Finding black spots on your Cactus can be very concerning, as black spots tend to be a symptom of larger problems. Cacti tend to have a reputation for being beginner-friendly houseplants. Still, they aren’t immune to problems, including black spots on their foliage and stems, and you should address these problems immediately.
The most common causes of black spots on your Cactus are fungal and bacterial issues due to overwatering or exposure to disease. However, your Cactus may also develop black spots due to leaf rot or chemical burns.
Once you find out what is causing the black spots, you should act quickly to save your Cactus before the black parts spread to the rest of the plant.
Cactus Black Spots
Black spots on your Cactus indicate that the tissue is dead or dying. In addition, the key to finding success in treating black spots on your Cactus will depend on how quickly you caught the problem, as Cacti with more dark spots have a poorer chance of recovery.
Fungal and Bacterial Issues
There are a few reasons why you might have fungal or bacterial problems with your Cactus.
- Pruning: If you prune your Cactus with unsterilized instruments, you could introduce bacteria into the wound, which can evolve into a fungal or bacterial problem that will create black spots.
- Other Physical Injuries: If your Cactus has fallen over or been knocked over, producing an open wound of some kind, it may be more susceptible to bacteria or fungal spores.
- Watering: If you give your Cactus water too frequently, you may unknowingly cause some root rot. Root rot occurs when the roots sit in wet soil for too long and begin to break down. This can expose your Cactus to a host of fungal and bacterial problems.
- No drainage: If your Cactus is in a pot with insufficient drainage or even a densely packed soil mix, this will prevent excess water from draining away from the soil. Not only does this put your Cactus at risk for overwatering and root rot, but it also allows fungus and bacteria to sit in the pot rather than being flushed out with water.
- Exposure to Elements: If you keep your Cactus outdoors seasonally or all year long, it may become exposed to certain fungi or bacteria from wind or rain. When brought inside, that fungus may spread to other plants.
Fungus Causing Black Spots
Fungus, when left untreated, can spread and kill your Cactus. The black spots caused by fungal infections are often mistaken for rot when they are the fungus consuming the dead tissue in the plant. While there are many different types of fungus, the two common types of fungus that may cause black spots to appear on your Cactus are:
This is a type of fungus that has a rusty copper ring spot with a dark center. If left untreated, this can evolve into dry rot patches. These black fungal patches will grow until they consume the entire plant, so it’s best to act immediately if you suspect your Cactus has diplodia fungus.
This fungus is also known as scabby canker disease and is common in Opuntia varieties. The appearance of botryosphaeria is precisely as the nickname suggests, and it’s the appearance of scabby sores on your Cactus. Like diplodia, if you suspect your Cactus has this fungus, you will have to act quickly to prevent it from spreading.
How Do You Treat Cactus Fungus?
There are a few ways that you can treat your Cactus for fungus problems, including home remedies such as baking soda and peroxide or apple cider vinegar treatments. Still, there are two methods of treatment that are the most effective.
The first option is to prune away the infected parts of the Cactus. This may not be possible depending on the variety of Cactus you have, but it is the best way to ensure that the spores do not travel to other parts of the plant or other plants in your collection.
The second option is using a high-quality fungicide solution. You may have to repeat this treatment several times to ensure that the fungus is not spreading. It’s also vital that you isolate any plants dealing with fungus problems as sometimes the spores can become airborne and infect other plants nearby.
What is the Best Fungicide for Cactus?
Your best bet for choosing an effective fungicide for your Cactus is to read the labels. Then, select a type of fungicide that will treat the specific type of fungus affecting your Cactus. Occasionally, you can find various fungicides that help treat cacti, which would also be a great option.
Bacteria Causing Black Spots
Cactus Black Tips
If you spray down your Cactus with water or even have hanging plants above that drain water down onto your Cactus, it can cause black spots to appear on the leaves and tips, especially where water has pooled on your Cactus.
In the wild, Cacti grow in warm, dry conditions, and while they do receive soaking rains, the excess water will evaporate reasonably quickly and will not linger on your Cactus. However, depending on your conditions, the water may not evaporate soon enough, leaving too much moisture on the foliage and creating black spots.
Bacteria resulting from root rot can cause black spots on your Cactus. This happens when your Cactus’ roots sit in wet soil for too long, causing the roots to rot and become more exposed to bacteria.
What Does An Overwatered Cactus Look Like?
If your Cactus is overwatered, black spots may appear, and if the root rot is severe, it may progress up the stem and crown. If you notice these symptoms, you should first check the soil. If the soil is completely saturated, gently remove your Cactus from its pot and inspect the roots. If the roots are dark, slimy, or squishy, then the Cactus has been overwatered, and the roots are starting to rot away.
How Often Should You Water a Cactus?
You should only water your Cactus when the soil has completely dried out, and often you can wait a little longer than that, thanks to how Cactus have been able to adapt to survive in desert-like conditions.
Cactus can withstand long drought periods depending on the amount of light your Cactus receives. If your Cactus is getting tons of sunlight during the growing season (or receiving artificial grow lights for 8-10 hours a day) and is actively growing, you can water it a bit more frequently. Depending on your variety of Cactus, you may notice signs of puckering or leaves starting to soften slightly, indicating that it is ready for a drink.
If your Cactus isn’t getting very much light, especially during the winter, you can withhold water for even more extended periods. This is because, in most cases, Cacti will go into dormancy in the winter and produce no new growth and, therefore, will not need water.
How to Fix Root Rot
Fixing root rot can be tricky but not impossible, especially if you catch it in its early stages. First, you will want to cut away any rotted roots using a sterile knife or pruning shears. Then, if there are still plenty of healthy roots left over and no damage to the stem or leaves, you can re-pot your Cactus in a well-draining soil mix.
If the root rot is severe, you may want to place the roots in distilled water and place your Cactus in bright light. You can also use a rooting hormone and re-pot your Cactus into the soil if using water isn’t an option.
If the rot has progressed up to the stem and crown of the Cactus, this means that the damage is quite severe and may not be reversible. If your particular variety of Cactus has leaves or sectoral portions that can be propagated, you may want to consider starting over.
How to Prevent Black Spots From Root Rot in Cacti
Growing a happy and healthy Cactus means that you give them the conditions they thrive in, where they grow naturally. To prevent root rot and bacterial problems causing black spots in the future, here are a few tips:
- Use a chunky soil mixture to drain any excess water away.
- Clay pots like terracotta work best for Cacti as they will leech any excess moisture away from the soil.
- Water thoroughly when it’s time to water. Giving your Cactus a small splash of water will not encourage strong root development, making them more susceptible to overwatering. This may sound surprising, but as long as your Cactus has well-draining soil and pot, you should mimic the soaking desert rains and thoroughly water it when it is ready for a drink.
Fertilizer and Chemical Burns
Sometimes black spots can occur from chemical burns, including using a fertilizer that isn’t sufficiently diluted or fertilizing during the winter. Chemical burns can also happen if you have recently treated your Cactus for pests using neem oil or insecticide solutions and immediately exposed your Cactus to the sun.
An easy way to prevent this is to make sure that your fertilizer is diluted accordingly (you can dilute the fertilizer, even more, to be safe) and treat your Cactus for pests in the evening so the chemicals aren’t getting direct sun exposure.
Will Black Spots on a Cactus Go Away?
Unfortunately, if your Cactus has black spots, the damage to the leaf is permanent and will not heal. However, if you catch the black spots early and treat them right away, you can prevent more black spots from forming and damaging the rest of your Cactus. It will also ensure that any new growth will be healthy and free of black spots.
If your Cactus has started to develop black spots, finding out the cause for the spots and acting quickly is the best way to prevent the problem from getting worse or taking over your Cactus entirely. The most common causes of black spots on your Cactus are fungal and bacterial issues due to overwatering or exposure to disease.
The two most common fungus that cause black spots are diplodia and botryosphaeria. To rid your Cactus of fungus, you must use home remedies like apple cider vinegar or a high quality fungicide. Bacteria from excess watering can also plague your Cactus and cause black spots and root rot. To help your Cactus recover from root rot, you must cut away the infected roots and consider repotting your plant in new soil.
While once your Cactus foliage is damaged, it is no longer reversible, but you can still save the rest of your plant and ensure that new growth going forward is healthy and robust.