Cacti are mostly known to be a rich green color, but they can also come in pink, yellow, orange or red, depending on the species. A cactus that has turned black, however, may signal a serious issue that should be addressed right away.
If your cactus has turned black, the most common culprit is often a bacterial or fungal infection. Often, such infections are irreversible and there is almost nothing you can do to save your plant.
However, if you are not ready to let your cactus go, you may still be able to save it, especially if you caught the disease in its early stages. Here, we will explain the most likely reasons your cactus has turned black, as well as what you can do to revive it.
Why has my cactus turned black
Root rot due to too much water
Healthy roots matter a lot to the health of your cactus. If the roots are damaged, it becomes difficult for the plant to absorb all the nutrients it needs to survive. Unfortunately, it is all too easy for a cactus’ roots to rot when it receives too much water.
Succulents like cacti are designed to thrive in the desert, where access to water is very limited. That is why these plants do not like to be watered very often. When the soil is too wet or soggy, then the roots will start to rot and cause your plant to turn soft and mushy. Soon, your cactus will struggle to absorb proper nutrition and oxygen, causing it to gradually turn black.
If overwatering is not the issue, then you may need to check the quality of your soil mix. In general, succulents prefer airy and well-draining soil. If your soil is too dense or has become compacted, it will be difficult for the water to drain and the soil will become soggy.
A container with no drainage holes can also hold too much water and soak the roots. Standing water, for any reason, is bad for succulents and can have the same effect as overwatering.
Drastic changes in temperature
Succulents are highly resilient and can survive unattended for a certain period of time. However, your cactus will not thrive in extremely cold climates, since it is more attuned to hot weather areas. Exposing it to such cold even for a few hours may cause it to start turning black.
The reason for this is that the plant’s surface tissue is highly sensitive to cold temperatures. This drastic drop in temperature will slowly kill the plant tissue, hence the appearance of black spots.
Scale Insects
Pests, in general, are not great for succulents, particularly scale bugs as they can wreak havoc on your cactus. This type of pest causes damage to your plant by leaving behind a honeydew-like substance that can become a breeding ground for black sooty mold. If not dealt with right away, the mold can spread rapidly to the body of your cactus and turn it black.
Fortunately, there are effective ways to stop these pests from harming your plant. These solutions are discussed in detail in the next sections.
Crown rot
Crown rot on a cactus can be as dangerous as root rot. It is caused by a type of soil-borne fungus that can affect not just cacti but almost any type of plant, including trees, vegetable crops, and shrubs. Heavy and moist soil provides favorable conditions for this fungus to thrive, and although cacti prefer dry and airy soil, they are not totally immune to the disease.
The infection is almost always subtle in the beginning. A healthy-looking cactus may already harbor the infection undetected. The issue only becomes obvious when the cactus starts turning black, at which point the damage is often already too much.
Unfortunately, there is almost nothing you can do to save your cactus once it has caught the disease. You can try pruning the affected stems or transferring the cactus to fresh soil, but this is not always a guarantee that your plant will survive.
Bacterial necrosis
Bacterial-related diseases may also be responsible for turning your cactus black. Just like fungal disease, a bacterial infection can go unnoticed for some time until the damage is too far gone, manifesting as tissue discoloration on your plant.
Bacterial necrosis is one particular type of bacterial disease that only affects certain species of cactus, namely the cholla, barrel cactus, saguaro cactus, organ pipe cactus, and the prickly pear. So, if you are growing other houseplants, you need not worry about it spreading and can focus your attention on your cacti.
So what causes bacterial necrosis? The Erwinia species of bacteria trigger the onset of the disease, which enters the plant through branch and trunk wounds, which later develop into necrotic pockets. These pockets are composed of dead plant tissue and provide a breeding ground for the infection to spread throughout the cactus.
Some cacti, like the saguaro cactus, can develop cork-like, patchy spots on their skin in an attempt to heal themselves. However, these corky areas can further damage any healthy tissue remaining on the cactus, turning it black. This rotting tissue may also crack and release a dark brown fluid.
Fortunately, bacterial necrosis is treatable. The earlier you catch the disease, the higher the chance your cactus will survive.
Phyllosticta pad spotting
Phyllosticta pad spotting is a type of fungal disease that affects not only the cactus but also other common houseplants like orchids, aloes, and agaves. Your plants can become infected by these pathogens via air or water.
Once the infection starts, it can take at least six weeks to see visible symptoms. The disease will not make the cactus turn black; instead, it will develop purplish circular spots. If untreated, these spots will then transform into diamond shapes. The lesions will continue to grow until they are big enough to cover the entire cactus, making it appear black.
What you can do to save your cactus
Despite being highly resilient plants, cacti can still be susceptible to many issues, which may be due to environmental conditions or a lack of proper care. A cactus that has turned black is an obvious sign that your plant is crying for help. Depending on the problem, you might still be able to return your cactus to its former health with the appropriate solutions.
Do not overwater you plant
Overwatering is something that most succulents hate, as it cripples the plant’s root system, causing dehydration. Waterlogged soil not only hurts the roots but also creates a favorable environment for bacteria and fungi to grow.
So, if your cactus has turned black, it is best to check first for rotten roots. If that is the case, trim off the affected roots and transfer the cactus to a new pot with sufficient drainage holes. Also, make sure that the soil mix is light and well-draining.
Avoid underwatering
Cacti are not totally zero-maintenance plants. Simply forgetting to water your plant for extended periods can cause the soil to become dry and crumbly. This will slowly kill the roots and prevent the plant from absorbing nutrients.
Before the soil completely dries out, make sure to water it to prevent dehydration. If the soil is too dry, give your plant less water but more regularly and see if it responds. If it does not recover, it is better to repot it in fresh soil.
Keep the cactus pads dry
Some owners may be tempted to mist their cacti as a way to hydrate their plants. However, this practice can create a perfect breeding ground for bacteria and fungi. Soil-borne fungi can spread to the stems and pads, causing the cactus to weaken and turn black.
To protect your plant from various diseases, make sure that the stem and pads are dry. Do not place the cactus near a water source or in areas with too much humidity.
Keep your cactus warm during winter
If you are growing your cactus outdoors, never forget to take it inside before the cold arrives. A sudden drop in temperature can injure the skin of the cactus, even if it is exposed for just a few hours.
If the cactus has already started turning black due to frost damage, place it in a warm room with a temperature of 75 to 80 degrees Fahrenheit. Monitor it for a couple of days, and you should see the black spots start to disappear on their own.
Cut down on fertilizer
Too much fertilizer, especially those rich in nitrogen, can be bad for your cactus. Never be tempted to over-fertilize, especially during winter when the cactus is dormant. Just use a small amount to boost your plant during its growing season.
Trim off any infected areas
Bacterial and fungal diseases, such as bacterial necrosis and crown rot, may be difficult to treat, especially if they have already spread throughout the body of your cactus.
But there may still be hope for your cactus if the lesions are only covering a small area. Simply cut away the infected parts with a sterilized knife. Make sure to remove more than what appears to be infected, to avoid any reinfection. Trimming off the infected parts is also highly recommended before applying any treatments.
Monitor your plant to see if it responds well. If not, you may need to accept the fate of your cactus and start all over with fresh cuttings.
Treat fungal and bacterial infections
Bacterial and fungal diseases are treatable as long as you can catch them in their early stages.
For bacterial necrosis, the only treatment may be to trim off the infected branches using a sharp, sterilized knife. However, the bacteria may still be present in the soil, so there is a chance that you might not save your plant. If your cactus has been black for some time, the damage might already be irreversible.
If fungal disease is the culprit, a fungicide treatment may be able to revive your cactus. Fungal treatments work wonders by penetrating deep into the plant tissue to kill the disease-causing fungi. These chemicals are mostly safe and may even help eradicate certain pests on the plant, too.
Some of the most effective fungicide sprays and treatments include:
- Southern Ag – Liquid Copper Fungicide – Fungicide
- BIOADVANCED 701287A 3-in-1 Insect, Disease, and Mite Control for Plants, 32-Ounce, Ready-to-Spray
- Bonide (BND148) – Infuse Systemic Disease Control, Fungicide Concentrate (16 oz)
Use neem oil to remove pests
If you have noticed bugs feasting on your weakened cactus, you will want to get rid of them as soon as possible to prevent further damage. The best remedy for these pests is to apply to rub alcohol on the affected areas. Dip a cotton bud or q-tip into the alcohol and gently wipe off any bugs you see on the branches. Then spray the cactus with neem oil once weekly at night.
Lastly, do not forget to keep your infected cacti away from other plants to prevent the pests from spreading. With the right treatments, your cactus should be back to its normal, healthy condition in no time.
Final thoughts
A cactus turning black is the plant’s way of crying for help. If caught early, you might still be able to do something to revive the plant and prevent any further damage.
Hopefully, all of the solutions we have presented will help you save your cactus and enjoy it for a longer time. Prevention is still better than cure, though, so try to make sure that your cactus receives the appropriate care and attention.
Image: istockphoto.com / photonewman