Although most succulents are hardy plants they are still prone to pests and diseases. Because of the large, water filled leaves and vibrant colours damage caused by various factors can become very noticeable.
But what does cause brown or black marks/ spots on succulents?
There are many causes to brown/black spots on succulents. Pests such as mealy bugs, aphids or slugs, fungus, rot, burns from the sun, warts from too much water, the equivalent of frost bite, any puncture marks or scratches.
It may be hard to identify what brown/black spot is caused by which factor. We have seen them all at our small plant nursery and will try our best to provide good photos and descriptions for each factor.
Pests that feed on succulents range from tiny aphids to bigger snails, slugs, caterpillars, grasshoppers to larger animals such as mice, birds, possums, kangaroos or deer. Large animals will take big chunks, but small insects and pests will only make tiny marks in the foliage and stalk which will end up darkening in colour and becoming brown or black as the plant heals (like a scab on a human body).
On a succulent the ‘scab’ is there permanently. The only way the plant will loose it is to grow out of it.
The worst culprits for causing black/brown spots on succulents are aphids, mealy bugs and young slugs and snails.
Aphids and mealies also secrete a sticky sap that can sometimes stick to the plants worsening the look. To make things even worse they tend to hide in the most tender growth (new leaves). By feeding on the new growth they make marks into something that is going to grow bigger and take longer to grow out of.
So what to do about small pests making ugly marks on your succulents? Unfortunately it is pretty much inevitable that at some of these little pests will attack your succulents at some stage.
They are extremely widespread and fast to invade. The most important thing is to act fast and not leave them, hoping they will eventually disappear. Even a day or two of delaying can mean many more hundreds if not thousands of aphids/mealy bugs waiting to hatch and unleash on your plants.
Aphids are quite easy to kill by using pyrethrum based sprays. If you notice them (they come in a green, brown, black and even orange) on your plants and there is not that many, they can just be squashed with a toothpick and washed off.
Isolate the plant from the rest until you are satisfied there are no more survivors. Large infestations may need to be dealt with pesticides. We would like to stress that a lot of pesticides can kill many beneficial insects too and this is not good.
Beneficial insects are struggling and we need to do what we can to protect them. Without beneficial insects and pollinators humans will have a tough time growing crops. Even small things like spraying after dark when the good bugs are no longer flying can make a big difference.
At the nursery I usually isolate the plants in a shed and spray there. Eco-neem or oil can be tried but so far it has not worked for us when quite a few aphids are crawling around.
Mealy Bugs are a tough one as they can become resistant to pesticides and have a woolly cover to protect them. They are also masters at hiding and can position themselves in the tightest of spots between leaves and stem, in curled up, dried leaves, under the lip of the pot, under the table, under any debris found around their food source.
They can lay their eggs (anywhere between 300-600) out of sight but close to the plants so when the little nymphs hatch, they don’t have far to crawl. In their early days, mealy bugs can be quite fast and can cover a fair distance, which will see them spread over several plants.
Controlling mealy bugs can be very hard. Early detection and isolation is best way to go. Also keeping your plant area clean, if your succulents are grown in pots.
When only one or two mealybugs are found, squashing with a toothpick/ thin skewer is your best option. The infected plant and any other surrounding plants will need to be thoroughly checked (pull leaves apart, take out of pot to make sure there are no mealy bugs on roots etc.).
Isolate the infected plant and monitor for a couple of weeks for any further outbreaks. If you have found multiple mealy bugs that are hard to reach/ there are too many to squash they can be sprayed with a 70% rubbing alcohol or isopropyl solution.
Each mealy bug will need to be doused and then washed off. We have tried numerous pesticides claiming to control mealy bugs but they seldom work and they are also horribly toxic to other insects as well as yourselves.
70% alcohol solution seems to do the trick, but may need multiple applications and monitoring as even this is not 100% effective. The majority of succulents are prone to mealy bugs and so if you have a nice collection with some expensive plants, regular checks and cleanliness are a must.
When talking about mealy bugs and aphids, we also have to mention ants as they are often a direct result of these pests appearing and thriving.
Ants spread and ‘farm’ these pests as they produce a sugary honeydew which ants collect. Ants also protect mealies and aphids (some scale insects too) from predators such as parasitic wasps or ladybugs. We have read some fascinating articles on ant behaviour and their relationship with pests here and here.
This relationship between ants and pests is however killing your (and our) plants. In our opinion ants are a pest as well and need to be stopped from reaching plants. This is harder than it sounds as ants always find a way and can infest pretty much any part of the garden very fast.
Chemicals will only kill the workers (and scores of other insects too). There are recipes online for borax and sugar lures which are meant to be collected by ants and brought over to the nest and fed to the queen (who produces all these gazillions of eggs under ground).
We have never had success with this or any other green solution. The best way for us is to keep the ground clean and spray any ants in our nursery area with a pyrethrum based spray to deter more from coming.
Slugs and snails will make little holes into succulents foliage that will then heal into a series of dark coloured marks. They can hide in tight crevices or under pots/rocks/moist dark spots and come out at night which makes them difficult to catch.
When it comes to slugs and snails the green elimination techniques work quite well. Our favourite is to bury a shallow dish of beer into the ground and the slugs will obligingly drink themselves to death.
Fungus, Rot & Water Warts
Fungus infection, rot and warts from over-watering in succulents are quite common, especially in humid and warm parts of the world. Many succulents come from dry and arid environments and dislike humidity. Even plants that are kept under cover and are not watered often can fall prey to a fungal infection if there is a lot of moisture in the air.
Fungal infections in succulents create black or brown, usually round spots that can grow to colonize the whole leaf which then tends to fall off and shrivel.
Sometimes there are only one or two spots per plant, sometimes many more. The spots can also develop a wart or kind of a scab. All of these need to be acted on by a fungicide which are easily obtainable from garden centres, online and even big grocery stores. Most fungicides can harm beneficial insects and so it is best to either spray in the evening when they are not active or make your own. We have found this recipe that is harmless and made out of ingredients found in most kitchens.
During high humidity seasons it is recommended that fungicide is applied every other week. Although limiting water in an already humid environment can be a good idea with some succulents, in others it can stress them too much and cause shrivelled leaves.
We would recommend that succulents get watered once a week when very hot. The potting mix should be allowed to dry between watering.
Do not spray succulents with water, this will not do them any favours as they are not able to absorb water through their leaves. It is best that water is administered around the root area in sensitive succulents.
Always use succulent potting mix or other well-draining mix in pot plants. Heavy potting mix that holds on to water will only make things worse.
Some succulents are more prone to fungus than others. We have had problems with many Echeveria, some Graptopetalum, Graptoveria, Pachyveria and Pachyphytum. Some plants in these genera such as Echeveria Romeo, Echeveria Purpurosum, Graptoveria Amethorum and Pachyphytum Fittkaui almost without a fail get fungus as soon as we’ve had a few consecutive days of rain, followed by hot and humid weather.
If succulents are kept under cover or a greenhouse, it is likely the fungal spots will be minimal or won't appear altogether. Curiously, some (but not all) sensitive plants such as the ones mentioned above do not seem to have fungus problems if planted in the ground. We do not know why.
If your plant is rotting and you notice the root, stem and leaves going black (sometimes mushy) there is not much that can be done. If the rot has not yet reached the top part of the plant and leaves are not affected, it can be cut off from the rotting bit and planted as a cutting.
Yes, unfortunately succulents can get sunburn. Different succulents have different tolerance levels to the sun, but most get some kind of a burn once the temperature starts climbing over 40C (104F).
It is important to remember that the weather forecast temperatures are shade temperatures. If you were to put a thermometer in full sun during the hottest part of the day, you’d record much more than what the forecast says.
This is what plants have to deal with, so there is little wonder that even tough plants such as succulents get burns. If a human spent an hour in such strong sun without protective clothing and sunscreen the burns would probably put them in hospital.
Our little nursery is west of Sydney and we regularly see temperatures climbing over 40C/104F which would cause burn marks on foliage of most of our potted succulents. Plants in the garden do remarkably well and have much higher sun exposure tolerance. We think this is because the roots can stay a lot cooler in the ground than in pots.
Succulents can react differently to extremely hot sun. Some may drop leaves, some can collapse in a heap of mush and some will get obvious dark coloured marks or spots. The marks will not disappear once the plant suffered a burn but it will, in time, grow out of the wounds.
To prevent burns pot plants can be moved to shade when heatwaves are expected. We pull a 30% shadecloth over ours. If pots are too hard to move, an umbrella or some kind of cloth can be pitched over. Same can be done for succulents in the ground. We leaves ours to deal with the weather but inevitably some do get dark marks as a result of sun damage.
Frost can have similar effects on succulents as the sun. Dark spots can develop on the foliage as a result of frost. If too severe, succulents will collapse.
To protect succulents from mild frosts a frost cloth can be placed over the plants. Apart from a few plants such as Sempervivum, succulents are not frost tolerant and so if you live in a country with cold winters that can see snow settle for several days, succulents may not survive outside even with a frost cloth over and should be brought inside until the danger of frosts passes.
Our plants stay outdoors as we very rarely get any frost and most succulents are ok if the temperatures do not dip below 0 Celsius (32F). Frost cloth is enough for us to get the plants through the coldest of winter.
To read more about how cold hardy succulents are, you may want to see this article.
Puncture Marks & Scratches
Just like tiny pests can cause brown/ black spots by feeding from succulents leaves scratching the surface of the leaves can result in dark spots forming on the plants. This can happen if something falls on the succulents, when the foliage is damaged while re-potting or through hail damage.
Some succulents are more prone to black/brown puncture marks and scratches than others. This is not always a rule but more frequently than not plants that have a good, thick coating of farina (the dusty coating on the leaves and stems) get marked more easily.
The prevention here is obvious- handle succulents with care and do not rub the leaves. Once the damage is done the only way the spots are going to go away is by growing out of them.
In conclusion, black or brown marks can be caused by different things in the succulents environment. Unfortunately, they do not heal and disappear, but will be covered over by new foliage.