You’re about to start watering your monstera when you notice rust-colored patches on its leaves. A quick Google search confirms your worries: your monstera has rust fungus.
Fortunately, rust fungus is rarely a fatal condition and often looks worse than it is. Still, you need to get rid of this fungal disease because it can spread quickly and reduce a plant’s vigor.
Several factors can cause rust fungus to grow on monstera. Luckily, there are also several ways to treat it.
So, in this article, we’ll provide you with an in-depth guide on what causes rust fungus on monstera, as well as how to identify and treat it.
What Is Rust Fungus?
Rust fungus refers to a group of fungal parasites that grow on a wide variety of plants, including the monstera houseplant.
Because these fungi are obligate parasites, they only live on living plants. So, they’re usually not fatal to their host plants.
However, they create a structure out of their cell wall and the host plant’s cell wall to absorb the food from the plant. As a result, rust fungus can severely limit the plant’s growth and vigor.
These fungi affect the leaves but can also spread to the plant’s stems, flowers, and fruits, giving it an unsightly appearance.
What Causes Rust Fungus on Monsteras?
Many factors can cause rust fungus to grow on your monstera, and determining what caused the infection in the first place can help you prevent reinfection.
So, read on to learn more about the most common causes of rust fungus on monstera.
Overwatering and Poor Drainage
Excessively and constantly wet soil serves as the perfect breeding ground for fungi. So, overwatering your monstera can lead to the growth of rust fungus and more destructive fungi, such as root rot.
Overwatering isn’t just caused by excessive watering. If you plant your monstera in soil with poor drainage, it’ll lead to waterlogged soil.
Misting at Night
Many monstera varieties prefer humid conditions. So, you may need to mist your monstera to maintain the necessary humidity levels.
That said, if you mist your monstera at night, the water droplets won’t dry fast enough due to the cool temperature. These moist conditions are ideal for fungi to grow and thrive.
Over-misting your monstera can lead to the same outcome.
High Humidity and Temperature
Monstera plants require high humidity and warm temperatures to grow and thrive. Unfortunately, so do fungi. That’s likely why many monstera varieties are more susceptible to rust fungus infection.
Poor Light Conditions
All plants need light to produce energy via photosynthesis. Monstera plants, for example, need 5–8 hours of bright indirect light to thrive. Otherwise, their health may decline, making them more vulnerable to diseases.
In addition, without light, water droplets on the leaves and water in the soil won’t evaporate, creating ideal conditions for rust fungus to grow.
Poor Air Circulation
Poor air circulation impedes water evaporation and transpiration, as well as promotes condensation on leaf surfaces. It also prevents enough light from reaching all parts of the plant.
All of these factors allow fungal infections to grow and thrive.
Infection Spread from Another Plant
Rust fungus spores can float through the air and spread from one plant to another.
That’s why you should quarantine new plants before you place them with your houseplants. If your new plant is infected, the quarantine period allows the rust spots to appear.
In addition, if your houseplant starts showing symptoms, you should isolate it from other plants. This way, you can keep the spores from spreading to your other houseplants.
Infection Spread via Infected Tools
Rust fungus spores can also spread from your gardening tools to your monstera.
One of the most dangerous gardening habits is working on multiple plants without disinfecting the tools between each plant. Bacteria and fungi can latch onto the tools and spread the infection from one plant to another.
How to Identify Rust Fungus on Monstera
Rust fungus gets its name from the rust-colored raised spots that appear on an infected plant’s leaves and stems. However, these rust spots may also appear brown, orange, red, yellow, or purple.
Yellow or white raised spots usually form on the upper side of the monstera’s leaves. Reddish to orange blister-like spots form on the undersides of the leaves.
These spots are often tiny and scattered across the plant’s foliage. Some rust fungi can also cause orange or yellow streaks on the undersides of the leaves.
Most of these spots on the infected monstera will form galls, which then become covered in white fluff-like spore masses.
In some cases, rust fungus can cause leaf distortion and premature defoliation.
How to Treat Rust Fungus on Monstera
The best treatment is prevention. So, as soon as you identify those rust-colored spots, you’ll need to take a few steps to treat your monstera.
Isolate the Infected Plant
In general, fungal spores can spread easily and quickly from an infected monstera to a healthy plant. So, ideally, you want to isolate the infected plant in a room separate from the other plants.
If that’s not possible, you want to keep the infected monstera a fair distance away from the other plants. You also don’t want to put it near a window or a fan because the wind can carry the spores to the other plants.
Prune the Infected Parts of the Plant
Using pruning shears, remove any infected leaves and other plant parts.
If most of the monstera is infected, only remove the most severely infected parts. Otherwise, your monstera may not be able to survive.
You can’t compost or mulch infected plants. So, after you prune the infected parts, dispose of them in a plastic bag and seal the bag well.
That way, the spores can’t spread around and to other plants.
Properly Clean Gardening Tools
If you have more than one plant, you should disinfect your gardening tools with 70% isopropyl alcohol between each plant. You should also wash your tools with antifungal-antibacterial soap and water before and after you use them.
Place Pruned Plant in Its Proper Environment
The most crucial step in treating a rust-fungus-infected monstera is to place it in an ideal environment. Otherwise, your monstera won’t regain its strength and the fungi will continue to thrive.
So, place your monstera in a well-ventilated room where it can receive bright indirect light. Prune it regularly to improve air circulation and allow light to reach all parts of the plant.
Only water your monstera when the top 1–2 inches of the soil is dry. If your monstera is in poor-draining soil, transfer it into the soil and a container that can drain excess water without an issue.
Treat With Natural Remedies
There are a few common household ingredients that you can use to treat and prevent rust fungus.
Spray With Baking Soda
Baking soda is a potent fungicide, so it can get rid of and prevent rust fungus.
However, the most effective way to use it as a fungicide is to combine 4 teaspoons of baking soda, an ounce of horticultural oil, and a gallon of water in a spray bottle.
You’ll need to spray this solution once a week for 3–4 months. Lightly spray the entire plant, making sure not to miss any part.
In time, the rust fungus spots will gradually fade until they disappear.
Wipe With White Vinegar
Like baking soda, white vinegar has antifungal properties. All you need to do is mix one part vinegar with one part water. Then, spray the infected monstera every 2–3 days.
If the white vinegar contains more than 20% acetic acid, you’ll need to dilute it more. Otherwise, the monstera may suffer from chemical burns.
Spray With Chamomile Tea
If you want a milder solution, you can use chamomile tea. Chamomile is antifungal, but it won’t cause chemical burns if used too much.
After boiling some water, steep chamomile flowers or sachets in the water and allow to cool to room temperature before transferring the tea to a spray bottle. Then, lightly spray your infected monstera daily.
Use Neem Oil
Another effective but safe natural fungicide is neem oil. It can get rid of rust fungus, as well as any other infections and infestations, without harming your monstera.
You can combine a teaspoon of neem oil and a liter of water in a spray bottle. Then, once a week, spray the infected plant.
After your monstera has healed, you can reduce spraying to once every two weeks. That way, you can avoid reinfection.
Prevent With Chemical Fungicides
Chemical fungicides aren’t effective after rust symptoms appear. However, they can control the spread of the infection.
That’s why it’s best to apply chemical fungicides when new growth appears or after leaf drop. The fungi can’t reproduce and spread this way, even if they’re on the plant.
Spray With Copper Fungicide
Copper fungicides are effective against rust fungus as well as a wide range of other fungal and bacterial infections.
Make sure to buy a copper fungicide from a reputable manufacturer and strictly adhere to the instruction label.
Most importantly, when spraying copper fungicides, wear protective clothing and eye goggles. You should also know that large amounts of copper can destroy plant cells.
Use Sulfur-Based Fungicide
Sulfur-based fungicides are also effective against the growth of fungal spores, especially rust fungus. Nevertheless, they’re less harmful to plants and humans.
They come in powder form, but some sulfur fungicides are for sprinkling over the plant while others are to be diluted in water and sprayed.
Now that you’ve learned all that’s to know about rust fungus on monsteras, you’ll be able to treat and nurse your monstera back to health.
Rust fungus can spoil the appearance of your beautiful monstera. Luckily, getting rid of the fungus and treating your plant isn’t difficult.
Still, if your monstera is a rust-prone variety, rust fungus may reappear on occasion. In this case, you can replace your monstera with a more rust-resistant variety.
Growing up with a mom who filled her home (inside and out) with all sorts of plants, Lisa got her start in gardening at a young age. Living now on her own with a home and yard full of plants (including an indoor greenhouse), she shares all the gardening tips she’s gained over the years.