Moon Cactus Brown Spots? 6 Problems [And Solutions]

A colorful moon cactus is always a great addition to your cacti collection. Aside from being a low-maintenance plant, it carries a good vibe through its bright colors. However, it can be alarming when portions of your colored plant begin to develop brown spots. Is this any sort of disease? What’s the cause, how to treat and prevent them?

Brown spots on moon cactus are due to 1) Sunburn, 2) Corking, 3) Rot, 4) Fungal Disease, 5) Insect Infestation, and 6) Animal bites. Brown spots can be prevented or removed by providing indirect sunlight, watering only when the soil is dry, and using fungicide and pesticides.

It’s frustrating to see your once-lovely moon cactus turning brown all over. The good news is that if you identify the cause of those pesky brown spots quite often, you can save your plant! Read on to understand how I do with my cactus when this happens!

The 6 Reasons of Brown Spots

Moon cactus discoloration is a usual problem that many owners have to deal with. Brown spots are common with moon cactus. Fortunately, it is not detrimental to your plants if figured out right away.

When you notice brown patches on your moon cactus, take the time to figure out what’s wrong and deal with it accordingly. Here are the possible reasons for brown spots on your plant.

Sunburn

Sunburn occurs when a moon cactus is left under direct sunlight for an extended length of time. It will appear as pale discoloration on the plant’s epidermis, which will soon dry out and turn brown. Sunburn on moon cactus looks like a hollow, crisp brown patch of burnt plant tissues.

While the rootstock prefers direct sunlight, the scion cannot withstand it. Usually, sunburn can happen on the top part of the moon cactus – the colored scion. It is prone to sunburn because it lacks protective pigment (chlorophyll) to absorb sunlight.

In some cases, severe sunburn may cause the outer layer of the skin to peel off. Spraying your moon cactus with chemicals like alcohol, pesticides, and soaps during hot weather will also cause burns on its epidermis.

Sometimes, sunburn can be mistaken as corking. So if you leave your moon cactus under too much sunlight and it develops a hollow brown portion, then it must be a sunburn. The damage is irreversible and permanent, but it does not mean death to your moon cactus.

Corking

Corking is a natural phenomenon that occurs in any aging cactus. It appears as a brown, solid, bark-like texture that usually starts at the base of the cactus and progresses upwards. The brown spot looks like a laid map. Corking is not harmful, but its appearance is unsightly.

A cactus will naturally begin to cork or woody over time to provide a firm base as it grows. It is a natural process typical to cactus as they age. Moon cactus starts corking at the bottom of the rootstock or the union point between the rootstock and the scion.

The moon cactus becomes top-heavy and prone to break as it grows. As a natural response, the aging moon cactus starts corking to provide support to the whole plant.

Corking is a slow process, so brown spots appearing overnight are not due to corking.

Determine what causes the discoloration by touching the affected part. If it seems firm and bark-like, then your moon cactus is indeed corking. However, if the brown part is mushy or soft to touch, it is more likely that the plant is rotting.

Rot

Like most cacti, moon cactus do not like excess moisture on their roots. Excess water is one of the significant causes of root rot among cacti. Brown spots that are squishy and transparent indicate root rot. These spots are dead tissues that have begun to decay.

Soil composition and pot drainage have something to do with waterlogging. Excess water on the potting mix hinders air circulation in the plant’s root system and results in root rot. Rotting took place first at the roots and progress up to the base.

Since root decay happens below the base of the plant, it is usually difficult to notice the signs. Damage may show on any part of the plant, but usually, it affects the rootstock first. Sections of the plant may develop edema or tiny brown blisters and bumps that become permanent scars.

It is possible to save a rotting moon cactus during the early stage of rot, but severe damage can be very challenging.

The brown spots due to sunburn, corking, and edema on moon cactus.

Fungal Diseases

Fungal diseases can also cause brown spots on moon cactus. Fungi are pathogens that can infect plants and be challenging to control once they have infected the plant.

Several fungi cause brown spots on the surface of the moon cactus. The damage develops as small circular spots and gets enlarged soon after. Overwatering, high humidity, and low lighting conditions promote the growth of these diseases.

They can be difficult to control after they have infected the host. Fungal diseases caused by Phyllosticta concava, Gleosporium, Diplotheca, and other fungi usually appear brown to black blisters on the cactus. Stem cankers or the brown and rusty patches surrounded by brown rims indicate that the moon cactus is decaying. The fungus can be detrimental if left untreated.

Insect Infestation

Aside from fungus, insects such as mites and scales are culprits in causing brown spots on the cactus. These insects stick to the surface of the plant and suck on the fluid for food.

Armored scales and red mites feed on the plant tissues, sucking juices with their needle-like mouthparts. They then leave tiny lesions on the plant’s surface, which develop into scars and appear like brown patches later.

Because they’re so tiny, they can be difficult to spot. You may not detect these minute pests right away until the plant begins to show signs of the damage it has caused.

Animal Bites

Animal bites caused by birds and worms may develop into brown scars. These are possible for moon cactus located inside a greenhouse or balcony where these animals can reach them.

The animals mentioned above are some who like to munch on a cactus. They can easily bite a moon cactus because it has short spines, especially the scion. The affected part will soon dry out and become brown scars. Unfortunately, this damage cannot be fixed and will become permanent.

How To Solve Brown Spots – 4 Tips

Brown spots on a moon cactus do not always mean death. As long as you act right away, you can still save your moon cactus. I will help you solve the brown spots issue based on my years of experience and keep them from affecting your precious plant once more.

No Direct Sunlight

Moon cactus prefers bright shade and only requires 4-6 hours of indirect sunlight. I wrote a complete guide on moon cactus lighting with even an image of a room!

You can do nothing to a severely scorched moon cactus but move it away from the direct sun to avoid further burns. However, if you notice that your moon cactus under intense light is losing color, you can still save it.

Losing of color is an early sign that your moon cactus is gradually getting scorched. The original vibrant color turns pale. Save your plant from the intense sunlight by transferring it to a shaded area and let it recover. I will not guarantee that it will return to its original color. But at least, it still has its color though a little paler.

The brown spots on the moon cactus are caused by fungal disease, insect infestation, and animal bites.

Water Only When Dry

Moon cactus hates wet soil. The best way to avoid excess moisture on your moon cactus is to use the soak and dry technique.

The first thing to do if you notice an overwatered moon cactus is uprooting the plant. Examine if either the roots or the base are rotten. If the rootstock is rotten, cut off the mushy part. Then let the cut heal and replant to a well-draining soil.

But if the rootstock is healthy, and only the roots were affected, just cut off the roots and air dry for a day or two. Once completely dry, trim off the dead roots and re-pot on a bone dry and well-draining soil.

To avoid overwatering, let the potting mix dry out between waterings. How to tell if it’s dry? Stick your finger 3-4 centimeters below the potting mix, avoiding the roots (so ideally close to the planter rim), and feel the soil if it’s wet or dry to touch.

Apply Fungicide

Fungi are hard to remove, and fungicide is the only way to eliminate them.

Moon cactus with fungus infection has a high risk of dying if not treated as soon as possible. They will most likely infect other nearby plants through their spores being dispersed in the air.

To avoid this from happening, you need to act quickly upon the sight of fungus. Isolate the moon cactus right away and disinfect the spot where the plant is previously placed to prevent pathogenic organisms from spreading.

Chemical control is the quick solution to fungal diseases. Use a curative fungicide to stop the infection and kill the pathogens. Before using any fungicide, it is crucial to read the label and use proper protection and application.

This multi-purpose fungicide below is effective and works well on fungus and pests.

For severely infected plants, I recommend proper disposal of the whole plant, including the pots.

Inspect and Use Insecticide

Insects like mites and scales are minute creatures that the moon cactus must be monitored regularly. They multiply like crazy, and you need to get rid of them right away.

Moon cactus infested with mites and scales get weak easily as these pesky pests suck on their fluid and leave the plant dry and desiccated. Isolate your plant away from other healthy plants and remove any traces of these insects.

You may use a soft bristle brush or a pressure water spray to terminate the pests. In case of severe infestation, uprooting the plant and washing its roots will remove the insects. An effective insecticide will help completely eradicate the presence of scales and mites.

Always remember to use chemicals responsibly and with utmost care.

Transfer Plants Indoor

To avoid birds pecking a moon cactus, transfer the plant indoors. Also, do not place a moon cactus near foliages which are the favorite of worms.

Moon cactus that animals bite will suffer cosmetic damage. You may sprinkle sulfur powder or cinnamon powder on the wounds to avoid infection and avoid getting wet as it may cause rot in the area.

Can A Moon Cactus With Brown Spots Recover?

Recovery mainly depends on the extent of the affected area. Moon cactus with too much damage can be challenging to save, especially if the rootstock and the scion are both mushy. Saving is impossible as none of them can survive.

If only the rootstock is severely damaged, the scion can be saved by grafting it again to a new base. However, if the scion gets heavy brown spots, you can let go of it and keep the rootstock. If the damage is minor, the moon cactus will continue to smile. Just follow the above remedies and observe your plant if the reaction is positive.

Takeaway

1) Take immediate action if you notice brown spots in your moon cactus. Determine the cause of the problem and apply an appropriate cure.

2) Environmental conditions (light), negligence (excess water), and biotic components (fungus and Pests) can result in the browning of the moon cactus.

3) Moon cactus with brown spots can be saved as long as the case is minor.

Sources

“Problems and Pests of Agave, Aloe, Cactus, and Yucca,” The University of Arizona

“Texas Plant Disease Handbook,” Texas A&M AgriLife Extension

“Preventative and Curative Fungicides,” Alison Robertson and Darren Mueller Iowa State University

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