One of the widely known indoor plants is the cactus. Aside from being unique in its appearance, this ornamental requires minimum care and is adaptable to extreme conditions such as drought and hot temperatures.
But despite its seemingly hardy characteristics, cacti do get serious problems such as black spots.
How to Get Rid of Black Spots on Cactus?
To get rid of black spots in cacti, you have to identify the real causes of the problem. From there, you can draw solutions to prevent further damage.
Quick solutions to this problem would range from adjusting light, regulating water and fertilizer, lowering humidity, using protection against frost, applying biological, mechanical, and chemical control to combat pests and diseases.
You need to follow specific steps to avoid those ugly black spots.
What Do Black Spots on a Cactus Mean?
When black spots start to develop in your cactus, that’s an obvious sign of a problem. Your plant is certainly under stress possibly caused by a lot of factors, either biotic or abiotic.
The unfavorable condition affects the normal physiology of your cactus and all these changes reflect on its outside appearance.
Causes of Black Spots on Cactus
Abiotic factors are causes of black spot problems that are non-living. In cacti, this includes excess light/scorching, high humidity, watering problems, fertilizer problems, and freeze damage.
On the other hand, biotic factors are those living organisms that cause problems in your cactus plant. It includes pests/insects, fungi, and bacteria.
We will discuss each factor further in the following sections.
You’re probably wondering how is it possible for cacti to develop sunburn given that it thrives in the deserts.
While it’s true that cactus love bright, sunny conditions, there are those that were grown in nurseries and greenhouses where light is filtered. It means that they’re acclimated to such a condition while growing.
When you try to suddenly move them under direct, bright light, they’ll find trouble adapting to it. As a result, your cactus will get a sunburn and turn purple.
Cacti that suffer from scorching turn yellow and in the later stage, it will start rotting, thus the black spots appear.
How to Fix
Put Your Cactus Back to A Shaded Place
Cactus showing signs of sunburn should be brought back to its original place to avoid further damage. The message is clearly saying that it’s not ready for abrupt changes when it comes to light exposure.
If you brought it from nurseries, mimic the environment there and create that condition within your home as close as possible. That will prevent the cactus from getting shocked by extreme light.
Transition The Cactus To Brighter Light Little By Little
Your cactus needs time to adjust and this adjustment period takes time. This means that you have to be patient in the transition period.
Expose your cactus to brighter light little by little. This will give them the chance to gradually adapt to a new environment.
Cacti naturally thrive in arid places where they’re accustomed to low humidity. They don’t need that much moisture because of their specialized stems where they store enough water.
Besides, they have a unique pathway for photosynthesis known as Crassulacean Acid Metabolism (CAM) which enables them to conserve water. (Source: Arizona State University)
With such, cacti can survive for a longer period even if you don’t give them water. It’s logical to think that they won’t benefit from extra moisture in the air.
A highly humid environment is actually a threat because it triggers the growth of pathogens that can potentially harm your cacti.
With the presence of bacteria and fungi, diseases will spread and infect cacti. These diseases are the reason why your plant is developing black spots.
How to Fix
Transfer Your Cactus to A New Location
This location should be less humid than its current environment. Avoid the kitchen and the shower room because these are highly humid places. If you place the cactus near a window, make sure to move it during the night to avoid cold drafts.
Run The Exhaust Fan.
If the humidity level gets too high, you may need the help of your exhaust fan to pull the moisture out of your room. This is more convenient to do when you have a lot of cacti and succulents at home.
Keep Them Away from Humidifiers.
Some of your indoor plants may love the extra moisture but your cactus won’t. You have to keep them apart.
Overwatering is the number one killer of cactus. No matter how informed people get about cacti not needing a lot of water, many still fall in the temptation. Water is both a friend and an enemy to plants depending on how much you give to them.
In the case of cactus, too much water will cause root rot and we know what happens next. The plant suffers from not receiving enough nutrients and water due to damaged roots. Cacti that are severely affected by this will slowly die and those black spots are signs of decay.
How to Fix
Drain The Soil Immediately.
Check the soil for signs of stagnant water and drain it out quickly. Poke the holes of the container to remove the blockage and allow air to flow.
Remove The Cactus from The Pot for Air Drying.
If the soil is very wet and soggy, you may consider removing the cactus from its pot for a quick fix. Set the cactus aside and let the roots dry as well as the soil.
Once dried, you can put it back in the same container but make sure to remove the damaged roots. Do not water the pot after this to allow the roots to have some sort of rest.
Skip Watering for Days Or Even Weeks.
Cacti that has suffered from overwatering should be spared from the same trouble. At this point, you can let it sit for a longer period without water until it gets back in shape.
Too much fertilizer can really bring serious harm to any plant including cactus. You’ve got to be extra careful with fertilizer application in cactus especially because it doesn’t require that much.
According to the University of Minnesota Extension, cactus and succulents will need fertilizer only once or twice a year. Even during these times, the concentration of your fertilizer should be reduced to half.
How to Fix
Leach Excess Fertilizer with Water
Too much fertilizer can result in the salt build-up in the soil. These salts are soluble so you can use water to leach it out.
Continuously pour in water into the soil until it flows out of the bottom. You can do this repeatedly every 2 to 3 hours. (Source: Penn State Extension)
Skip The Next Fertilizer Application Schedule
If your cactus has suffered from overfertilization, it would be wise to refrain from adding fertilizers in the coming days.
Your plant surely needs time to recover from the damage. Aside from that, the soil will also have a rest period.
Always Dilute The Fertilizer Before Use
Diluting the fertilizer helps in lessening the strength of the concentration. Since potted plants have limited soil, it’s very easy to saturate the medium with water and nutrients.
You can dilute the fertilizer to half or even a quarter of the original strength. It’s already sufficient to supply the needs of your cactus since it’s not considered as a heavy feeder.
Some cacti species are frost-tolerant but some are not. You have to know before buying if the particular species you own is hardy during winter. If not, then it’s susceptible to getting freeze damage.
Extremely cold temperatures are an enemy of most cacti species especially if it’s exposed for a long period. The exposed parts turn black which eventually dries out and becomes crisp.
How to Fix
Place Your Cactus in Areas Where Heat Is Radiated
It’s like placing yourself near a fireplace when it’s winter. Your cactus may not necessarily need a literal fireplace, but it does need extra heat for warmth.
Provide Insulators like Cotton Sheets
You can use this as a cover when the temperature is extremely low especially at night. Be sure to cover it completely to avoid any open space through which cold air could pass through.
Use Artificial Sources of Light to Add Extra Heat
Light brings life to your cactus, especially during the cold seasons. LED bulbs not only compensate for the lack of light during winter, but it also provides additional warmth that would help dissipate the coolness.
Just maintain enough space (12 inches at least) between the light and your cactus to avoid scorching.
The presence of insects may also be a reason why your cactus is developing black spots. These pests would create damage by feeding on certain portions of the cactus, leaving lesions that turn yellow, brown, or black.
The common pests in cacti are the cactus longhorn beetle (Moneilema gigas), cochineal scale (Dactylopius coccus), plant bugs, coccid scale, soft scale, and mites. (Source: University of Arizona College of Agriculture and Life Sciences)
How to Fix
Remove The Pests Manually Once Spotted
You can control most of these insects manually. All you need is to be vigilant and extra patient in picking out any foreign organism you see roaming around your cactus.
Spray Water to Wash off The Insects
If you don’t want to pick them one by one, using water spray is a better option. The pressure that comes from spraying would be enough to wash the pests off the surface.
Just make sure to dry the plant after this. We know excessive moisture is an enemy of cactus.
Apply Insecticidal Soap
If these pests get a little persistent and keep coming back, you can apply insecticidal soap to completely kill them off. You just have to dilute it in water and spray it directly to these insects.
You may need to do this repeatedly in order to completely get rid of these insects.
Leaf Spot Diseases
This is also known as the Phyllosticta pad spot because it’s caused by a fungus Phyllosticta. They appear as black lesions on the pads of prickly pear cacti.
Once this fungus gets in touch with the tissues, they produce pycnidia which are small black reproductive structures.
How to Fix
Isolate The Diseased Cactus
You need to separate healthy ones from plant that needs treatment. The reason is self-explanatory. We don’t want any disease to spread like wildfire.
Remove Infected Parts
If the extent of damage is minimal, you can remove only the infected portions. Discard it properly. However, if the infection is severe, you have to discard the whole plant.
Expose Your Cactus under Bright, Direct Sun
The warm temperature will help the pads dry. This will help mitigate the action of fungi by weakening them. The goal is to dry the pads so that the fungi will become inactive if not dead.
Though we observe it less often, cactus also suffers from fungal diseases. Some fungi destroy a specific host while others infect various species.
Fungal diseases develop when the cactus is exposed to prolonged cold and wet conditions. Spores of fungi develop during this favorable time.
According to the Texas Plant Disease Handbook, there are five major fungi species that can infect your cactus. These are the following:
- Colletotrichum (Gloeosporium) spp.
This fungus causes the cactus anthracnose disease which appears as moist, brown spots at the surface of your cactus. The spots would look like rotten portions that are small at first but enlarges as time passes by.
In severe cases, these brown spots can turn from light to dark black. Removal of the infected plant is necessary to prevent the spread of disease.
- Stevensea (Diplotheca) wrightii
This fungus specifically targets Opuntia and is the culprit behind the charcoal spot disease. Spots appear as large as a quarter of an inch and are surrounded by a ring.
Charcoal spots tend to enlarge as well through time. Once the cactus is infected, you have to get rid of it for good.
- Phyllosticta concava and Mycosphaerella spp.
Once these fungi infect the plant, they cause dry rot. The spots started showing as small black, circular dots but later developed to quite big sizes, one to two inches in diameter.
Callus tissues also tend to develop on the infected parts. You have no choice but to discard the plant as well.
- Hendersonia opuntiae
This fungus mostly targets Opuntia (again) and causes scorch or sunscald. Affected portions show reddish-brown to black spots which grow larger in time.
Infected cacti eventually dies because controlling this disease is quite difficult. You have to dispose of the plant too.
- Phymatotrichum omnivorum
This is another fungus that causes disease in cacti. What is devastating about it is that infected plants experience immediate death no matter how healthy they seem.
Yellowing and bronzing of leaves are the common symptoms followed by wilting. Some wilted parts may also show black portions.
- Drechslera cactivorum
Stem rot easily kills your cactus and it’s caused by this fungus. The rotten portions can appear either from the basal or apical portions of your cactus.
Dark brown to almost black spots are notable in the shrunken, soft stems. Once infected, you can assume your cactus is good as dead.
How to Fix
Discard Infected Cactus Plants
Most of the diseases caused by fungi are difficult to control. To avoid the spread of the disease, you have to discard the infected plant as soon as possible.
Seal the diseased plant and dispose of them properly or burn them. Do not leave it lying around your garden or it might spread the pathogens.
Apply Fungicides to The Remaining Cacti for Prevention
If one of your cacti had been infected, there’s a high chance that the others will contract the disease as well. It’s best to apply fungicides as a way of prevention.
Move Your Cactus to Warmer Location
A moist environment is a breeding ground for fungi so make sure that your cactus isn’t exposed to such a condition for a long time. Moving your plant to a warm location will help prevent these fungi from maturing.
Bacteria are another biotic factor that leads to your cactus developing diseases and showing black spots. Like fungi, bacteria are another kind of pathogen that can destroy any plant like your favorite cactus.
Some common bacteria behind diseases in cactus are the following:
- Erwinia carotovora
A cactus that has been cut or wounded is the main target of this bacterium. Once it enters the tissue through the open wounds, it will multiply and infect the whole plant.
If the environment is moist, this bacterium will reproduce faster. Visible signs of infection include the appearance of watery, soft, black tissues.
- Erwinia cacticida
This is another bacterium that can infect cacti, specifically bacterial necrosis of saguaro species. This bacterium is spread by roaming insects and the use of contaminated soils.
The spot where the insects suck is the first site of infection where the tissue under it becomes brown or black. Severe infection can kill the whole cactus plant.
How to Fix
Dry The Wounded Portions under The Sun
Infected wounds are now inhabited by bacteria. They work their way to destroy the tissue and reproduce there as fast as they can.
By exposing the infected cacti under sun, you’re lessening the amount of moisture around it. This will render the bacteria inactive.
Cut Back Infected Parts And Discard Properly
At the onset of infection, immediately remove the damaged portions. This will help prevent the spread of the disease.
If the infestation continues and the damage is irreparable, it would be best to just let go of the plant. Make sure to leave no trace when you do so.
How to Prevent Black Spots on Cactus?
Black spots don’t just appear randomly. From what we’ve understood earlier, it’s caused by many possible reasons. Most of them are preventable if only we knew better.
So here are a few tips to help you prevent black spots in your cacti plants.
1. Know Your Cactus First
Whenever you acquire a new plant, ask for a leaflet or growing guide. This will serve as your reference about the basic growing conditions your plant needs.
Different species of cactus would require different treatments. To avoid the consequences of going trial-and-error, know your cactus first.
2. Purchase A Good Quality Cactus
Always inspect the plant before purchase. Check for possible presence of pests and diseases.
Does the plant look vigorous and healthy? Never ever buy a diseased plant.
3. Mimic Its Natural Growing Conditions
Even when your cactus is brought indoor, you have to stick as close as possible to its natural growing conditions. This includes bright light, low humidity, etc.
There are ideal conditions for certain cacti to grow well. Try to maintain these conditions as much as possible.
4. Watch Out for Drastic Changes in The Environment
High humidity, cold temperatures, extremely bright light are things that you should watch out for. Certain diseases tend to develop when the conditions get favorable so you should avoid that.
5. Use Sterilized Soil And Gardening Tools
Fungal and bacterial diseases often spread by using contaminated soils and tools. Make it a habit to disinfect your pruning shears, shovels, scissors, and other things you use in gardening.
This simple act of prevention can do a lot to save your cactus from diseases. Always use clean, sterilized soil.
6. Check Repeatedly
Inspect regularly if the soil is dry enough before watering. Check if there are foreign organisms lurking around. Check if it receives enough light.
Remember, prevention will only come if you’re vigilant enough to detect the problem early on. So, always check on your cacti.
You May Also Enjoy: Cactus Soil for Peace Lily (And How to Make Your Own)
1. What causes black spots on cactus?
There are a lot of factors that may trigger the appearance of black spots in your cactus. In general, we can categorize them as abiotic and biotic factors. The abiotic are the non-living organisms while biotic are the opposite.
It would be difficult to assess the problem at first glance but it helps to be familiar with the signs and symptoms of each cause. This will help you decide on which solutions are appropriate.
2. What do white spots on a cactus mean?
White spots in cactus are possibly caused by pests/insects that have found shelter on the plant. Specifically, the Cochineal scale (Dactylopius coccus) appears as white cottony tufts on the surface of the cactus pads.
Powdery mildew is another reason for the white spots you see on your cactus. It’s a disease caused by a fungus.
3. How do you get rid of white fungus on cactus?
To remove white fungus from your cactus, you’ll need to apply fungicide on the infected portions to eliminate the fungus. You can also use baking soda and soap water as natural alternatives.
If infection persists, you’ll have to cut the damaged portions. Provide a better air circulation around your cactus. To slow down reproduction of spores, avoid putting your cacti in a wet environment.
Cacti plants may have established the reputation of a hardy plant but it doesn’t mean it can’t be vulnerable. Every plant has a weak point and your cactus surely has its own. But it doesn’t mean you can’t be successful in keeping it indoors.
Black spots may appear as normal in the beginning. However, it does speak a lot about the general health of your cactus. It’s best to investigate and find solutions early than make regrets in the end.