What Causes Black Spots on Monstera Leaves?
The majestic fenestrated leaves of a Monstera plant become much harder to appreciate when they’re riddled with black spots. Today, I’ll explain the source of those spots and how to treat them.
What causes black spots on Monstera leaves? Black spots on Monstera leaves are usually caused by overwatering but can also be attributed to underwatering, pest infestations, diseases, nutrient deficiencies, low humidity, and sunburn.
Ahead, I’ll guide you through each cause of Monstera leaf blackening and provide plenty of actionable solutions so you can get your plant looking and feeling healthier soon!
The 7 Causes of Black Spots on Monstera Leaves
The Cause: Overwatering
Jumping right in, the primary cause of black spots developing across the green or variegated leaves of your Monstera is overwatering.
Most Monstera plant species like soil with at least a little bit of moisture.
The problem with some indoor gardeners–especially beginners–is that they assume that if a little bit of moisture is allowable that surely a little bit more is, then a little bit more.
Before you know it, you’ve created conditions that lead to root rot.
If you need a refresher on root rot, let me provide it to you here.
Root rot is a fungal disease that’s often caused by overwatering. Soil compaction or non-aerated soil can also trigger root rot.
When a plant is a candidate for root rot, it has more water than it does oxygen. The standing water can’t drain fast enough, usually because you’re replenishing the water at such a frequent rate.
The plant does what any of us would do without enough air. It begins to die.
The plant death occurs at the root level first. The roots will become black or brown and give off a terrible odor.
If your Swiss cheese plant has black spots across its leaves, this is a sign that the root rot is quite advanced. Now the rest of the plant is dying just as the roots have.
Besides foliar blackening, you may also see symptoms like stopped growth, leaf yellowing and/or browning, and wilting if your Monstera has root rot.
What to Do About It: Cut Back on Watering
I’m sure you’re wondering what you can do for your Monstera if it has black spots caused by root rot.
Honestly? By that point, not too much.
I’m not trying to be Debbie Downer here, but if the fungal disease has progressed to the point where your Monstera’s leaves are dying, your plant is in altogether very poor shape.
You can always try removing your Monstera from its pot, cutting away its dead roots, repotting, and hoping for the best.
In the future, better managing your Monstera’s watering habits is a good course of action.
The Monstera does like moist soil, but the soil only has to be slightly moist. By putting a clean finger or two deep in the plant’s pot, you can determine how much moisture is left.
You do want to allow the soil to dry out moderately before you water the Monstera again. In other words, don’t let the soil get bone-dry, but allow the moisture in the soil to vanish.
The Cause: Underwatering
Another watering error you could accidentally make when it comes to Monstera care is underwatering.
You know from the last section that watering this lovely fenestrated indoor plant is a careful balancing act, like walking perfectly across a tightrope.
Too much water will lead to potentially deadly root rot. So what happens when the Swiss cheese plant receives too little water?
It’s not at risk of root rot, but something else entirely that’s almost equally as deadly: dehydration.
Without water in the root system, the Monstera begins to dry out. Its leaves will become crispy and brittle. If you touch them, it will feel like they could snap right off (which, they probably could!).
The coloration of the Monstera leaves will shift as well. Gone will be the bright greens or the creamy whites and pinks. Replacing that will be yellow spots that will later turn brown or even black.
If you’re still looking for confirmation that your Swiss cheese plant is being underwatered, just feel the soil. It will be dry as a bone.
By pressing your finger on the surface the soil, you’ll leave a visible footprint, so to speak. That indentation from your fingertip will not go away.
In properly moist soil, that wouldn’t be the case.
What to Do About It: Water More Frequently but Not Excessively
If you’re watering your Monstera on a schedule, I’d strongly encourage you to stop.
The issue with following a watering schedule is that it doesn’t accommodate for factors such as heat and humidity.
If you live in a consistently hot region, then you can’t wait more than a few days without watering the Swiss cheese plant. Its soil will become too dry otherwise.
Even if you live in a more temperate region, you will need to water your plant more in the summer than in the winter.
The fingertip test is your best gauge for measuring even very minute quantities of water. Please use it and water your Monstera when the soil begins feeling mostly dry but not bone-dry.
What can you do about the deadened leaves of your Monstera?
Unfortunately, those leaves are done for. A plant’s leaves that are damaged and discolored will never magically turn green again, even if you’re caring for your plant perfectly.
I’d recommend pruning the black or brown parts of your Monstera’s foliage with gardening shears. Please disinfect the shears using 70 percent isopropyl alcohol or bleach.
The Cause: Pest Infestation
If not caused by a watering issue (be that overwatering or underwatering), then your problem with the Monstera’s leaves turning black could very well have to do with pests.
No indoor plant is entirely immune to pests, although some plants are less appealing to small insects than others.
The Monstera is not one of them. Here’s a rundown of the insects you might see crawling along the plant’s large holey leaves.
- Whiteflies: A relative of the mealybug and the aphid–which also invades Monstera–the whitefly in the Aleyrodidae family does not include solely white bugs. There are, after all more than 1,500 species, so they come in an assortment of colors.
- Aphids: Since they’re related to whiteflies, it’s unsurprising that the aphid is sometimes called the blackfly or greenfly. Aphids also come in plenty of colors and can propagate quickly due to the maturity of female nymphs shortly after birth.
- Scale insects: Hungrily noshing on plant sap, scale insects might be practically microscopic, but you should not underestimate them and the damage they can cause when they invade your indoor garden.
- Spider mites: The web-weaving spider mite has more than 1,200 unique species. All will settle under a plant’s leaves, where they’ll drink sap as much as they want and weave webs so they’re nice and cozy.
What to Do About It: Identify and Remove/Treat Pests
The black spots you’re witnessing on your Monstera’s leaves can be one of two things if the cause is pests.
You’re either looking at the pests themselves or the ruinous damage they’ve left behind.
Although spider mites usually weave white webs, they themselves are dark in color.
By flipping up your Monstera’s leaves one at a time and looking underneath, you should be able to see a serious spider mite infestation.
You already know that whiteflies, aphids, and scale insects can be dark in color too, although that varies by species.
Scale insects are tough to see individually, but they usually cluster together. That makes them a lot easier to see!
You’re sure your Swiss cheese plant has been afflicted by pests, so the question becomes what to do about it.
You can try manually removing the pests from your Monstera, which works especially well for tiny scale insects. You need chopsticks and a bucket.
Be sure to catch the bugs in the bucket once you dislodge them or they could find their way back to your plant!
If manually removing insects is too unpalatable or just not working, then you can kill the bugs.
You only need water and dish soap or rubbing alcohol to do it. Transfer the liquid to a spray bottle and thoroughly mist the Swiss cheese plant where bugs have been.
After doing this consistently for about a week or two, you should see far fewer insects.
Are a couple of stragglers still hanging on? You can always upgrade to neem oil or insecticidal soap, but these are harsher solutions than dish soap or rubbing alcohol.
The Cause: Disease
Root rot is not the only disease that can afflict a Monstera and cause its leaves to turn black. Anthracnose and bacterial leaf spot can do it as well.
Here’s an overview of both plant diseases.
- Anthracnose: Anthracnose is a group of fungal diseases that cause plant leaves to develop dark spots. The leaves may drop earlier than usual and begin curling or cupping. You’ll have to remove the affected leaves of the Monstera and cut back on watering. If you begin overwatering your plant again, anthracnose will rear its ugly head.
- Bacterial leaf spot: Monstera leaf spot disease is technically a bacterial leaf spot. The disease can sometimes be viral and/or fungal in nature as well. The prior use of herbicides, insect infestations, and/or injuries can cause bacterial leaf spot.
What to Do About It: Treat the Disease If Possible
If you suspect your Swiss cheese plant has either bacterial leaf spot or anthracnose, you do not want to be inactive for long.
The black spots that are caused by bacterial leaf spot are lesions. The lesions are killing the cells in the leaves, leaving nothing that’s salvageable.
Let’s talk treatments then.
Going back to what I said about anthracnose, this fungal disease often doesn’t have a treatment. You’ll have to remove the infected Monstera as well as any other adjacent indoor plants that might be infected.
Using a fungicide with copper as a primary ingredient can help. Reducing watering frequency is a prevention method rather than a treatment.
To treat bacterial leaf spot, prune any foliage with dark spots, even if that’s most of the Monstera’s foliage.
Then, use neem oil or a fungicide and keep your plant mostly dry (no water on the leaves especially). It may just recover.
The Cause: Nutrient Deficiencies
Many indoor plants require the same balanced ratio of nutrients in their fertilizer.
This may cause an indoor gardener to think that every plant in their garden has the same fertilizer requirements. That’s simply not the case.
The Monstera is a foliage plant, so it requires a fertilizer with a 3-1-2 ratio. The 3 is for nitrogen, the 1 is for phosphorus, and the 2 is for potassium.
If your Monstera receives less nitrogen than it needs or less of any of the other macronutrients, what will follow are nutrient deficiencies.
Leaf yellowing is common when a plant is starving for nutrients, but you may also see foliar browning and/or blackening. The leaves can appear reddish or purple too.
The leaves will take on a burnt appearance and can develop small holes throughout. No, these are not Monstera fenestrations!
Any new leaves that develop will be stunted.
What to Do About It: Increase Fertilization or Use a Monstera-Friendly Fertilizer
Nutrient deficiencies in indoor plants have two causes. You’re either not using the right type of fertilizer or you are, but you’re not fertilizing your Monstera often enough.
To reiterate, the Monstera requires a 3-1-2 fertilizer with three parts nitrogen, one part phosphorus, and two parts potassium.
You should fertilize the Swiss cheese plant during its active growing season, which begins in the spring and ends in the fall.
I would recommend fertilizing once every two weeks to start. Then, as your plant gets healthier, you can scale back to applying fertilizer monthly.
If you want to continue with the two-week schedule and your Monstera shows no adverse effects, then that’s another option.
The Cause: Low Humidity
Humidity is a measure of moisture in the air, and you already know what happens when your Monstera doesn’t receive enough moisture.
The plant begins drying out, becoming brittle, crispy, and blackened.
The spots that you saw when you were overwatering the Swiss cheese plant can come back with a vengeance if you’re not meeting the humidity requirements for this plant.
What to Do About It: Raise Humidity Levels
What are the humidity requirements of a Monstera?
Most Monstera species prefer humidity over the relative average. They need between 60 and 80 percent humidity.
Your home or office might have 50 percent relative humidity at best, so what’s an indoor gardener to do?
That’s simple. For those who are growing the Monstera at home, you can put the plant in your bathroom. It will certainly get enough humidity in there.
If your bathroom is too tiny for a Monstera or if you’re growing the plant in a cubicle, then a small humidifier will make conditions warmer for the Swiss cheese plant.
The Cause: Sunburn
The last reason your Monstera could have developed blackened foliage is due to sunburn.
That’s right, indoor plants can burn in the sun just as you and I can.
There are two causes of sunburn in indoor plants.
The main cause is flooding the Monstera with too much sunlight. This plant, despite the size of its leaves, does not like direct sun exposure.
Direct sunlight, by the way, means sunlight that comes directly from a window to your plant.
Another cause of sunburn in your Monstera is excessively high temperatures.
All plants have an upper temp tolerance. Once that tolerance is surpassed, the plant can experience heat stress.
The leaf edges will become crispy and blackened, and they’ll wilt too. You may even notice your Monstera is trying to roll or cup its leaves to preserve water on the leaf surface.
I should note that sunburn can cause white spots on the leaves. These areas look bleached.
However, they don’t stay white for long, quickly become dried out and black as the cells in the leaf die.
What to Do About It: Reduce Sun Exposure and Temperature
The Monstera prefers temperatures between 60 and 80 degrees Fahrenheit.
At work or at home, you shouldn’t have to touch your thermostat to keep this plant happy, which is nice.
As for the Swiss cheese plant’s ideal lighting, that’s bright, indirect light.
Let me explain how bright, indirect light differs from direct light.
The big difference is the inclusion of a curtain or another medium for the light to pass through before it reaches the Monstera when you provide bright, indirect light. This filters the light so it’s not as strong and less likely to cause sunburns.
The Monstera prefers at least five hours of bright, indirect sunlight per day but ideally eight hours.
If your Monstera is variegated, without proper sunlight, its coloration will disappear for good!
