How to Treat Black Spots on Cactus Plants (An Easy Guide)

By

Black spots on cactus plants are a problem that many people face.

The black spots can cover the entire plant and eventually lead to its death.

Fortunately, there are some easy ways to treat black spots on cacti, so the black spot doesn’t get any worse.

This article will outline these treatments in detail, so you know how to treat black spots on cactus plants with ease!

What Causes Black Spots on Cactus Plants and How to Get Rid of Them

Cactus black spot is a common problem with cacti and succulents. It can affect the plant’s health, appearance, and even its survival if not treated in time.

Black spots on cactus plants are typically caused by fungi or bacteria that infect through wounds in the plant’s skin.

However, black spots may also be caused by chemical damage from fertilizers or soil additives.

Below, you will learn how to identify black spots on cactus plants and the best treatment for this problem.

Fungal Diseases

Fungi most commonly cause black spots on cactus plants.

The fungus can penetrate through any wounds in the skin of a plant, such as when it’s been handled with dirty hands or pruned carelessly with shears that haven’t been sterilized after use.

There isn’t one type of fungus that causes black spots on cactus plants, and usually, there are multiple types of fungi at play.

The most common types of fungus on cactus plants include species of Botryosphaeria, Diplodia, Ramularia, and Cladosporium.

How to Treat Fungal Diseases on Cactus Plants

Fungal diseases are typically highly treatable if caught early.

For smaller infestations, it may be sufficient to trim away the infected parts of the plant with clean shears or a sterilized blade and dispose of them in a sealed bag so that they cannot infect other plants.

You can also use a cotton swab soaked in rubbing alcohol to disinfect the area.

It is best to use a fast-acting fungicide for larger infestations that you can purchase from your local garden store.

Treatments may vary depending on the type of fungus causing the black spots, but a good general course of action is to mix equal parts hydrogen peroxide and water.

Then, spray or paint this solution on infected plant areas using a cotton swab or brush until they are completely saturated.

You can also add one part white vinegar into two parts hydrogen peroxide to reduce any risk of damage to the plant’s skin.

Bacterial Diseases

Black spots on cactus plants may also be caused by a bacterial disease known as Xanthomonas or Pseudomonas, depending on whether they have been introduced through wounds in the plant’s skin or from infected water and soil additives.

Though these types of bacteria are less common than fungi, they are even more challenging to treat.

How to Treat Bacterial Diseases on Cactus Plants

Bacteria typically infect a plant’s vascular system and so require much harsher treatments to be cured.

The most effective treatment for bacterial black spots on cactus plants is a combination of the antibiotics oxytetracycline and streptomycin.

You will need to mix one part of each antibiotic powder with five parts water, then apply this solution using a spray bottle on all infected areas until they are soaked through.

Be sure to cover the whole plant, including healthy areas where the solution may drip.

Leaf Spot Damage

Black spots on cactus plants may also be caused by leaf spot disease, which attacks the plant’s leaves.

These lesions are typically brown and circular with a black border around them, making it easy to identify this common problem.

Though they can appear anywhere on the plant, leaf spot lesions tend to cluster near each other and grow larger as time goes on.

If left untreated, leaf spot disease eventually spreads to the veins of the cactus’ leaves which will cause them to become brittle and fall off easily.

How to Treat Black Spots Caused by Leaf Spot Damage on Cactus Plants

The best way to treat black spots caused by leaf spot is to prune off the affected plant leaves and move the plant into a sunnier area.

This will help prevent new infections while giving it more access to sunlight, which helps promote healthy growth.

Chemical Damage

Black spots on cactus plants are most commonly caused by chemical damage from fertilizers or soil additives that contain metals such as manganese, zinc, and iron.

These chemicals burn through a plant’s skin tissue causing black spot lesions.

However, it is also possible for chemicals to cause black spots on cactus plants indirectly through water runoff.

If the soil has been repeatedly watered with contaminated water, such as from a pool or hot tub, the chemicals may be carried by water droplets onto the plant’s skin.

How to Treat Black Spots Caused by Chemical Damage on Cactus Plants

Treating black spots caused by chemical damage is fairly straightforward.

You will need to spray the plant with a mixture of one part milk and two parts water, which has been boiled for at least five minutes.

This solution should be sprayed on all affected areas until they are thoroughly soaked.

The milk helps neutralize the chemicals that have burned through the skin tissue while also providing nutrients that may help promote new growth in the affected areas.

Sun Damage

Black spots on cactus plants that fungal, bacterial, or chemical agents do not cause may be a sign of sun damage.

In general, any black spot on a cactus plant is an indication that the skin tissue has been burned and should therefore receive extra protection from sunlight for at least one year while it recovers.

How to Treat Black Spots Caused by Sun Damage on Cactus Plants

The best treatment for sun damage is to keep the plant out of direct sunlight while it recovers.

If possible, place the cactus in an area that receives indirect light and water it sparingly, allowing the soil to dry out completely before watering again.

Pest Damage

Cactus plants may also develop black spots on their skin from pest damage.

Though cacti are usually resistant to most pests, they may occasionally be attacked by mealybugs, aphids, scale insects, and spider mites.

How to Treat Black Spots Caused by Pest Damage on Cactus Plants

Pests can spread easily from plant to plant, which means that if your cactus has been infected, it will need to be quarantined for several weeks while the infection is treated.

To kill the pests, you can use Insecticidal Soap which contains fatty acids that disrupt the pests’ cell membranes.

Insecticidal Soap is effective against a wide range of insects but should not be used on any plants in direct sunlight as it may damage them.

You can also use a solution of one tablespoon liquid dish soap with two cups of water for smaller infestations. You will have to repeat this treatment every five days until the problem goes away completely.

Rot

Black spots on cactus plants may be a sign of rot if soft, mushy areas accompany them on the plant’s epidermis that has a foul odor.

Rot is usually caused by fungal or bacterial infections and may also be triggered when cactus plants stay wet for too long.

Root rot can be caused by any type of fungal or bacterial infection that makes its way into the plant’s root system and prevents it from absorbing water properly.

In addition, crown rot is caused by an infection that starts at the roots and works its way up through the lower portions of the cactus plant.

Cacti are typically not very susceptible to rot unless they have been previously injured or infected with pests such as scale insects and mealybugs, which weaken the plant tissue, making them more vulnerable to secondary infections.

How to Treat Black Spots Caused by Rot on Cactus Plants

The best way to treat black spots caused by rot is to ensure that your cactus plant does not stay wet for long periods.

Rot usually happens after the soil has been kept moist, so it’s essential to allow the surface layer of potting mix to dry out before re-watering.

If you suspect crown rot or root rot, it will be necessary to repot the cactus plant in new potting soil that contains some sand and perlite, which helps improve drainage.

If you opt to use a cactus mix, make sure not to pack it too tightly around the roots, as this can suffocate them while making it more difficult for water to reach the root system.

Make sure there are at least two inches of sand and perlite helping the potting mix drain properly.

How To Prevent Black Spots on Cactus Plants

Cacti are very resilient plants, but they are still vulnerable to spider mites, pests, and diseases.

One of the most common problems new cactus owners face is black spots on their plants.

Several different things can cause black spots. However, regardless of what is causing your problem with black spots on cactus plants, there are some ways that you can prevent this issue.

Here are some tips to prevent them from appearing:

  • Only water the plant when it’s dry or just about to be dry.
  • Do not overwater your cactus plant because this will cause rot, and then you’ll have a dead plant on your hands.
  • Ensure that there is enough sunlight coming in through the window where you’re growing your cacti. If they don’t get any light, they won’t grow properly.
  • Make sure to use cactus soil instead of regular potting soil. Cactus soil is specially designed for these plants, and it contains nutrients that are necessary for their growth.
  • Make sure that the temperature of where you keep your cacti is between 70 and 90 degrees Fahrenheit. If it’s cooler or hotter than this, there will be problems with black spots on cactus plants.
  • Keep them away from any drafts because these can also cause damage to them and lead to more problems such as rot and fungus, which will lead to black spots.
  • If you notice that any of your cacti are developing black dots, make sure to prune them off right away. Do not wait for the problem to get worse because it will continue spreading, and then there’s no way that you’ll be able to save your plant anymore.

If you follow all these tips, you should be able to prevent black spots from appearing on your cacti.

Final Thoughts

Black spots on cactus plants are not hard to get rid of, but you should act quickly if you see any black spots on your cactus plants.

If you treat them in time, most of the spots will go away, and they won’t affect the plant’s overall health too much.

However, if you wait too long to deal with these black spots, it can be a bit harder to get rid of them since they spread so quickly.

Taking care of your plant so it stays happy is essential for the health of your plant.

Black spots on cactus plants are not something you want to see, but it’s easy enough to treat them and make sure they don’t come back!