Why Does My Cactus Have Black Spots? [What To Do Now]

The appearance of black spots is common in cacti and succulents, especially in the USA. However, if left untreated, they can jeopardize your cactus’s health and cause it to die. But why do they appear in the first place? Read on to discover the reasons cactus plants develop black spots.

Table of Contents

Fungal Infections

The fungus can penetrate through a plant’s wounds when handled with dirty hands or pruned with unsterilized shears.

Sadly, there isn’t a single type of fungus, but various fungi at play. The most common types of fungus in cactus plants are Cladosporium, Diplodia, Botryosphaeria, and Ramularia.

Treatment

Use sterilized blades and shears to cut away the infected parts for smaller infections. The infected cutters and other tools must then be disinfected before use on other plants. This helps prevent the spread of fungal disease.

You can also clean your cactus with a cotton swab drenched in isopropyl alcohol while the best remedy is a high-quality fungicide from the local garden store.

However, you can also mix equal parts water and hydrogen peroxide to make your fungicide. Once you have your fungicide of choice, you can spray the solution onto the infected cactus plant.

Another option is to mix one part vinegar and two parts hydrogen peroxide to protect the plant’s skin from further fungal infections.

Bacterial Disease

Bacteria affect a plant’s vascular system, and the way to deal with bacterial infections is quite harsh for the plant. Two common bacterial diseases in cactus plants are Xanthomonas and Pseudomonas.

Both of these diseases are inflicted through wounds. These diseases are more dangerous than fungi, making them more difficult to treat.

Treatment

The best way to treat bacterial diseases in cacti is by using a mixture of antibiotics, streptomycin, and oxytetracycline.

Mix one part of each antibiotic with five parts of water. Fill the concoction in a spray bottle, and apply it to the affected areas. Make sure to cover the entire plant to prevent the mixture from dripping.

It is better to quarantine the plant until the black spots start to lessen, or the disease can spread to other plants.

Leaf Spot Damage

Leaf spot damage is the formation of brown lesions on your cactus’s leaves. A black border surrounds these brown marks, and they can appear anywhere on your plant.

These spots cluster one another, and they grow bigger with time. If left untreated, the lesions can spread to the roots, eventually causing the plant to die.

Treatment

The best way to fight leaf spot damage is by pruning the damaged areas and moving the plant to a sunnier location.

Doing so will prevent further damage, and sunlight will allow the plant to grow while recovering from the disease.

Chemical Damage

Soil additives and fertilizers contain metals like manganese, iron, and zinc. These metals can cause chemical damage to the cactus plant.

They can burn through the skin tissue of a plant, causing black spots. Chemical damage can also happen to cacti because of water runoff.

If the plant is repeatedly watered using contaminated water from tubs and pools, the water droplets can get absorbed into the plant’s skin, which results in chemical damage.

Treatment

Mix one part of the milk in two parts of water in a spray bottle. You should boil the water for three to five minutes before making the mixture.

Next, spray the affected area with the milk and water solution, and keep spraying until the plant is adequately soaked.

Milk neutralizes the chemical that burns through the plant’s skin while allowing the plant to grow properly, especially in damaged areas.

Pest Damage

Like any other plant in the USA, the humble cactus is also prone to pest damage. The most common pests harming your cactus are mealybugs, scales, and spider mites.

These bugs feed on the plant’s sap, which results in malnutrition, and a common symptom is the formation of black spots on the plant.

Treatment

Pests can quickly move from one plant to another. What you need to do if you suspect a pest infestation is quarantine your cactus.

Insecticidal soaps help deal with a pest infestation in plants. The fatty acids contained within these soaps tend to disrupt the cell membrane of these pests, thus preventing them from doing further harm.

However, you shouldn’t use insecticidal soaps in direct sunlight or they will damage your plants. You can also make your insecticide by mixing one tablespoon of liquid dishwashing soap in two cups of water.

Spray the affected areas with this insecticide, and repeat the process after three to five days if the problem stays.

Rot

Sometimes, along with black spots, you notice your plant turning mushy. Your plants will also give off a foul odor. This means the cactus plant has started rotting.

Rot occurs when a cactus plant remains wet or moist for too long. It is also a product of fungal and bacterial diseases.

The rot spreads across the plant and reaches the roots, making it difficult for the roots to absorb water.

Rotting is rare in cacti unless they have had a problem with fungus and bacteria in the past. Pets and bacteria weaken the plant’s tissue, thus making them more vulnerable to secondary infections.

Treatment

First, ensure your cactus doesn’t remain wet or moist for too long. As mentioned above, rot usually occurs when the soil is allowed to stay wet for too long. Therefore, letting the soil dry completely before watering the plant is better.

If you notice root or crown rot, repot the plant right away. Add some perlite to give your plant ample drainage along with the new soil.

The layer of sand and perlite should be two inches thick to allow the potting mix to drain properly, thus keeping the plant dry.

Ways to Prevent Rot in Cactus Plants

Regardless of what’s causing these black spots in your cactus plant, here are some easy ways to prevent black spots in cactus plants.

Water the plant only after the soil has dried completely or is about to dry.
Do not overwater the cactus, or it will cause rot that will eventually kill off your plant.
Instead of using regular potting soil, use special cactus soil that you can buy in various shops in the USA. This soil has all the nutrients a cactus plant needs to grow healthily.
The water temperature should be between 70 and 90 degrees Fahrenheit. Anything outside this range will damage your cactus.
Keep your cactus away from any drafts. Drafts can cause fungal infections and rot, thus resulting in black spots.
Prune the plant immediately if you notice black spots appearing. If these spots are left to stay, they will soon damage the entire plant.

Final Word

Black spots in cactus plants aren’t uncommon and are perfectly treatable. As a devout plant lover, your job is to stay vigilant and closely watch your plant. The moment you see black spots, sense messiness, or smell something foul near your plant, use the tips mentioned above to save your plant right away.