Cacti are great plants for busy plant lovers. They can be grown with little hassle. Cacti and Succulents are often low maintenance because they need less care, water, and food compared to many other houseplants. But they do have few issues, and the prominent one is the sudden appearance of white spots or web-like spots.
What Causes White Spots on Cacti?
In most cases, white spots on Cacti are caused by a pest infestation. The white spots are protective shields of mealybugs or scales. Another reason for white spots on cacti can be fungus growth due to powdery mildew or the necrotic spot virus. Treat white spots as soon as you spot them and you have identified the cause as yellowing and abnormal foliage is often following after white spots emerge.
Pests
Indoor Cacti plants are highly vulnerable to cochineal insects such as mealybugs and scales. Most plant owners mistake the soft and cottony white fuzz for mold. But the white fuzz acts as a shield for the pests.
Scales are brown insects covered in white fluff. They feed on the sap, and as the Cacti dries, it develops yellow or white spots.
Mealybugs are small, slow-moving pests that measure between 1/20 to 1/5 inch (0.13 – 0.5cm) only. They happily feed on your Cacti under the white fuzz.
Let’s go through the growth cycle of these bugs to better understand the timeline of a mealybugs infection on Cacti.
Mealybugs have a lifecycle of about ten weeks (maximum). The eggs take anywhere from one to two weeks to develop into nymphs. From the sixth week onwards, the nymphs mature into fully grown adult bugs.
The short lifecycle is the reason mealybugs can spread fast on indoor Cacti plants. They can easily hide in a spot for several weeks and continue to feed and multiply.
Initial symptoms of a mealybugs infections include twisted foliage and yellow leaves.
Fungus
Fungus rarely causes white spots on a Cacti plant. Fungus usually grows if you keep your Cacti in very humid or wet conditions. The area under the fungus mold will be soft.
White spots are not detrimental, but they cause stunted growth, yellowing, and deformed foliage.
The white fuzz can be powdery mildew. Cacti are infected with Powdery mildew because this fungus thrives in the same environment. As the fungus spreads, the leaves will deform, and the foliage will turn pale yellow.
Another reason behind white spots is the white-rot or impatiens necrotic spot virus. This virus causes wilting, spots, and decay in Cactus plants. The color of the spots can vary depending on the species.
Treating White Spots Due to Pests
Most pest infections on Cacti are noticed after they have caused immense damage. The first step is always to quarantine the infected Cacti.
As mealybugs are not picky, they can spread and start feeding on any plant growing nearby. I would suggest spraying the other healthy plants with neem oil as a precaution.
If the infection is spreading fast, you should consider repotting your Cacti. For a small population, get rid of the mealybugs by destroying them with hand using tweezers.
There are several methods to deal with pests on Cacti, but the two common methods are discussed below.
Isopropyl Alcohol
- Dip a cotton ball in the alcohol and dab it on bugs that are clearly visible on the plant. This method is time-consuming, but it immediately destroys the insects.
- You should also inspect the hidden areas on your Cacti, such as the base of the plant, the undersides of the leaves, and leaf joints.
- The mealybugs can even survive in the potting soil. I would suggest removing the upper soil layer to check for mealybugs hiding under the soil.
- If the roots also have white fuzz, wash them with lukewarm water and dip them in pesticide. Dry the roots and repot your Cacti. Check the bottom and sides of the container as well.
- The treatment using alcohol should be repeated every day for 1 week to ensure all the bugs are killed. Irregular treatment allows the bugs to multiply.
- However, test a small spot on your Cacti before using isopropyl alcohol on the whole plant. Never apply a lot of alcohol directly to the plant.
DIY Insecticide
- If you do not want to spend time killing each bug on your Cacti, you can try the homemade insecticide. It is a quicker solution to treat the mealybugs or scales. However, please know that the insecticide can be harsh for your Cacti.
- Create the insecticide by mixing 1 teaspoon of liquid soap in 1 liter of water.
- Take your plant to the shower or sink, rub all the infected areas with the homemade insecticide. Now rinse your plant with cold freshwater.
- Do not leave the water sitting on foliage. Dry your plant before moving it to a bug-free location.
- Repeat this treatment every 2-3 days. If you notice bugs roaming on the plant during that period, treat them with alcohol since it’s gentle to use.
- Inspect your plant daily because even after rinsing the plant, some bugs can hide in joints.
- If the infestation is huge, you should use a combination of neem oil, alcohol, and insecticidal soap. Thoroughly wash your gardening tools and hands after handling an infected plant.
To treat mealybugs on outdoor Cacti, you should use the help of beneficial insects like ladybugs or lacewings. You can get these insects from specialist breeders of beneficial insects online.
For small infections, these insects can easily eliminate the bad bugs without harming your plant. For heavy infections, you should use insecticidal soap or insecticide that contains acephate or bifenthrin.
Depending on the population of mealybugs, you will have to continue treating your Cacti for weeks or months.
Treating White Spots Due to Fungus
Start by removing the infected areas. Treat your plant with a milk (1 part) and water (3 parts) solution. Spray this solution on the infected parts using a spray bottle.
Initially, repeat the application every 2-3 days later, repeat after 14 days.
What to Do if the Pests Keep Reappearing
If the bugs keep reappearing even after the above treatment, you have missed a hidden spot on your Cacti. You should do the following
- Replace the top inch of the soil with fresh potting soil.
- Relocate your plant because the environment at the current spot might be helping the bugs thrive.
Frequently Asked Questions about White Spots on Cacti
How long should I quarantine infected Cacti with white spots?
I would recommend isolating cacti for at least 3 weeks, even after the treatment is over. This helps you protect other plants. You can also identify bugs faster if they return when the issue of white spots was caused by a pest infestation.
Should I sterilize the pot after treating Cacti with white spots?
If your plant was potted in a terracotta pot, you should sterilize the pot because the pores in the pot can make a home for the pest eggs. Create a solution using bleach and water for sterilizing the pot.
How can I ensure pests are causing the white spots on my Cacti?
Take a gardening tool and scrape the white stuff from the stem or the leaf. Make sure you scrape gently without injuring the foliage. Squeeze the fuzz if it produces red juice. Your plant is facing a mealybugs infection that needs immediate treatment.
Daniel has been a plant enthusiast for over 20 years. He owns hundreds of houseplants and prepares for the chili growing seasons yearly with great anticipation. His favorite plants are plant species in the Araceae family, such as Monstera, Philodendron, and Anthurium. He also loves gardening and is growing hot peppers, tomatoes, and many more vegetables.