8 Reasons for Black Spots on Cactus (Solutions)

Black spots on cactus plants can be alarming, but they don’t always mean your plant is in trouble. From sunburn to disease, black spots on cacti have many different causes, and several easy solutions exist.

In this article, we’ll look at eight of the most common reasons why black spots may appear on your cacti and how to fix them. Whether you’re a seasoned plant enthusiast or just getting into the green thumb hobby, this guide will provide the information to identify and rectify black spots on your cactus.

black spots on cactus

8 Reasons for Black Spots on Cactus

Since a variety of issues can cause black spots on cacti, it’s essential to understand the various reasons why they appear. Here are eight of the most common causes and solutions for black spots on cacti.

1. Fungal Infection

Fungal infections are a common cause of black spots on cacti. These fungi can be caused by too much moisture around the plant or poor airflow in the cactus growing environment. Fungal infections can appear as black spots, yellow spots, white cottony patches, or slimy coatings. Such fungal diseases include bacterial necrosis, crown rot, and phyllosticta pad spotting.

To prevent fungal infections on cactus, keep the plant in a dry area with good air circulation and avoid watering it too frequently. If the cactus already has a fungal infection, treat it with a fungicide formulated especially for cacti, and follow the directions on the label carefully. If the cactus is in a state beyond saving, it is crucial to remove any affected areas and take precautions to prevent the spread of the infection.

cactus with black spots

What does cactus fungus look like?

The cactus fungus can appear in various forms. It can look like black or yellow spots, white cottony patches, or slimy coatings. It can also be identified if you see the stems wilting or falling off the plant.

Fungal lesions on cacti can manifest in various shapes and tones, from round to irregular and from yellow to black. Some may have a corky texture, while others may be weepy and oozing fluid like brown, rusty, or black liquid – indicating severe infection.

The severity of the infection will determine how the lesions appear and the color they take on.

2. Bacterial Infection

Bacterial infections can also be a common cause of black spots on cacti. Bacterial necrosis is the most common type of bacterial infection, and it appears as black spots or streaks on the stems of the cactus. Bacterial infections can be caused by overwatering, poor air circulation, or contaminated soil.

To prevent bacterial necrosis on cacti, ensure that the soil is well-draining and keep the environment dry. If the cactus already has a bacterial infection, treat it with a copper-based fungicide formulated for cacti and follow the directions on the label carefully. If the affected areas are severe, it is vital to remove them to prevent the spread of the infection.

3. Leaf Spot Damage

Leaf spot damage can also cause black spots on cacti. Leaf spot damage is typically caused by environmental factors like too much sun exposure. The cactus may look scorched or have dark spots, which can be caused by direct sunlight. Moreover, nematodes, fungi, or bacteria can cause leaf spot damage. The toxicity of the environment can also be a factor in leaf spot damage.

Leaf spot damage is characterized by yellow to brownish spots on the cactus. In extreme cases, the whole plant may die if the infection spreads. Cacti should be kept in an environment with indirect sun exposure and adequate ventilation to prevent leaf spot damage. It is also important to avoid overwatering the cacti, as this can cause the fungus to spread.

Proper fertilization and regular pruning can also help reduce the risk of leaf spot damage. Additionally, inspecting the cactus regularly and removing any infected parts as soon as possible is vital. Finally, if leaf spot damage is still not cured, applying a fungicide may help to treat the infection.

black spot on cactus pad

4. Crown Rot

Crown rot is a common infection on cacti; it manifests as black spots or yellowing of the stems near the soil line. The fungus responsible for crown rot enters the cactus through wounds and spreads throughout the plant. Crown rot can be caused by overwatering, poor drainage, or contaminated soil.

To prevent crown rot, keep the cactus in an environment with good air circulation and avoid overwatering it. If your cactus already has crown rot, remove any infected areas or parts of the plant to prevent it from spreading. Additionally, fungicides formulated for cacti can be applied to treat crown rot. Always follow the directions on the label carefully when using any fungicide treatment.

What does cactus rot look like?

Cacti suffering from rot typically display signs such as sunken spots, discolored scabs, soft and squishy parts with fruiting bodies present, and various colored dots on the surface of their skin. In severe cases, there may be oozing or leakage from the affected areas.

black spot on cactus crown

5. Frost Damage

Cold temperatures below the freezing point can cause irreparable damage to your cactus. As this desert succulent is naturally accustomed to hot climates, it cannot tolerate frost and may develop black spots that look wet. Over time, as the cactus dries out due to the cold weather, its pads become brittle and begin to fall off, resulting in tissue damage. Unfortunately, frost can severely impact your cactus, and it may take some time to restore it to its healthy state.

To prevent frost damage, keeping the cactus indoors in a warm environment is best. Additionally, you can use frost covers or blankets to shield your cactus from extreme cold conditions. Finally, make sure to monitor your cactus for any signs of frost damage so you can act quickly to reduce the impact.

6. Fertilizer Burn

Fertilizer burn is a condition where the roots of a cactus become damaged due to excessive exposure to fertilizer. This can happen when too much fertilizer is applied to the cactus or when it is not diluted correctly. The symptoms of fertilizer burn include yellowing and wilting of the cactus and brown or black spots on the cactus.

To prevent fertilizer burn, always dilute the fertilizer according to the instructions on the label. Additionally, keep track of when and how often you fertilize your cactus, as this will help ensure that it is not over-fertilized. You can also reduce the risk of fertilizer burn by mixing an equal amount of soil and water when fertilizing the cactus.

Finally, inspect your cactus regularly and remove any damaged parts due to fertilizer burn. If you notice any signs of fertilizer burn, reduce the amount of fertilizer you use and follow the instructions on the label.

7. Sunburn

Cacti grown in nurseries or greenhouses and not used to intense sunlight can suffer from sunburn when suddenly exposed to direct, bright light. This is often indicated by a purplish discoloration of their skin, followed by yellowing and, eventually, rotting accompanied with black spots.

The side effects of overexposure to the sun can damage cacti in the long run and should, therefore, be addressed promptly. Thus, it is crucial to ensure that a sudden shift in bright light exposure for cacti is done gradually. Moreover, any cactus exposed to too much sun should be monitored, and appropriate measures must be taken.

black spot on cactus stem

8. Pest Damage

Various pests, such as mealybugs, aphids, whiteflies, scales, and spider mites, can infect a cactus. These pests can cause damage to the cactus by sucking its sap, leaving behind a sticky residue, and causing discoloration on the cactus. They can also introduce diseases to the cactus and cause further damage.

To protect your cactus from pests, inspect it regularly for any signs of infestation. If you do notice any pests, take immediate action to remove them, either by manually removing them or using an appropriate pesticide treatment. Additionally, you can use natural pest repellants, such as neem oil or other plant-based oils, to keep pests away. Finally, ensure the cactus is kept in a clean environment to reduce the chances of pests infecting it.

How Do You Treat Black Spots on Cactus?

Black sports can be due to one of the mentioned issues and can be treated according to the cause. In cases where black spots are due to frost damage, the best way to restore the cactus is by bringing it indoors or covering it with a blanket during cold weather.

For fertilizer burn, reduce exposure to fertilizer and make sure it is diluted before use. For sunburn, minimize exposure to direct sunlight and increase the amount of shade. Finally, remove any pests manually or use an appropriate pesticide treatment for pest damage. Following these tips, you can ensure that your cactus stays healthy and free from black spots.

Can Cactus Rot Heal Itself?

Yes, in most cases, cactus rot can heal itself. No need to fret over treating cacti wounds—these resilient plants can heal themselves! Move it into a location away from direct sunlight and allow it time to air dry. For larger-sized cacti (with diameters of 5 cm or more), let them sit for several weeks before watering again. However, the best way to prevent cactus rot is to take appropriate action as soon as you notice any signs, such as discoloration or mushy spots.


In conclusion, proper care for cacti is essential to ensure their health and well-being. Different issues can arise that can cause damage to cacti, such as frost damage, fertilizer burn, sunburn, and pest damage. These can be identified by looking out for certain signs and symptoms.

It is essential to take prompt action when these issues arise to prevent further damage. Additionally, it is crucial to keep cacti in a clean and suitable environment to reduce the risk of pest infestation. With proper care, cacti can thrive and be a wonderfully lasting addition to your home.