Cacti are hardy, drought-tolerant plants, perfect for novice gardeners looking to add to their low-maintenance collections.
They come in all different shapes and sizes and are very easy to grow and care for. However, this does not mean that cacti do not have their share of common problems.
One of the most common problems observed by cactus owners is white spots appearing on the plant. If a cactus has white spots on it, the most probable reasons are fungal infections or pests.
In this article, we will discuss the common causes of white dots on a cactus and how to resolve each one. So, if you are experiencing this problem with your cactus and you wish to learn more, just keep on reading.
Why are there white dots on my cactus?
The first possible reason your cactus has white dots on it is because of fungi. This is commonly observed in cacti that grow in damp or humid conditions.
Cacti are desert plants, used to growing in dry, arid environments, so if they are kept in a wet or humid environment they are completely out of their element. These conditions are also conducive to fungal growth.
Mold will start to grow on the surface of the cactus and, the longer the infection is left untreated, the more damaged the affected areas will become. The parts that are covered in mold or fungi will also feel soft.
White spots caused by fungal growth may not kill your cactus, but they do alter the aesthetic of the plant, causing yellowing of the affected areas and stunted growth.
Other than mold, white spots on the surface of a cactus can also be powdery mildew. Powdery mildew also develops in wet and humid conditions, and the more the mildew spreads, the more the foliage turns yellow and becomes deformed.
If you suspect a fungal infection in your cactus, the first thing you need to do is to take the plant to a different room, away from your other plants. These infections are very contagious and quarantining the affected plant can avoid the spread of the fungus.
Use a sterile knife or pair of scissors to cut off the infected cactus parts. Dispose of these cuttings properly; you do not want to accidentally leave them near your other plants. Clean your gardening tools with rubbing alcohol after using them on the infected plant.
You can use a commercially available fungicide on your plant, but if you do not want to use chemicals, you can also spray it down with a neem oil solution.
Mix two tablespoons of neem oil into a spray bottle of water and spray the solution on the entire plant. Repeat the process once a week for as long as necessary until the cactus has recovered.
Pests that attack cacti can also be the source of white spots on your plant.
The most common bugs observed on cacti are scale insects and mealybugs.
At first glance, clusters of these insects will look like fuzz from mold. This fuzz is white and cottony, and it acts as a protective barrier for the pests.
Scale insects are covered in this white fuzz but they are actually brown-colored insects. They suck the sap from the body of the cactus, drying it out. These bite marks will also leave white or yellow spots on the surface of the cactus.
Mealybugs, on the other hand, are smaller than scale insects and also produce a white fuzz to hide under as they feed on the cactus’ juices.
Mealybugs take about two weeks to develop from egg to nymph, and after four weeks as nymphs they become mature mealybugs. It takes no time for these bugs to reproduce, and you might have a hard time catching the infestation in its early stages because they are so small.
If you do not keep a close eye on your cactus, the infestation can grow extensively and the foliage will become deformed and yellow.
If you suspect that your cactus has pests, take it to a different part of the house immediately so that the infestation does not spread to your other plants.
This is especially true for mealybugs, which are not discerning when it comes to the plants they attack and consume. They will happily hop from one plant to another to feed on them.
You can spray the above mentioned neem oil solution on the infested cactus, or even use it as a preventative measure to protect it from mealybugs.
If the infestation is spreading very quickly, repotting the plant can help.
If you are able to catch the infestation in its early stages, you can use a pair of tweezers to literally pick the mealybugs from the plant one by one.
You can also use rubbing alcohol to kill the pests. Take a cotton ball, soak it in alcohol, and wipe down the cactus. This process may take some time because you have to actually look for the bugs to kill them with the alcohol. You should also be careful not to prick yourself on your cactus if it has spines.
Check the base of the plant near the soil, because pests like to hide from danger in the nooks and crannies there. If your cactus has leaves, check under them as well as in the leaf joints because these are a favorite hiding place for pests.
For mealybugs, remove the top layer of the soil in the pot because they may be hiding where they often lay their eggs, which is in this layer of soil. Discard this soil properly and do not reuse it for other plants.
If the white fuzz produced by these pests is on the roots, you might need to wash the roots with water and spray them with pesticide before replanting the cactus in a fresh potting mix.
Wipe the plant down with rubbing alcohol every day for a week to make sure that all the bugs are dead. Try not to miss a day, because it only takes these bugs a short time to effectively reproduce.
Another homemade insecticide you can use against scale insects and mealybugs is a solution of one tablespoon of mild dish soap to a liter of water.
Take your infested plant to the sink or the shower and apply the soap and water solution on the cactus. Rinse the soap off with water.
Do not let the water stay on the foliage. Dry the cactus with some paper towel and place it in a pest-free spot where it can get lots of light.
Repeat the soap-and-water process every three days until you can no longer see any bugs on the plant.
Another method of getting rid of mealybugs is by using beneficial insects such as lacewings and ladybugs. This is a good option for cacti that are grown outdoors.
These beneficial insects can be bought from breeders that specialize in them, and having them around can help take care of smaller infestations before they become full blown.
How long should I keep a cactus quarantined?
Just to be sure, keep the cactus in isolation for at least three weeks after completing the treatment. This gives you enough time to observe whether there are any more bugs remaining on the plant. It is possible for pests to hide for a few weeks and then come out of hiding to reproduce, so three weeks after treatment is a long enough period to catch any bugs that might have survived the treatment.
Cacti are low-maintenance houseplants that come in all shapes and sizes. They are great plants for novice gardeners who are scared of plants that require too much attention, because cacti can survive drought and neglect for weeks.
This does not mean that cacti are invincible, however. These plants have their share of problems, one of which is the appearance of white spots on their foliage.
The most probable causes of these white spots are fungal infections or pests.
You can treat fungal infections by cutting off the infected parts of the cactus and wiping it down with rubbing alcohol or spraying it with a neem oil solution.
For pests, you can use a soap and water solution, rubbing alcohol, neem oil solution or beneficial insects.
Image: istockphoto.com / Engin Ozber