Does your cactus develop rusty or orangey spots that look nasty and menacing as it spreads through the entire stem? Do not fret, as I will help you identify, eliminate and control them.
The rust or orange spots on the cactus are caused by 1) rust fungus or 2) red spider mites or 3) fungal scabs. Rust spots must be removed or controlled by using :
- neem oil solution,
- sodium bicarbonate, or
- organic fungicides.
Although cacti are tough, they are still vulnerable to diseases and harmful insects. I will discuss what causes a cactus to develop rusty spots and how to take care of them.
Rust or orange spots on cactus are caused by 1) rust fungus, 2) red spider mites, and 3) scab. If left unchecked, the rust will kill the cactus, infecting other neighboring plants.
The appearance of rust spots may differ depending on the cause. They vary in texture, color intensity, and size. Some rusts appear as spots from the beginning and gradually become more prominent as it spreads through the epidermis. Here are the reasons why a cactus develops rust spots.
Rust fungus is caused by the Puccinia fungus species. The spots appear as irregular light brown patches with orange recessed areas on the cactus epidermis. The fungi’s growth on the plant is triggered by warm temperatures and low air circulation in a moist environment.
Rust fungus requires specific conditions before infecting and spreading. This fungus is triggered by warm temperatures and low air circulation in a damp environment. The fungal spores attach to the moist cactus and start germinating during these settings. Acidic soil hastens spore development.
The first contamination in a cactus is typically caused by fungal spores blown in by the wind or carried by insects such as mealybugs and aphids. But established spores may disperse during watering, infection through plant touch, and via the use of contaminated instruments.
Rust fungus is contagious and may infect others if not treated right away.
Red spider mites cause the tiny orange dots on the cactus stem here they puncture the epidermis and feed on the plant’s juice. They thrive in a dry and warm environment. These arachnids are not conspicuous but their existence on the plant is indicated by messy webs.
As they start building colonies, red-orange blotches become visible and multiply throughout the plant. Red spider mites are minute arachnids that you cannot identify through your naked eye. Their infestation is sometimes mistaken as a fungal disease, but looking closely, you’ll notice their movements.
If you’re unsure what might cause the orange spots, look for messy webs around your cactus stem. The presence of white strands indicates that the red spider mites caused them. Usually, they start in the folds and crannies on top of the cactus stem, where there are soft tissues.
A scab is a superficial rusty patch on the cactus skin. The rust color is due to fungal spores forming just below the waxy epidermis. Its texture resembles a hardened flat bump that can be peeled off.
A scab is initially a form of edema where cactus cells increase in size due to too much moisture and poor ventilation. As the cells are filled with excess water, they will eventually disrupt and leave a microscopic opening on the stem that serves as a threshold for the fungal spores.
As the fungus develops, it infects the surrounding cells and becomes patches of scabs. Although edema is not contagious, the fungus that inhabits inside the wound has orange spores that are dispersed by the wind and may infect other plants.
Rust spots on cacti are permanent unless the affected portion is cut off. However, they can be controlled and prevented using DIY remedies such as 1) neem oil solution and 2) sodium bicarbonate. Fungicides containing 1) sulfur and 2) copper can also help curb fungal disease.
Let’s discuss how to apply these remedies and how they work.
Using a sterile cutting tool, cut off the portions affected with fungal rust and dispose of them properly. Peel off fungal scabs with a disinfected knife. After cutting or removing the infection, sprinkle the wounds with cinnamon or sulfur powder to inhibit pathogens.
Even though trimming the affected parts may leave scars on the cactus, it promotes healthy new growth and prevents fungal spores from spreading. To avoid future outbreaks, always sanitize your surroundings, get rid of dried leaves and damp surfaces, and regularly check your plants.
Under severe infection, I recommend getting rid of your cactus so that other healthy plants will be spared from the disease.
Mix 2 tablespoons of cold-pressed concentrated neem oil in a gallon of water and stir until the solution is homogeneous. Apply the solution by spraying the affected area and the soil. Use only during sundown or temperatures below 30 degrees centigrade to avoid scorching.
The azadirachtin content in neem oil acts as a pesticide for red spider mites and prevents fungal spores from germinating and reproducing. Once red spider mite nymphs ingest neem oil, they will lose interest in feeding and finally die.
Moreover, studies show that neem oil interferes with the hormones making adult mites unable to lay eggs and become infertile. Although neem oil does not directly kill spider mites, it does prevent them from developing and propagating, eliminating the infestation without affecting beneficial insects.
Make your DIY neem oil solution using the product below.
Only take this route if the disease is not controlled and has become too much of a problem. Effective fungicides are those that contain sulfur, copper, and permethrin.
Sulfur can be applied as a liquid or dust fungicide. The recommended application in liquid form is to mix 3 tbsp of sulfur powder in a gallon of water and spray to the rust spots. Re-apply at 14-day intervals until the infestation is eliminated.
Direct application of powder sulfur is an effective method in controlling fungal scabs. Simply dust off the affected spots with sulfur powder. When fungal spores contact and ingest sulfur dust, it disrupts their cells and kills them. Sulfur also repels pest mites due to its strong odor.
Below is a ready-to-use sulfur fungicide.
Copper fungicide is the most effective pesticide in controlling fungal rust on cacti. It comes in a ready-to-use spray bottle. But always read the back label before using any fungicide and use protective gears upon application.
Apply copper fungicide to prevent spores from further reproduction during early signs of rust infestation. It works by disrupting the pathogen cells on the cactus epidermis.
But when the infection enters the cactus’ tissues, copper will no longer have an effect. Below is an excellent copper fungicide to control rust on your cactus.
Aside from the ugly appearance, rust can also hinder growth as the cactus loses its vigor. It can reduce the photosynthetic activity of the cactus due to the patches of rust covering the plant stem.
Cactus rust due to red spider mites need immediate action as they can reproduce rapidly and spread from plant to plant. Fungal spores are readily dispersed, so chances of infection are high. If you notice the early stage of infestation, isolate the plant right away and apply a suitable method to control the disease.
- Rust spots on cacti can be due to rust fungus, red spider mites, or fungal scabs.
- Rust disease is not curable but controllable. The best method to eliminate rust is by removing the infected part. Other than that, it can only be controlled using the following ways: use of neem oil solution, sodium bicarbonate solution, and organic fungicides such as sulfur and copper.
- Although rust disease won’t kill the cactus right away, it can reduce plant growth and vigor due to lower photosynthetic action.
- “Spider Mites Management,” University of California Agriculture and Natural Resources
- “Horticultural oils- What A Gardener Needs To Know,” University of Nevada
- “Neem Oil Antifeedant And Insecticidal Effects On Oebalus Poecilus (Hemiptera: Pentatomidae) Males And Females,” by Patricia Valle Pinheiro, Eliane Dias Quintela, Pennsylvania State University
- “Neem Oil,” Oregon State University
- “Baking Soda,” by Linda Chalker-Scott, Ph.D., Washington State University
- “Lime-sulfur: A fungicide (lime-sulfur),” by Don Janssen, University of Nebraska Lincoln
- “How Copper Sprays Work and Avoiding Phytotoxicity,” Cornell University
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