactus-only disease. Everything else in your garden, from shrubs and indoor trees to vegetable plants, can impact crown rot.
Crown rot, which is also known as crown rot disease, occurs when a fungus born within the soil wreaks havoc on your houseplant. If the soil used is too heavy for your plant or you overwater or water too often, the fungus develops. The deadly thigh about crown rot is that once the fungus is in your soil, it’s highly impossible to remove it from the soil.
This fungal disease is very subtle at the start, as you can only see symptoms at the cactus soil line. It will look like dry rotting. Eventually, these affected areas of your cactus turn tan, then dark brown or black. At this point, some of the tissue would have been dead, hence some serious damage has been caused.
The younger the cactus is, higher is the risk of death from crown rot, specially if it gets to the advanced stages where the plant is now black. Even adult plants may not survive from crown rot, but that depends. Like the bacterial necrosis, it’s possible for your cactus and other houseplants(as this crown rot affects other plant also along with cactus) to start leaking a blackish sap if the infection progresses past a certain point. Then it will be too late for any remediation.
3. Phyllosticta Pad Spotting
Yucca plants, cacti, aloe, agave, orchids, and many more are all susceptible of developing phyllosticta pad or leaf spotting. This disease has two fungi types that likely affect the houseplants. They are ascospores and conidia.
Ascospores is caused by air and travel via wind, where they can pass from one houseplant to another. Even if your plant is never outdoor, all that it takes to affect your plant is opening the window on a nice day, this is all it takes for ascospores to get in and begin their germination causing phyllosticta pad spotting.
While, Conidia transports itself from one houseplant to another via water. When you water your plants or for that case even if you open your window to let the rain do the watering for your plants, the conidia can spread to unaffected houseplants. They may pass phyllosticta pad spotting to them as well.
Unlike the other diseases, the 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 called as lesions, and if unchecked they will become bigger. In such case shape too changes. While they were circular spots before, now they are streaky and look more like diamonds.
It can be said that after six weeks of the infection, your cactus to visibly start to display the symptoms. By this time, the spots could have spread to a whole cactus arm and eventually the entire plant.
Does Freezing Temperature Change a Cactus Color to black?
Other than fungal and bacterial disease, to prevent cactus from blackening, another thing we must look into is the plant’s temperature.
There no secret that cacti prefer warmer environments. After all, they are desert succulents. Even though they can be adapted in indoor and outdoor growing, cacti do not do well if the weather turns too cold, specially when temperatures dips to freezing or below.
Within just few hours of consistent cold, your cactus will be injured by the very low temperature. These injuries manifest as black spots which look as wet at the beginning. As they dry, they may also turn crispy or brittle, often fall off from the plant. This is often caused due to tissues damaged from the freezing cold.
Unlike the fungal or bacterial diseases, that can infiltrate your cactus, freezing temperature injuries are not usually that serious. Cactus plant can restore their tissue, but it will take some time. Once this happens, the black spots will disappear naturally. Just keep in mind going forward, not to leave your cactus outdoors when it’s too cold.
Even if the plant is grown indoors exclusively, keep all the windows closed during winter. While in your home or away as well, one way to avoid this situation is to leave your thermostat set to a range of 700 to 800 F.
How to Treat a Cactus with Black Spots
Let’s say your cactus has been affected with either a fungal disease or an injury from freezing temperatures. Either way, it’s not good looking.
If you notice large black spots, especially covering whole swathes of your plant, then the disease is quite advanced already. This means that saving your houseplant is likely going to take long time and will be quite difficult, but you can still do all that you can to keep your cactus alive.
1. Isolate the Cactus
First thing is to move your cactus from that place, 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. While with phyllosticta pad spotting, the disease can transfer via wind or water, so it’s not at all worth risking your other healthy plants.
So isolate the infected cactus and put it far away from the rest of plants in your indoor garden until its condition begins to get better.
2. Remove Damaged Arms
Since the cactus disease will keep on going without any intervention, you will have to stop it in its tracks only. This means you will have to get rid of the infected bits of cactus. You can begin to cut the cactus where it is black with a pruning shears or even a gardening knife. Before starting the process check whether you have sterilized your pruning tool and even after you’re done with the cutting.
Cut in a layer-like way so you can get far into the cactus arms. Fungal diseases can surprisingly go very deep, so keep cutting until no more rotten parts are remaining. The rotting and infection will just not affect the outside of the plant, but internally as well. Your cactus may look black inside when you cut it open, and this is expected.
You must cut until there are no more brownness/blackness left, either inside or outside of the cactus. Only then we can confidently say that plant is in its best possible condition it can be. This means that there is not much left in your cactus, but what remains has a much higher chance of survival. By leaving even one rotting bits in your plant, your cactus might not survive.
3. Change the Soil
As said earlier, diseases like crown rot start and remain in the soil. You could treat the crown rot by cutting away at your cactus, but if the soil intact and not changed, the plant is likely to get crown rot again.
Even if your cactus is not suffering from a crown rot, it’s still not a bad idea to replace the soil & consider adding additional nutrients to the new potting soil or potting mix after the development of a fungal or bacterial disease.
4. Cut Again if Necessary
Once finished doesn’t necessarily mean your duties are done. You will have to watch your cactus carefully over the days and weeks upcoming. It’s possible that you have potentially missed a few even though you thought you have cut away all the black and rotted areas. Unfortunately, if that’s the case, then what is left of your cactus will probably begin rotting once again.
In such cases, then prune yet a second time. Also don’t forget to disinfect your shears both before and after cutting just like last time to limit the spread of plant diseases. If your cactus does not rot after a few days or weeks, then the parts where you have cut before should begin to harden and form a callus. It means the cactus should begin to regrow from the cut spot.