Cacti are amazing plants and are quite hardy. But even though cacti are hardy, some small mistakes can lead to getting diseases and pests. This happens to most cacti hobbyists, and today we will discuss all cacti diseases, how to recognize and treat them. Caring for cacti’s health is very important for them to live longer and look appealing.
Most cacti diseases are caused by bacteria, viruses, fungi and other microorganisms. It is quite hard to treat a cactus for pests and especially diseases, so it is the best practice to monitor growing conditions regularly. Prevention is always easier than treatment. Now, let’s look at the common cacti diseases and pests.
General tips on preventing and treating cacti for pests and diseases:
- Check your cacti’s roots every 2-3 months. Many changes take few months to show, and your cactus might be already suffering from pests/diseases. You can find out about many problems by looking at your cacti’s roots.
- If you use alcohol to treat any pests/diseases on a cactus, make sure to shield it from direct sun for 3-5 days after application. Alcohol and sun can burn cacti’s skin, so shielding it from the sun can help avoid phototoxicity.
- When treating your cactus for any diseases/pests, make sure to disinfect the pot and the soil (fresh one) before planting your cactus. Otherwise, there is a big chance of reinfection.
- Quarantine (isolate) your newly purchased or affected cacti. Newly bought cacti might be carrying pests and will cause infection of other plants. Same goes with infected plants – quarantining is a must.
- Don’t overwater your cacti, as this might cause many problems. Read about watering cacti correctly here.
Cacti diseases
Cactus rotting – dry and wet rot caused by bacteria and fungi
Your cactus might rot due to different causes – mainly bacteria and fungi, but can be also viruses. There might be few reasons that lead to cactus rotting. Rotting is a very common problem that new cacti hobbyists might face.
- First of all, if you live somewhere with cold winter, you will need to provide your cactus with rest period in a cold spot.
- What is more, you will need to water you cactus much less in winter. If you overwater your cactus in winter, it won’t use the water, and the plant is likely to start rotting.
- Another problem that can lead to rotting is underwatering your cacti in spring and summer. Yes, this can lead to dry roots and your cactus won’t grow throughout the active season. This will lead to poor root system in your cactus and it is unlikely to survive winter and will rot.
- Your cacti need rocky and well draining soil + a pot with drainage holes. Don’t fertilize the soil with manure or other fertilizer with high organic content. If cacti’s soil remains wet for prolonged periods of time, it will cause rot. Stagnant air, high humidity and low temperatures can cause slow water absorption. Read about making the cacti soil here.
- When repotting your cactus, be careful not to damage any roots and the stem. Any small cuts on the roots or the stem might cause rotting. Read on repotting cacti here. A very important rule is not to water your cactus before and after repotting for up to a week!
- If you accidentally cut your cactus, its roots or cause any mechanical damage resulting in open wounds, use activated charcoal like this to disinfect the wounds. Otherwise, it can cause bacterial and other infections.
- Drying your cactus after washing roots is a must. Hang your cactus vertically or place it in an empty pot.
How to spot if your cactus is rotting?
Rotting can be a slow process if it’s a dry rot, and will be much faster with soft rot. If you notice that your cactus has not grown through the active growing season and doesn’t respond quickly to waterings – there might be a problem with roots.
You should also be alarmed if you notice drying out of the stem. Most rotting processes start from roots and work their way up. Regular checks of cacti roots can help you spot rotting early. Wet or soft rotting (black or white spot on the roots and stem) progresses faster than a dry rotting.
Soft rotting will feature wet roots and even stem, with shriveled stem and black/brown spots. A dry rot looks more like a brown spot that spreads through the stem. With a dry rot, the stem becomes dry from the inside, and it is usually hard to notice early.
There are few types of fungi and bacteria that cause cactus rotting. For example, Phytophtora cactorum fungus causes a rot to the roots and the plant stem. Rot progress quickly, so you need to act fast. This type of rot has a ‘classic’ look – wet and black roots and dying of the plant. Your cactus is likely to get this infection if you leave any untreated cuts on the plant.
Dry cactus rot caused by fungi
Diplodia, Phyllosticta concava and other fungi cause black spots on the cacti, leading to dry rot. You can also notice prominent brown/rust-colored spots (causing dryness on the cactus) often surrounded by brown rims, all of which also mean rotting of the cactus. Act fast and remove affected parts of the plant.
Otherwise, you will notice the black/brown spots get larger. If you can’t cut affected parts, then quarantine and the best thing is to get rid of the cactus completely. You can also try fungicides, but they usually fail to treat the plant fully.
Soft cactus rot caused by bacteria
Wet, or soft rotting is usually caused by bacteria in the genus Erwinia. Wet rot usually follows any damage or cuts to the cactus, and that is how bacteria enter the plant. The main symptom is softening of the stem, which can also become dark in color, coupled with black spots. You will need to cut off affected areas to treat your cactus.
Yellowing of a cactus
You might also notice yellowing of your cactus, and there might be few causes of this. First of all, yellowing of a cactus might be a symptom of nutrient deficiency. If you notice yellowing of the cactus, which often affects top parts of the plant, use some fertilizer to feed the plant.
But what is more, yellowing might be the symptom of viral/bacterial disease, and is usually called ‘cactus jaundice’. There is no treatments for cactus jaundice, which leads to rot and dying of the plant. If fertilizers don’t help, it is probably the jaundice. Affected cactus can die in few months or few years.
How to prevent and treat cactus rotting
To prevent cactus rot, make sure to follow the tips mentioned above (mainly watering and winter care). What is more, make sure to spray/mist your cactus once a day in the evening (during its growth period). Cactus is mainly opening its stomata at night – to reduce water loss. Misting the plant will remove any dust, prevent any mold and help your cactus breathe and evaporate water.
Check your plant’s roots once a month to make sure there is no rotting. If you see your cactus rotting, you can still save the plant if the infection hasn’t spread heavily.
If the roots have become black and thin or with any type of rotting, you will need to clean the cactus, remove dead roots and cut off affected parts. For this, you will need to sterilize shears or scissors (use fire or rubbing alcohol), and cut all the dead roots and all rotten parts, even up to a stem if needed.
When cutting, make sure there are no black or brown spots on the stem, and keep cutting until you are left with clean, green stem. Then, wash your cactus and its roots in a warm water and make it a warm bath.
Corking of a cactus vs. sunburn (phototoxicity)
Corking of a cactus can look similar to a burn. While corking is a natural process of an aging cactus, sun burn is negative for your cactus.
Corking will look like brown cork around the plant’s base, and there is nothing to worry about. If it’s corking from the top, there is another problem, such as mites. Corking can also be a result of improper conditions and care.
Sun burn is caused when you let your cactus face direct sun light straight after rest period, without shading it beforehand. Same happens when you use any pesticides or alcohol on the cactus and let it sit under the sun. Too much sun will burn plant’s epidermis, preventing it from breathing and growing. Any burnt areas become brown and can’t be treated.
Please note that if your cactus becomes purple or red, this can be normal if you have prepared your cactus for direct sunlight. This can happen in prepared cacti or if your cactus has been experiencing cold winds (which means that your cactus is too cold).
Underwatering/overwatering cacti
Both underwatering and overwatering cacti is a big problem and can kill plants. Many new hobbyists make these mistakes, and end up kill the cacti.
Overwatering: all cacti are succulents and they store water in their cells. If you overwater your cactus many times (especially need to be careful in cold weather and winter), it will start rotting. You can also cause rot or rupture of cacti cells.
To treat overwatering, you need to take out the cactus out of the pot and let it and the soil dry out before potting it again. Give your cactus roots a warm bath and hang it vertically to dry for 3-4 days before repotting. If it started rotting, cut the affected areas and use the fresh soil for repotting. Some cacti can burst if overwatered, so you will need to wait for the scar to heal.
Underwatering: There is a big myth that cacti don’t need water, but that is not true. If you don’t water your cacti, especially during the growth period, it will shrivel and go dormant to survive. In this case, don’t expect any growth or flowering.
Make sure to water your cacti right throughout the year. Read about watering cacti here. Many underwatered cacti lose their roots, so you will need to water them with little water and increase amounts gradually. Make sure that your cactus dries between waterings.
How to treat sun burnt cactus
While corking is natural, you will need to treat sun burnt cactus. If you notice that your cactus becomes weak and brown, take it to the shade immediately, spray it with water and cover with a small bag. Leave it there for few days and take it out. Shield your cactus from direct sun and prepare it for summer. Read more about sunlight requirements and preparing your cactus for summer here.
With mild burns, your cactus will continue growing and will have to grow out of the burn. The brown spot will remain and heal. But with severe burns, your cactus might stop growing at all and die. Make sure to increase time in the sun gradually, especially after the rest period.
Frost damaged cactus or damage caused due to cool temperatures and wind
There are different reasons why your cactus might get sick due to cold weather or water. For example:
- You have left your cactus on a cold windowsill
- There is conditioning in the room and your cactus has been cold
- You have watered your cactus with cold water. Please remember that you need to water your cactus with warm water (read about how to water cacti here).
Your cactus might also become ‘sick’ if it experiences cool weather and watering, which can cause rotting. Anyways, you will notice if your cactus has become frost damaged – it will shrivel and turn dark brown to black after few days of frost.
Rotting in case of cold weather or water will have classic symptoms. Not all cacti can survive cold temperatures, and some can only survive them for short periods of time.
There is nothing you can do to treat affected areas of damaged cactus – you will need to change the conditions immediately and wait for your cactus to ‘grow out of it’.
Pests affecting cacti
Red spider mites and cacti
Your cacti might also become infected with mites. Red spider mites are ones that can attack your cactus and kill it by sucking its juices. Spider mites are very small and red in color.
They can live and multiply in the soil all year round. You might not see spider mites, but they will also create small webs around the plant. Your cactus will start becoming dry and brown, because mites will drink the cactus juice from the stem.
Affected spots will have a rusty brown color. Spider mites like attacking fresh parts of the plant, which is usually from the top. This can look like corking of the cactus, so you can use a magnifying glass to examine the cactus for mites.
Cactus will need to grow out of the infestation, and after you have treated it for red spider mites, it will have fresh growth from the top and will remain brown part on the bottom. Mature red spider mites can be yellow-greenish in color, but also red (especially those that are brumating).
How to treat your cactus for red spider mites?
Red spider mites reproduce quickly in dry areas with stagnant air and high temperatures. They hate higher humidity and fresh air, so make sure to have fresh air in the room. As they reproduce quickly, you need to quarantine your plant. Check all the plants with a magnifying glass and transfer affected ones to a different room.
Then, start treating your cactus or cacti. Length of the treatment will depend on the season and how affected the plant is. You will need to get a pesticide or arachnicide to kill the spider mites.
Most insecticides won’t work as mites are not insects, so make sure to pick one that targets spider mites, such as this pesticide. When treating your cactus, make sure to shield it from the direct sun to avoid burns. In summer, when it is hot, you will need to continue the treatment for a bit longer.
Mealybugs on a cactus
Mealybugs are very common pests that attack different parts of the cacti – roots, stem, spines etc. They reproduce quickly and will suck its juices. If untreated, they will kill your cactus. Mealybugs on a cactus will have a white residue or spot look that can also look ‘fluffy’. As soon as you notice this fluff, start the treatment immediately. Otherwise, your cactus will become weak and die.
How to treat cactus for mealybugs?
Mealybugs will reproduce quickly, so act as soon as you notice them. You will need to quarantine your cactus and spray it with pesticide until the plant is free of them. The ‘fluff’ on mealybugs protects them from pesticides, so you will need to start the treatment by water spraying the cactus with high pressure spray, to physically remove them.
What is more, you can use cotton buds dipped in alcohol to physically remove them. Alcohol will dissolve their outer layer and will kill them quickly. After spraying the plant and removing mealybugs physically, use pesticide spray.
If your plant is weak and has stopped growing, you might check its roots – some mealybugs hide in the soil and affect roots. You will see the same white ‘fluff’ on the roots. In this case, you will need to wash the roots in warm water (gradually move to hot water), then dip the roots in the pesticide solution and repot your plant.
Make sure the roots are dry before potting the plant. If roots are affected, try giving a few hour warm-hot bath and drying your cactus by hanging it vertically before repotting. Make sure to disinfect a pot and use fresh soil.
As mealybugs are very common pests, you can use a systemic insecticide to prevent any mealybug infestation at the start of the growing period.
Aphids on a cactus
Aphids are small green bugs, but sometimes they can be grey or yellowish. These small bugs are often found in groups and suck the juices out of cacti’s stem and kill the flowers. Aphids are usually seen with ants.
How to treat a cactus for aphids?
It is possible to kill aphids without using any chemicals. Start the treatment by quarantining your new or affected plants. Use a high pressure water spray and try to remove all the bugs from the cactus. If spraying the cactus with water doesn’t help, you can make a DIY solution at home at clean your cactus with it.
To make it, mix some soap with water and spray your cactus with it. If nothing helps, you can use an insecticide. You can also rub your cactus with rubbing/methylated alcohol.
Cactus scale
Scale can attack many types of cacti. Scale insects infect cacti and start sucking out the juices. They move in colonies and also have a whitish coating, similarly to mealybugs.
Scale bugs multiply very rapidly and usually cover specific spots on the cactus, sucking the juices. These bugs move around very slowly, and it can seem like they are still.
Scale bugs are brown in color (females have white coating). They can also be yellow, and there are many other types of scale bugs. Affected areas of cacti will become dry and have white or yellow spots. The whole plant will have a white fuzzy look, but if you look closely – bugs will be most probably brown, covered in white fluff.
Sometimes you can mistaken the rotting for scale bugs. To check that, use tweezers to dig under the brown spot. If you can see a wet residue but the skin remains untouched- these are scale bugs. If the infection is under the skin and you can’t remove it – these are fungi or bacteria.
How to treat a cactus for scale bugs
Scale bugs multiply very quickly, as well as suck plant’s juices and leave scars. That is why it is important to act quickly. Make sure to quarantine an affected cactus. First of all, start the treatment by spraying your cactus with high pressure water from a sprayer or a hose.
Your can also try to remove scale bugs physically by using tweezers or cotton buds soaked in rubbing alcohol like this. If you rub alcohol on the cactus, make sure to shield it from the sun for around 2-3 days after application. Otherwise, it will cause burns, also known as phototoxicity.
Scale bugs tend to come back easily, to repeat the treatment until they are all gone. Also, at the beginning of the growing season, you can use a systemic insecticide to prevent scale bugs, especially if your cactus had them before.
Nematodes on a cactus
Nematode bugs are very parasitic and affect the plant’s health drastically. They hide in the soil, so owners usually find out about the problem quite late. This is why it is important to check your cacti’s roots from time to time, to make sure they are healthy. Nematodes are very small bugs and are not visible to an unaided eye.
The symptoms of nematode infection is the slow growing cactus and changes in the stem color. Cactus becomes weak and loses its vivid green coloration. And when you look at the roots – they will look swollen (also called galls), thick and often brown because rotting has started to progress.
Nematodes hide in the roots and block the movement of water and nutrients to the cactus, making it weak. Another way to check for nematodes is to soak the roots in the jar with water – and if you notice small black dots left on the walls of the jar, this indicated nematodes.
How to treat a cactus for nematodes
Reinfection is very common with nematodes. This is why you will need to get rid of the soil and disinfect the pot with boiling water and soap.
As nematodes are not insects, no insecticides or pesticides will help to get rid of nematodes. You will need to cut all the affected roots on the cactus to help it get better. Make sure to cut the roots with disinfected shears or scissors.
After cutting the roots, put some ground charcoal on the plant and pot it in a fresh soil. Before potting your cactus in the fresh soil, you can give it a bath with water temperature of around 131 Fahrenheit (55 Celsius) for 20 minutes, and drying it by hanging vertically for 4-5 days.
To prevent nematodes, make sure to quarantine new plants and check them before adding to the collection. What is more, you must disinfect the soil before using it for a cactus. Read about making and disinfecting the soil for cacti here.
Thrips on a cactus
There are thousands of thrips types, but not many affect cacti. Thrips are usually either green or grey-black in color. One of the thrips that can affect your cacti is called Heliothrips haemorrhoidalis, also called greenhouse thrips or black tea thrips. Thrips are very small in size, around 1.5 mm in length and can live all year round in the soil of indoor plants.
How to treat a cactus for thrips
Thrips are very small and will be problematic to kill. The best solution for treating a cactus for thrips is to use cotton buds soaked in alcohol, removing them manually.
This is not likely to remove all of them completely, so you will need to use pesticides and systemic insecticides during the growth season to get rid of the thrips. Make sure to dilute pesticides greatly to avoid burning the plant.
Fungus gnats on cacti
These fungus gnats are small black flies. They look similar to common flies, but are much thinner and smaller in size. Adult fungus gnats are harmless and don’t cause any damage to your cactus. But you should be careful, because gnats’ larvae in the soil are ones that cause damage to your cacti.
Fungus gnat larvae look like white worms with black heads, and are around 4 mm in length. Larvae feed on organic matter in the soil as well as roots of the cacti. Because of fungus gnats, cactus stop flowering, becomes weak and stops growing.
How to treat a cactus for fungus gnats?
As fungus gnats enjoy living in the moist soil, you will need to dry the soil a little. Don’t use any manure or other fertilizer with high organic matter. To kill adult fungus gnats, use sticky traps that you can hang around the cactus.
To get rid of larvae, you will need to either dry the soil + use an insecticide, or repot your cactus into fresh soil. Avoid overwatering your cacti.
Woodlice/slugs/snails/caterpillars/worms on a cactus
Snails as well as slugs are small molluscs that are mainly active at night. There is a small chance that you will get snails or slugs indoors, because they prefer staying outside to survive and to avoid drying out.
You will know that your cactus was damaged by snails or slugs if you notice a slime lines across the plant. Snails and slugs mostly enjoy eating flowers, seedlings as well as fresh grown parts of the cactus.
Woodlice also become active at night and enjoy eating fresh parts of the cactus. Caterpillars and worms often affect prickly pear cacti, hiding in the plants and laying their eggs on a cactus – which later hatch and damage the plant.
How to treat your cactus for woodlice/slugs/snails/caterpillars/worms
Make sure to check around and under the plant to find any snails and slugs. To kill them, you will need to physically catch and remove them by hand. If plants are outdoors, you can come out at night and look for them – they are very likely to come out when the sun goes down.
Another good way to get rid of snails and slugs is to put some bait around the cactus or directly in the soil. Make sure to use a non-toxic bait, such as this Garden Safe Slug and Snail bait. Same goes with worms, caterpillars and woodlice – you will need to catch and kill them or use a bait.
Mice and birds attacking cacti
Some mice and birds might attack and eat your cactus. This is very common, and many rodents and birds are not afraid of cacti spines. Mostly, only outdoor cacti suffer from bird or rodent attacks, and this might happen when you are not looking. If a big piece of your cactus is gone, or you can see multiple small holes overnight, this is most probably an animal attack.
How to prevent rodents and birds from attacking your cacti
The most disappointing thing with rodent or bird attacks is that you won’t be able to bring your cactus back to its normal condition. The only thing that you can do is to fight them by covering the cacti or setting up traps & poison. With poison, be cautious if you have any pets or children.