It looks awful.
Your beautiful healthy cactus has got black spots. And you’re panicking.
What can you do? Don’t worry.
Your cactus is turning black due to overwatering. You need to stop the watering and make sure the soil gets dry. Cactus is a desert plant so it does not need much water. The plant may also turn black due to poor sunlight, low temperatures, lack of nutrients, or excess fertilizer.
I’ll help you find out the exact reason that your cactus is facing this problem. And what you can do to solve and prevent it from happening in the future.
So keep reading.
1. Age of the cactus
A cactus that has grown old can develop black spots on its surface. This is normal and you don’t need to worry about anything bad happening to the plant.
If the spots are due to age, you find they don’t harm the cactus and will remain on the plant.
2. Genetic factors of the cactus
You get certain cactus varieties that are black. So don’t get confused that the plant has a problem. It’s just the natural color of varieties such as ‘Black Prince’, ‘Zwartkop’, ‘Chinese Jade’, ‘Arachnacantha’, and ‘Black Knight’.
You may see these varieties when purchasing the cactus from a nursery or garden center. They will be almost fully black and it’s just their natural color.
3. Overwatering the cactus
Cacti are made for the harsh weather of the desert. So they don’t need watering as often as other plants. They would prefer infrequent watering, as it would encourage them to grow and thrive.
If the plant gets too much water, it can invite fungal diseases such as root rot that will rot the roots and lower part of the cactus, turning it black.
You notice that the rotting part of the cactus smells. It will become soggy and if you dig around the base, you will also see roots having the same problem.
You need to stop watering as soon as you figure out that overwatering is the problem. Allow the soil to completely dry out.
Dig around the roots of the cactus and take it out if possible. If all the roots have turned black and soggy, the plant will die.
If it’s only a few roots with the problem, you can trim those out. The plant the cactus back into the soil or pot.
Make sure that the soil is completely dry before you think about watering the cactus. Typically, you wait at least a couple of weeks before watering it.
You can stick your finger 2-3 inches into the soil and check for moisture. Water the soil only if your finger comes out completely dry with no soil sticking to it.
Overwatering is probably the most common reason that cactus turn black. When a cactus is overwatered, the roots are unable to get the oxygen they need to survive, and the plant will start to die. If you think your cactus has been overwatered, the best thing to do is to let the plant dry out for a few days. Once the roots have had a chance to dry out, the plant should start to recover. – John Dave, Founder, AgroFever.com
4. Lack of sunlight for the cactus
Cacti thrive in desert environments where they can get a lot of direct sunlight. If you place them in a location that has poor light, they may start turning black or brown.
If you move the cactus to a location that gets plenty of sunlight, the plant will slowly recover and the black spots will be gone in some time.
You need to help get more sunlight to the cactus plant. If it’s outdoors in a shaded area, you either need to remove the cause of the shade, such as branches. Or you need to move the plant to a better location.
This is easier if your cactus is growing in a pot. If it’s in the soil, you need to dig it up and transplant it to another location.
When you get yourself a cactus plant, pick a suitable location either indoors or outdoors that gets plenty of sunlight.
Check there are no branches, walls, or fences that can block the sunlight throughout the different seasons.
If your cactus is turning black, it’s possible that it’s getting too much sunlight. Cacti are native to desert environments, where they are exposed to intense sunlight for long periods of time. However, when grown in a domestic setting they might not be able to tolerate the same levels of light. If it is placed in direct sunlight for extended periods of time, it can cause the plant to become sunburned– resulting in blackened areas on the skin of the cactus. – Jeremy Yamaguchi, CEO of Lawn Love
5. Cold damage to the cactus
Cacti prefer warm weather that would be around 70 to 85 degrees to thrive. So, if the temperature drops during the cold season, it may not suit them and their color may turn black because of cold damage.
If the damage is light, the cactus will recover once you move it to a warm location. The black spots should also resolve once the plant recovers. If the damage is heavy, the affected parts are dead and the cactus will probably die.
If you live in a region that gets cold during the winters, you need to move the cactus indoors so it can get the required heat.
When growing the cactus, pick a location that will get good sunlight and stay warm during the growing season. If it gets cold in winter, you can grow the cactus in a pot so it’s easier to move it indoors once the temperature drops.
The two most common reasons why a cactus in your home turns black is cold damage and over watering. In colder climates like here in Michigan, the exposure of moving a plant from a store to your car and to your home in the middle of winter can easily cause cold damage. If you buy a cactus in the winter, you need to wrap it well and move it into your heated car quickly. – Shane Pliska, President, Planterra
6. Disease infecting the cactus
There are several fungal and bacterial diseases that would cause the cactus to turn black. I’ve written some of them below,
Root rot: A fungal disease that attacks cacti’ s root systems, causing them to decay and turn black. This can lead to the plant becoming weak and eventually dying.
Fusarium wilt: This is a fungus that attacks the vascular system of the plant, resulting in wilting and turning black.
Anthracnose: This is a fungal disease that causes dark, sunken lesions on the surface of the plant, which may eventually turn black.
Cactus blight: A bacterial disease that causes dark, water-soaking lesions to form on the plant, which can eventually become black.
Cacti crown rot: This is a fungal disease that affects the top part, or crown, of cacti plants. It causes them to wilt and turn yellowish, and may develop brown spots or sores on their surfaces.
Diseases can be fungal, bacterial, or viral, so you need to figure that out based on the signs and symptoms. You can get help from an expert gardener to find this out.
Once we know the disease, we can also find the potential solution. Often, there is no cure for a bacterial or viral disease and the plant will eventually die.
If it’s a fungal disease, you can move the cactus to a sunny location so that the excess moisture can dry out and help the plant. Organic fungicides specific to the disease are an option to try out, such as neem oil, horticultural oil.
If it’s a fungal or bacterial disease, it might help to trim off the affected parts as early as possible to prevent the spread of the disease.
There’s not much you can do to protect the cactus from a viral or bacterial disease. But sterilize any tools such as pruners that you may use to trim the plant. If there are other cactus or plants infected, move the healthy ones away from them.
You can prevent fungal diseases by avoiding excess moisture on the cactus. Avoid overwatering the plant and only water the soil. Don’t splash water on the foliage.
I prefer to water plants in the morning so any excess water evaporates soon after. If you water in the evening, it can remain on the cactus longer and increase risk of fungal issues.
7. Pest infestation in the cactus
Pests such as aphids, mealybugs, whiteflies, or spider mites suck the sap out of cacti and cause tiny black spots appearing on them.
The solution will depend on the type of pest attacking the cactus plant, so get help from an expert if you’re not sure.
For pests like aphids, mealybugs, whiteflies, and spider mites, my suggestion is to use diluted neem oil spray until they are completely gone.
You just need 2-3 ml of neem oil in 1 quart of water. Mix 1-2 drops of liquid soap so they mix well. Spray it on a small part of the cactus and check after 24 hours to ensure there is no adverse reaction. Then you can spray the entire plant to get rid of the pests.
Keep doing this every 3-4 days until you don’t see the pests any more on the cactus.
Avoid splashing water on the foliage when watering the plant. Humid conditions attract many pests.
If you find your cactus plant is often infested with pests, I would advise spraying the diluted neem oil every couple of weeks as a prevention measure.
It might be because you are giving it too much love! Oftentimes, we unintentionally kill our cactus with kindness, by over-watering it! Taking cactus care to the extreme can actually result in a cactus turning black! As with all plant care, it’s important to not overdo it or deficiency in the cactus’ needs. With a good balance of light, water, and soil you will have a happy and vibrant cactus that won’t need worrying about turning black! – Ren Lenhof, Founder, HouseFur
8. Physical damage to the cactus
Any sort of physical damage can cause black spots on the cactus. This could be due to fall damage or attacks from wildlife or pets.
Check how much damage is caused to the cactus. If the damage is less, the plant will recover. If there is a lot of damage, you can try to trim some parts if possible.
Keep the plant in a location that is away from the reach of wildlife or pets. If it’s in a pot, you can keep it at a height so they can’t reach. Or you can use a wire fence or cage to protect the plant growing outdoors.
9. Nutrient deficiency in the cactus
Your cactus plant will need nutrients like any other plant. If there is a deficiency of some nutrients, it can turn them black. For e.g. if there is iron deficiency, it will start with chlorosis, turning the cactus yellow to brown and finally black as it dies.
Use a cactus specific fertilizer that contains the required nutrients for the plant. If the damage is not too much, the plant will get better.
Follow the instructions on the fertilizer packet so the cactus gets the right amount of nutrients and at the right time.
To prevent nutrient deficiency, add the cactus specific fertilizer as per the schedule mentioned by the manufacturer.
It’s also good to add a little compost every month that can give a boost of nutrients to the soil.
10. Over-fertilization of the cactus
While cactus needs nutrients, it does not need too much of them. Cacti thrive in harsh, desert environments with limited supply. If you provide too much fertilizer, it can cause toxicity, where the cactus suffers chlorosis and eventually turns black as it dies.
First thing would be to stop adding any fertilizer to the soil and let the plant recover. You can water the soil well so the excess fertilizer can leach out from it.
Always follow the instructions on the fertilizer packet on how much you need to use and how often. This goes for whether you use an organic or synthetic fertilizer.