Why Would a Cactus Turn Black?

A cactus on your office table or your room’s windowsill gives a cool look. It’s a fun plant because you don’t have to pay too much attention to its maintenance.

That doesn’t mean you forget to water it for months or keep it indoors 24/7.

Cactus is a resilient succulent. It can survive on its own for a long time. However, it does develop some hard-to-miss problems, such as black spots.

Black isn’t the right cactus color. Any spot on your succulent is a red flag indicating that it is unhealthy.

The spots appear first and then slowly spread, making the cactus appear black. No matter what color the marks are, they are a cause of concern.

They appear mostly due to the environment. This is why you need to acclimate your cactus so that it can adjust to direct sunlight.

To save your succulents, avoiding incidents such as underwatering and overwatering is important.

These actions can lead to your cactus turning soft and squishy, which invites fungus and bacteria to grow on your succulent and lead it to its death. Pests are another reason why your cactus turns black.

Now that you know what’s causing your cactus to die, let’s take a look at diseases and their solutions in detail:

The Cause of Fungus on Your Cactus

The black color on your cactus is because of a fungal infection. What causes it is another story.

All you need to know is that overwatering, underwatering, and too much sunlight can lead to your cactus’s death.

The fungus simply appears in different variants. It can survive for a long time and spreads quickly, even when dormant. 

The fungus, in this case, might look like black spots or mold on the leaves. This is common in all plants and usually happens when you don’t take care of them.

Before you try any remedies or use fungicides to kill the bacteria, you need to know what type of disease you are dealing with to take appropriate action.

The most common cactus type that is susceptible to bacteria is Opuntia, Saguaro, and Agave.

Types of Diseases in Cactus

Scale Insects

Succulents are not a fan of pests. In fact, they hate the little critters because they suck the life out of them, literally.

Some of the most common insects that like to drink cactus sap include scale bugs and mealybugs.

These insects leave behind honeydew, which creates a breeding ground for mold. If you don’t take any steps to remove the mold, it will spread and affect the entire plant.

Bacterial Necrosis 

Bacterial necrosis only attacks certain cactus species, including the prickly pear, saguaro cactus, the cholla, barrel cactus, and organ pipe cactus.

This disease is limited to succulents and only attacks a cactus in its weakened state. Apart from the black spots, a few other signs that tell you your cactus has bacterial necrosis are:

  • The soft and mushy texture
  • A bad smell

Erwinia Bacteria is a disease that got its name from a well-known American plant pathologist called Erwin Frink Smith.

It triggers necrosis in the cactus by entering through a wound in the trunk or branch and spreading bacteria.

As a result, your cactus develops necrotic pockets that kill plant tissues in certain areas. Once the bacteria enter the cactus, it travels through the stem and to the leaves.

Some cactus species, such as the saguaro cactus, try to heal the damage caused by bacterial necrosis. It develops a patch over the wounds that become cork-like in shape and feel.

While the cactus attempts to stop the bacteria from forming further, some organisms get left behind, affecting the other cactus.

The cork-like spots eventually turn black by killing any healthy tissues. The cactus then starts to rot, making it beyond salvageable.

When the cactus is on its last life, the surface tissue starts to crack and releases a dark brown liquid.

Saving your cactus from this disease is possible only if you catch the signs early. The survival rate is 80%, which falls every day the longer the disease is unnoticed.

It’s important to dispose of the plant in a plastic bag and ensure it’s far away from your succulents, or the disease will invade all your plants.

Crown Rot

Crown rot is a fungus born in the soil and travels through it. Since it is advised to focus on your soil when watering a cactus, this disease thrives in a wet environment.

The good news is that cactuses enjoy airy and fast-drying soil. However, this does not mean that your cactus is safe from crown rot. 

The bad news is that crown rot affects all plants, fruits, and vegetables. Crown rot first appears on the soil and is dry in texture.

You might not know that your plant has crown rot until it’s too late when black spots appear on the stem. By this point, your cactus is past saving.

This is why crown rot is the deadliest cactus disease. Once it invades your succulent, it’s impossible to save your plant.

The disease is almost undetectable, and since the rot settles deep into the tissues, no amount of trimming the black spots or replacing the soil will bring back your cactus.

It’s better to throw it away safely so that your other plants are safe.

If your cactus is in its initial growing stage, crown rot will spread faster. Once it reaches the advanced stage and your cactus turns black, there is no saving your succulence.

No matter how resilient adult cactuses are, they too can’t survive crown rot. The moment you see a brown liquid leaking from your cactus, stop trying any remedies to nurse it back to health.

Phyllosticta Pad Spotting 

Phyllosticta pad spotting is a fungal disease that mostly attacks prickly pear. Hence it is also called prickly pear leaf spot.

The disease starts with tiny spots that are full of bacteria. They colonize the cactus pads and cause lesions. If you see the spots on your plant, remove them immediately from other plants because this infection spreads quite fast.

The good news is that this disease is treatable. All you need to do is remove affected plant parts. Any damage from this disease is cosmetic.

It might distort the appearance of your cactus but won’t affect it on a cellular level or kill it.

Phyllosticta concava is commonly called dry rot. The lesions caused by this disease are 2 inches wide. The black spots are a result of pycnidia.

This reproductive structure releases the spores. Later, the black spots fall out and leave scars on your cactus.

This disease is weather dominant, which means that any damage won’t stay for long on your cactus.

Once the weather turns dry and warm, your cactus’s health will improve on its own. However, it would be best if you still inspected it closely to make sure the plant is recovering well.

Treating Fungal Infections

Fungal and bacterial diseases are treatable if you spot the signs early. In the case of bacterial necrosis, the best treatment is to cut off the infected branches with a sterilized and sharp knife.

The bacteria might also be present in the soil. The damage is irreversible if the cactus is covered with black spots from top to bottom.

In the case of phyllosticta pad spotting, it’s possible to revive your cactus by using a fungicide treatment.

The fungicide penetrates the cactus’s tissues and kills the disease-causing fungus. Chemicals in these commercial fungicides are safe and help eradicate pests as well.

Some of the best fungicides for treating phyllosticta pad spotting are:

  • Liquid Copper Fungicide by Southern Ag
  • 3-in-1 Insect, Disease, and Mite Control for Plants by BIOADVANCED
  • Infuse™ Systemic Disease Control Fungicide Concentrate by Bonide
  • Sulfur Plant Fungicide by Bonide
  • Neem Oil RTU by Monterey Lawn Garden
  • Regalia Fungicide

DIY Remedy – Use Neem Oil

In its weakened state, a cactus becomes a buffet for bugs. It’s important to get rid of them as soon as possible.

Since bugs leave behind wounds, the best way to close them is to rub alcohol all over the affected area.

You can then spray Neem oil on the cactus to keep insects away. Do not spritz the cactus with water because this will create a breeding ground for the bacteria.

The strong smell of Neem oil acts as a deterrent, and its anti-fungal properties keep your cactus safe from fungus.

Final Thoughts

In conclusion, fungal infections and bacteria that invade cactuses can damage them from the inside.

While scale insects and phyllosticta pad spotting are treatable, crown rot and bacterial necrosis kill your cactus immediately.

If you spot any signs of the latter two diseases, do not waste your time reviving your cactus. It’s better to remove it from the root and dispose of it along with the soil.

Since the organisms in these bacteria multiply fast, they might infect your entire garden if the cactus is planted outdoor.