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Why Is My Cactus Turning Black? | 3 Diseases & 4 Ways to Heal!

There are many colors for cacti, but black shouldn’t be one of them.

Now…if you’ve seen your cactus turning black, you’re not alone. It’s a common problem that many people face in cacti. Either some parts or possibly the entire plant can turn black.

So, stop wondering “Why is my cactus turning black?” and head straight ahead!

What might be the reason for your cactus to turn black, and how to treat black spots on cacti?

Fungal infections, such as Crown Rot, Bacterial Necrosis, and Phyllosticta Pad Spotting, cause cacti to turn black. Once detected, the only possible solution is to remove the afflicted regions and prevent the cactus from turning black by preventing the infection from spreading to the remainder of your cactus.

Now you might also have queries like what is Phyllosticta pad spotting, and can you always treat the black spots on the cactus?

In this article, we’ll try to answer all your queries related to why is my cactus turning black. So, read the article fully if you have a cactus or intend to add one to your collection of houseplants eventually.

Why is my cactus turning black
Why is my cactus turning black? – Image via Reddit.

Why Is My Cactus Turning Black? | The Diseases!

The plant pleads for assistance when a cactus turns black and soft or is completely disfigured. While the damage may have been occurring internally for some time, you are only now noticing it outside.

Several bacterial and fungal infections could be the cause of a blackened cactus. Let’s get into the details of each.

1. Bacterial Necrosis

You won’t ever need to be concerned about bacterial necrosis if you grow houseplants other than cacti in your indoor garden.

Only some species of cacti, specifically, Cholla, Saguaro, Prickly Pear, Barrel, and Organ Pipe, are affected by bacterial necrosis.

Cacti experience bacterial necrosis due to the Erwinia bacteria, which got its name from renowned plant pathologist Erwin Frink Smith. Any wounds in your cactus’ branches and the trunk could help bacteria enter the plant and cause bacterial necrosis.

  • The cactus subsequently develops necrotic pockets or dead plant tissue.
  • These pockets make it much easier for the illness to spread by targeting the weak spots all over the cactus.
  • Some cactus species, like the Saguaro cactus, won’t always have these exposed patches from bacterial necrosis.
  • These patches quickly develop into cork-like patches.
  • Although the cactus is trying to repair itself, the corky patches still contain bacteria that lead to the spread of the disease.
  • These cork patches continue to develop, and finally, the cactus turns black.
  • Any healthy tissue is already dead and entirely withering away at this point.
  • The tissue may even degenerate to the point that it fractures and releases a dark brown liquid.

Although curable, the earlier you diagnose bacterial necrosis in your cactus, the better.

Your plant has an average survival rate of about 80% in its early stages. Remember that as the disease progresses, the number does indeed slowly decline.

2. Crown Rot

Crown rot is a disease that can harm your cactus and cause it to turn black

Compared to bacterial necrosis, crown rot affects all plants, not just cacti.

If you still need awareness about crown rot, now is an excellent time to be informed. It can affect anything from shrubs and indoor trees to vegetable plants.

“Crown rot, sometimes called crown rot disease, affects indoor plants due to a soil-born fungus that creates chaos.”

The fungus grows if you use too heavy soil for your plant, overwater it, or water it too frequently.

Crown rot is particularly dangerous since it is nearly impossible to get rid of once the fungus is in your soil. This fungus initially appears quietly, as the symptoms only appear at the soil line of your cactus. It will appear to be dry and decaying.

Your cactus’ impacted regions eventually tan, then turn dark brown or black. Some of the tissue will die already, causing significant harm.

Your cactus’ chance of dying from crown rot increases when young if it has already reached the later stages of the disease, where the plant is black. Crown rot may even kill adult plants, but it depends.

Your cactus (and other houseplants) may bleed a brownish sap if the infection worsens past a particular stage, just like bacterial necrosis.

If so, there might be less time to make any corrections.

3. Spotting Phyllosticta Pads

All kinds of plants, including Yucca, Cacti, Aloe, Agave, Orchids, and more, are likely to develop Phyllosticta pads or Leaf spots. The two fungus strains that cause this disease are Ascospores and Conidia.

Ascospores are airborne particles that move with the wind and can spread from one indoor plant to another.

Though you open the window on a good day, even if your plant never goes outside, Ascospores may be able to enter and start germination there.

On the other hand, Conidia travels through water.

Conidia can travel to unaffected houseplants and spread Phyllosticta pad spotting when you water them or open the window to let the rain do it.

The entire surface of your cactus won’t appear black with a Phyllosticta pad spot, unlike the other illnesses we’ve covered. Instead, the plant starts to form patches that might occasionally be purple or black. These are lesions, and if left untreated, they will enlarge.

Their appearance also varies.

They used to be circular spots, but now they are streaky and resemble diamonds.

Your cactus may not show symptoms for up to six weeks after the illness first appears. By then, the spots might take over an entire cactus arm and, ultimately, the entire plant.

Black spots on cactus—why is my cactus turning black?
Why is my cactus turning black? – Image via Reddit.

Can Freezing Temperatures Affect the Color of a Cactus?

In addition to bacterial and fungal infections, the plant’s temperature is another factor that can answer why is my cactus turning black.

  • Watch out for cold temperatures to avoid cactus blackening.
  • We all know that cacti like warm climates as they are desert succulents.
  • Despite being able to grow both indoors and outdoors, cacti struggle in extreme cold, especially when the temperature drops to below zero degrees Celsius or close.
  • The low temperatures can harm your cactus immediately after a few hours.
  • The wounds appear as black spots on the cactus that initially appear moist.
  • They may become crispy or brittle as they dry up and frequently fall or break away from the plant.
  • Due to tissue damage brought on by the extreme cold, this frequently happens.
  • Freezing temperature injuries are typically not as severe as those caused by fungi or bacteria.
  • Be patient; your cactus plant can regrow its tissue, but it will take some time.
  • After regrowth, the cactus’s black spots should vanish.

Just remember to put your cactus inside in the future when it’s cold.

Even if you cultivate your plant inside, close all windows during the winter. One approach to prevent this when at home and away is to leave your thermostat set between 70- and 80-degrees Fahrenheit.

Related article: Why Is My Cactus Turning White? Reasons and 4 Easy Solutions

How to Heal Black Spots in Cactus?

Now you know the reasons why is my cactus turning black—which is either a fungal infection or a wound from exposure to chilly conditions.

Whatever the reason, it’s not good anyway!

Vast black spots, especially those that cover large areas of the plant, indicate an advanced stage of the illness.

You can still make every effort to keep your cactus alive, even if it might mean that keeping your houseplant will probably take some time and be challenging.

Here are some actions we advise you to take.

1. Keep the Cactus Alone

First and foremost, you need to reposition your cactus.

Fungal and bacterial infections can quickly spread to nearby plants in your indoor garden, whether there are only two or three other plants or dozens.

It would be best to remember that Phyllosticta pad spotting can spread through the wind or water.

Therefore, it is not worth risking your other healthy plants.

While you wait for your sick cactus to recover, isolate it from the rest of your plants in your indoor garden and place it somewhere else.

2. Remove Infected Areas

You must halt the cactus disease in its tracks since it will continue to spread if nothing is done. This step also includes removing the infected cactus parts.

  • Start cutting where the cactus is darkest using pruning scissors or a gardening knife.
  • Ensure your pruning tool is clean before and after cutting the tree down.
  • Dirty tools easily source fungus or germs that could destroy other houseplants.
  • Cut in layers so that you can get deep inside the cactus arms.
  • Cut until you can no longer see the decaying portions as fungal diseases enter shockingly deep.
  • Remember that rot and illness will impact the plant’s interior and exterior.
  • It’s normal for your cactus to appear brown when you cut it apart.
  • It would be best if you kept chopping until no brown or black material appeared on the cactus’ exterior or interior. Only then can you be confident that the plant is in the finest state possible.

Although there might not be much of your cactus left after the cutting, what is left has a far better chance of surviving.

Your cactus might not survive if you leave even the smallest rotting material on it.

3. Change the Soil

Let me inform you that diseases like crown rot begin in the soil and stay there.

Cutting away your cactus parts could help treat a case of crown rot, but if you leave the soil unchanged, the plant will probably develop crown rot again.

Related article: How to Make the Best Potting Mix for Indoor Plants According to the Nature of Your Plants Themselves

Even if you don’t think your cactus has crown rot, replace the soil and add extra nutrients to the fresh potting soil just after a fungal or bacterial disease appears.

4. Repeat the Cut

Although you might think you’re done by cutting the infected parts of your cactus once, your work might still not be finished.

Keep a close eye on your cactus throughout the upcoming days and weeks.

You might have missed a few black and rotten patches even though you removed them all.

Unfortunately, in this situation, the remaining parts of your cactus will probably start to deteriorate once more. If so, perform a second trimming if necessary. To prevent the spread of plant diseases, kindly disinfect your scissors before and after cutting.

After a few days or weeks, if your cactus did not rot, the areas you previously chopped should start to harden and develop a callus.

The cactus should then start to grow again from the cut area.

You can always combine different cactus species to create a super-cactus if you’re unhappy with your barely-there cactus.

To prevent rot or illnesses from spreading to the other arms and components, ensure that all the cacti you are grafting together are healthy.

Watch this video to learn to repot cacti:

Why is my cactus turning black? | HOW TO REPOT A CACTUS LIKE A PRO? | CHOOSING A RIGHT POT – YouTube

What Is Causing My Cactus to Turn Gray?

While you’re looking for why is my cactus turning black, the actual cactus’ problem might be turning grey rather than black or brown.

This grey hue has a metallic sheen on some cacti, resembling silver, but not on others.

It could be somewhat puzzling if your cactus looked different from when you initially brought it home.

Given that some cactus species are meant to turn grey, yours may have reached maturity. Just continue to take care of it and enjoy its distinctive hue, as a grey cactus doesn’t indicate a fungal disease or rot like a brown or black one does!

Or you may have mistaken your cactus for another type of plant.

While purchasing or adopting a new plant, people often believe they are getting a cactus when, in reality, they might not be doing so.

Final Thoughts!

Why is my cactus turning black, you asked? Hope you got the answer!

Look after your cactus, even though it is an easy-growing plant and requires little watering duty. Please pay attention to the unusual spots on the cactus.

It may differ from many flowering plants, but it requires your attention.

The sooner you notice the black cactus spot, the better, and soon it will revive. Now you know how to treat black spots on cacti.

Happy planting!