How to Get Rid of Brown Spots on Cactus (And Prevention)


Have you been having problems with your cactus particularly brown spots? No doubt because it’s one of the common issues you’ll encounter with cacti plants and they can be pretty alarming. We know cactus is a tough one but there are times when it gets susceptible. 

Brown spots on cactus are the result of the changes in physiological processes. It can be due to changing environmental conditions (light, humidity, temperature, frost, poor air circulation), malpractice (overwatering, overfertilization), and biotic factors (pests, fungal diseases).

To understand why brown spots appear in your cactus, it’s critical to know the mechanism behind its causes.

This article will help you better combat the problems that in turn minimize the chance of your cactus acquiring brown spots.

How to Identify Cactus Brown Spots?

Brown spots may appear differently depending on the cause of browning, the length of time of its appearance, and/or the intensity of the damage. They differ in size, the shade of brown color, and even texture. 

Some brown spots appear larger through time. Some are barely noticeable to the naked eye. Some become dry lesions while others look like blisters.

Why Does My Cactus Have a Brown Spot?

Excess Light

Light is a basic necessity for any plant, cacti is not an exception.. They do love very bright light to maintain a vibrant color.

Light also helps them reduce excess water on the soil to prevent the dangers of overwatering. 

However, if your cactus is exposed to a light intensity that’s higher than it previously received, there’s this possibility of experiencing scorching.

Too much light can hurt the surface or skin of your cactus creating sunburn spots. Excess sunlight may also cause cactus to turn purple.

Damage from excess light usually happens when you move the cactus out of your home.

Since it’s used to having indirect light exposure, the sudden transition to direct light brings shock to your plant. 

How to Treat

Move Your Plant Indoors

If your plant is situated outside, bring it back indoors. The excess light is definitely causing trouble for your cactus. By bringing it inside, you’re reducing the impact of extreme light exposure. 

Provide Additional Shade

At times, light intensity inside may also cause scorching. In this case, you’ll need to provide extra shade to the plant to minimize the intensity.

Shades such as curtains, cotton sheets, and larger plants can be used to block some of the light.

Temperature Stress

Extreme rise and fall in the temperature also bring stress to your cactus. The ideal temperature ranges from 45oF and 85oF (7oC-29oC). Sudden changes in temperature aren’t good for your cacti because it will shock the plant. 

Heat and cold stress can damage the tissues of your cacti. The result is often rapid. That’s why in just a short period of exposure, you’ll be able to find the damage such as brown spots.

How to Treat

Relocate the Cacti Pots

It’s possible that the current location of your cacti is exposed to heat or cold drafts. If the pot is near a window, check if the glass is emitting too much heat during the day or cold during the night.

You can minimize the impact of temperature stress by finding a location where the temperature is more stable.

Provide Insulation When Cold

Insulating your cactus is an important step to provide protection. Temperature is lower during the night but it can drop to an even lower level when the cold season comes.

Providing extra warmth will help the plant cope with the drastic temperature change. 

Pest Infestation

Although we rarely see pests infecting our cacti, we cannot do away with the possibility that they’re the reason for the brown spots in our plants.

Pests normally infect the cacti by sucking on the plant tissues. They then leave small wounds on the surface of the plant which later develops as scars appearing as brown spots. 

An example of these pests is armored scales (Diaspididae) which use their straw-like mouthparts to suck plant fluids.

They can be difficult to detect because they’re quite small. Sometimes, you’ll only notice them when the plant starts showing off the damage it caused. (Source: NC State Extension Publications)

Another potential pest is mites. They also feed on different plant parts by sucking fluids.

If they manage to produce a large population, they’ll surely bring damage to the plant usually leaving infected portions to turn yellowish or reddish-brown. ( Source: Agriculture and Natural Resources, University of California)

How to Treat:

Inspect the Cacti for Any Presence of Pests

Since these organisms are too small to notice, you have to make a deliberate attempt of inspecting your cacti for roaming insects.

You can do this once in a while in order to immediately prevent the spread. Extend the inspection to your other indoor plants as well because these pests may have found a home in another host.

Fungal Disease 

Aside from pests, diseases are also a reason why brown spots appear in your cacti. Fungi are pathogens that can cause diseases in many plants. Once they infect the host, they can be difficult to manage.

Fungal diseases create permanent damages to the plant. In cacti, the common fungus is Colletotrichum (Gleosporium) spp. This causes what we call cactus anthracnose.

The site of infection initially appears as moist, light brown spots. Through time, the spots enlarge until it covers the entire plant. Cacti plants with fungal diseases have a high chance of dying. 

How to Treat:

Discard Infected Portions

To avoid the spread of the disease, it’s best to immediately cut off the infected portions and discard it properly.

There will even be times that you have to dispose of the whole plant itself. Controlling anthracnose is difficult. 

Apply Fungicide

Chemical control is an immediate way to combat the fungal problem. Fungicides can be preventive or curative. (Source: Ohio State University Extension)

You should apply preventive fungicides before planting while curative after the symptoms started showing. 

Avoid Humid Conditions

Pathogens like fungi grow in moist environments. If the moisture level is high, there’s a high chance of developing fungal diseases.

Cactus never wants high humidity so you have to keep the environment in check. 

Rust Spots on Cactus

One of the diseases that develop in cactus, especially the cactus pear, is rust. It’s caused by Puccinia opuntiae which is a species of fungi.

According to a report of FAO, this fungus “produces yellow-orange pustules on the surface of cladodes or fruits” of the cactus.

You will find a hole when the rust spot eventually dries. The site of infection will continue to spread until the plant is severely infected. Lack of intervention may cause the death of the whole plant. 

How to Treat:

Remove and Bury the Infected Clalode

This is the most recommended way of managing rust disease on cacti. Once you notice the signs, you have to immediately remove the sites of infection to prevent the spread of disease.

Isolate the Infected Plant

Diseases in plants spread like wildfire. If you’re not cautious, other plants will be in danger of getting infected as well. Isolating the plant will help contain the disease. 

Apply Preventive Fungicide on Other Cacti

If you have other cacti that has not been infected, better apply fungicide on them so that they’re protected.

Good thing because there are already organic and inorganic options to choose from in the market.

Just be careful in handling fungicides and always follow the instructions indicated in each label. 

Fertilizer Problem

Cacti aren’t heavy feeders so they don’t require frequent fertilization. If you’ve applied too much fertilizer to the plant, the injury will most likely develop in the form of brown spots on the stems. Excess fertilizer will burn the tissues of the cacti.

Salts also build up in the soil if you’re using excess fertilizer. These salts alter the pH and negatively affect the population of good microorganisms in the soil.

Roots also get damaged and can not function properly. As a result, the cactus will turn brown.

How to Treat:

Leach Or Rinse Off Excess Fertilizer

You can quickly fix the excess fertilizer by leaching it off using water. Thoroughly pour in water in the pot until it drenches. Wait for a few hours and do it once again. 

Repot in New Potting Media If Needed

There are times when you need to repot a cacti to do away with overfertilization.

If leaching isn’t enough, you’ll have to prepare fresh soil where to repot the plant. Check the roots for any signs of damage and trim it off before putting it in the soil.

Poor Air Circulation

Poor air circulation is a common problem in an indoor, closed setting. The lack of good airflow will prolong the damp condition especially after you’ve watered your plants. We know that a moist environment serves as an invitation for pathogens to grow. 

How to Treat:

Open the Doors and Windows

To encourage air circulation, you can open the windows and doors for a few hours. Not only will it be good for the plant’s health but for you as well. By letting in air from the outside, you’re pushing the air indoors to flow. 

Take the Cacti Plant Out

Let your cacti enjoy the fresh air outside. Place it in a wide area where the light is available. It will help hasten the evaporation of excess moisture in your cacti. 

Use the Exhaust Fan

Exhaust fan works by pulling out fumes, odors, and moisture from the inside of your home to the outside.

It helps in promoting air circulation in an area. When the stuck air is pulled outside, the fresh air can now enter the room.   

Humidity

As we’ve mentioned previously, high humidity is not good for your cactus. In fact, it prefers dry arid places like that of the deserts. When the moisture level in the air is high, cacti will be prone to rotting.

High humidity coupled with overwatering is a number cause of death. Pathogen growth is also triggered. These unfavorable conditions will compromise the health of your cacti.

How to Treat:

Place the Cacti in a Dry, Arid Place

There must be an area in your home where the air is relatively dry. You can put your potted cactus there. You should avoid humid places such as kitchens and bathrooms.

DoN’t Water the Cacti Yet

If the environment is highly humid, you may skip the watering schedule. Because of the moisture available on the air, transpiration rate will surely be slow.

If you add water to the soil, your cacti will have difficulty utilizing it and the result will be overwatering.

Improper Watering

Improper watering could mean two things: either overwatering or underwatering.

Cacti can survive drought conditions so underwatering is not much of an issue. What you need to be careful of is the application of excess water. 

The quality of potting mix, frequency of watering, temperature and humidity are factors that affect water application.

If you’re not careful, you will end up drowning your cactus. This opens the door for root rot to occur.

An indication of root rot is the appearance of brown spots that tend to be mushy.

These spots are the dead portions of the plant that have started rotting.

This is due to the lack of nutrients and water that are supposed to be channeled by the roots that have not been damaged.

How to Treat:

Check How Moist the Soil Is.

Just by dipping your finger at least an inch below the soil, you’d know if the soil is overwatered or not.

A soil that’s soggy and feels wet means that it received excess water. Ideally, the right condition for cactus soil should be dry to touch. 

Expose the Cactus Under Bright Light.

To hasten the release of moisture, you can take advantage of the heat that the sun radiates.

This will help dry the soil faster. You may also bring the potted cacti outside to receive natural sunlight.

Frost Damage

Cacti are not frost-tolerant. They can be damaged by extremely cold temperatures especially during winter.

Because they have high moisture content inside their bodies, they can easily freeze at very low temperatures. 

Frost will damage the tissues of the plant. When it gets thawed, you’ll observe the plant to develop brown spots on different portions. Frost damage can kill the cacti.

How to Treat: 

Relocate the Cacti During the Night.

The temperature at night is relatively lower than the temperature during the day.

If the plant is near a window glass, the coldness of night will surely radiate and affect the plant. In that case, you’ll need to move it somewhere warmer.

Keep the Cactus Plant Insulated

In times of cooler temperatures, you may use cotton sheets to cover the cacti. It helps create a warm environment and will prevent the plant from freezing. The thickness of the insulator should depend on how low the temperature gets. 

Add Light to Warm the Place

Light helps emit additional warmth for your cacti. During winter, you can group your plants together and install a 100-watt lamp above them. Create enough space and don’t let the lamp touch the sheet covering. 

How to Prevent Brown Spots on Cactus? 

Start with Disease-Free Cacti

Always choose a healthy and vigorous cacti whenever you buy. Inspect for any presence of brown spots and if you see any, don’t make a purchase. Starting with healthy plants will save you a huge amount of work.  

Use Clean and Good Quality Potting Soil

The kind of potting mix that you use is very important for the success of your cactus plant.

If you’re unsure of how to do the right mixture, make use of the readily available cacti mixes in garden stores. What’s important is that it’s sterilized (pathogen-free) and is well-draining. 

Find the Best Location

It has to be well-lit, relatively less humid, and warmer. Those conditions are ideal for your cacti to thrive.

There are times when these conditions drastically change so you have to pull a few tricks like using exhaust fans, artificial light sources, etc. 

Regulate Water and Fertilizer

Cacti won’t need so much of these two inputs. It’s better to give them in regulated amounts than overdoing it. Any excess will take a toll on the health of your cactus. 

Use Fungicide as Preventive Measure

You can apply fungicide on the soil to prevent growth of fungi. This will help eliminate the chance of developing fungal diseases.

You can make use of baking soda as a natural fungicide or the commercially available organic and inorganic fungicides.

Check and Remove Pests Regularly

Visit your cacti regularly and inspect for any present foreign organisms. If you’re early to spot them, you can prevent them from bringing further damage.

There are many ways to control pests such as mechanical, chemical, biological, and physical methods.

FAQs

How Do I Get Rid Of Black Spots on My Cactus?

Black spots on cactus appear as a result of undergoing stressful conditions brought about by abiotic or biotic factors.

There are different possible reasons why your cactus is having black spots. The first thing you need to do is find the exact cause. The treatments for black spots vary depending on what causes it.

Abiotic factors such as excess light, overwatering, over-fertilizing, high humidity, and cold temperatures can be managed by applying simple techniques. If the problem is caused by pathogens, it will be more challenging. 

Can You Revive A Brown Cactus?

It depends on the extent of browning. If the majority of the cactus body is already brown, you might as well let it go. It would be difficult to try to save an already lost cause. 

However, if the browning is somewhat minimal, you can still save the plant. Investigate how the cacti responds to the treatments you’ve applied. If the browning lessens, you’re on the right track. 

What Does An Overwatered Cactus Look Like?

An overwatered cactus would have brown portions that are mushy when touched.

The roots are damaged and the base of the plant is usually rotten and weak. When you try to pull it out from the soil, the base would easily detach from the roots. 

The appearance of brown spots on the cactus indicates an internal problem. It’s a sign that you must not ignore. Otherwise, you’d be too late to notice the damage. 

Final Words

In keeping your cacti healthy, prevention is still the best way. Keep in mind the basic growing conditions that it needs and try to stick to it as much as possible. Be quick to determine environmental changes and make adjustments if necessary. 

Arifur Rahman

I'm the owner of gardenforindoor.com. After completing my bachelor of science in agriculture, I'm serving as a civil service officer at the Department of Agricultural Extension, Bangladesh. I started Garden For Indoor to make your indoor gardening journey easy and enjoyable.

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