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The one thing that I really want to impress on you is that bacterial infections in house plants are best thought of as being a symptom that your plant has an issue rather than a problem you need to solve.
Oeople ask me to help them treat black spots on their house plants that they believe to be bacterial infections.
Whilst they might not be wrong, we need to sort out what caused the bacterial infection.
Only treating the leaf spot means that the problem will keep reoccurring.
I usually say that bacterial infections in house plants aren’t common.
That isn’t strictly true, but I want people to investigate a little further, and find out why their plant succumbed to a bacterial infection in the first place.
What is a house plant bacterial infection?
There are dozens (probably hundreds) of bacteria that can infect house plants, the most common probably being Phytophthora, which is the main cause of root rot.
However, as with humans, many of these bacteria are often present in the soil in mall quantities, and will only multiply and cause issues if the plant is weakened or given incorrect care.
As well as root rot, we also have:
- Fungal leaf spot
- Bacterial leaf spot
- Sooty mold
- Powdery mildew
Symptoms of bacterial infections in house plants
Infections like sooty mold and powdery mildew are fairly easy to diagnose because you can see the mold on the leaves. Powdery mildew tends to be white/grey, sooty mold is black. If the leaf has powder than can be rubbed off, then you likely have a bacterial infection.
However, leaf spots can be far more difficult to diagnose.
One of the most common symptoms of bacterial infections in house plants is a black spot on the leaf with a yellow edge to it. Like this one:
This plant, however, doesn’t have a bacterial infection. It was underwatered, and then in its weakened state, thrips turned up.
A house plant beginner might be tempted to rush out and buy some fungicide, and not realise that brown spots with a yellow edging CAN be a bacterial infection, but could also be any number of things.
Most brown spots on plants will develop a yellow edging over time – regardless of what caused it. The yellow part creates a border between the alive and dead parts of the leaf to protect the rest of the leaf (in case it’s something that can spread).
The plant removes the chlorophyll around the black spot (basically killing that part of the leaf) to create a barrier.
Luckily, most bacterial infections can be prevented or eradicated with care tips that won’t damage the plant if you misdiagnose, so check out these tips before running out and buying chemicals to treat. Bacteria/fungus are living organisms that can be often eradicated by destroying their habitat, rather than the actual offender.
How to prevent house plant bacterial infections
- Don’t overwater
Overwatering can lead to root rot, but it can also lead to other bacterial infections as well.
As the roots start to decay, the leaves begin to show signs of dying off – the foliage blackens and wilts.
The dead parts of the leaves remain quite moist (as the plant tries to take up as much moisture as it can to save the roots) and become a nice little breeding ground for bacteria
Overwatering is usually caused by watering too often (it has little to do with how much water is given at one time) and providing no drainage holes. However, too big of a pot or soil that’s too dense can also cause it.
- Don’t wet the leaves
Well, you can wet them (most fungi won’t b able to survive in water), for example, if you need to give the leaves a clean, but make sure you dry them afterwards.
If you have plants with a lot of little leaves, then putting them somewhere warm where they can dry out on their own might be easier than drying each individual leaf.
Also, clean your plant leaves in the morning. It’s colder at night, and wet, cold leaves will be a breeding ground for bacterial infections.
- Don’t keep your plants too close together
I have a whole article on how house plants do NOT like to touch each other. The main reason is that…they simply don’t like it.
In the wild, many trees practice a thing called canopy shyness, where they don’t touch each other, and one of the main reasons (we think – the jury’s still out) for this is to avoid the spread of leaf-eating larvae and other diseases.
I get that this isn’t always easy, especially in winter.
My plants touch more in winter because I have to move all the plants that live in the cold parts of my house into the warmer parts (and there’s more every freaking year!). I can’t help that.
However, since plants need less watering/fertilising in winter, I concentrate my time on making sure they stay clean.
As I mentioned before, try to keep the leaves dry – dust with a dry cloth if you can (I like the makeup eraser microfiber ones) and when you deep clean the leaves, make sure to dry them off afterwards – paper towels are great for this.
- Reduce humidity in cold weather
This might sound counterintuitive, but good plant care requires that the conditions you provide for your house plants are balanced.
Great light and humidity are all well and good, but if you don’t balance those out with warmth (between 15oC/60oF and 24oC/75oF) then your plant may struggle.
Bacterial infections thrive in high-humidity/low-temperature environments. And since your house plants will struggle in those conditions, it’s no surprise that bacterial infections can quickly take over.
Obviously getting a heater would be best, but they can be quite expensive to run, so a dehumidifier might be a better option if you’re on a budget.
Bear in mind that many types of mold can grow in these cold, humid conditions, so investing in a dehumidifier is as much for your own health as it is for your plants.
- Keep your plants pest-free
Some house plant pests, such as aphids and scale, produce a substance called honeydew, designed to attract other pests that will help to protect them (such as ants).
Some house plant bacterial infections, such as sooty mold, use honeydew as a nutrition, so will rock up to infected plants. I mean, can you blame them?
I know that it’s pretty much impossible to keep plants pest-free at all times. What I really mean is that you need to keep an eye on your plants so that you can start treating any pest issues as quickly as you can – before any bacterial infections show up at the party.
- Increase air flow
A lot of bacterial infections thrive in still, stagnant air, BUT there are some that like a bit of aeration. Don’t get bogged down in which bacteria like what conditions – it doesn’t really matter. Whatever conditions you have, at least one type of bacteria will love it – fitting into whatever conditions are available is bacteria’s strong suit.
What does matter is that your plant will benefit from air movement because it will help keep moisture from forming on its leaves and blocking the stomata.
How do you treat a bacterial infection in a house plant?
First things first, make sure your plant is in a good position to fight off infections.
This is also the case with treating pests.
Plants aren’t as useless as we might think at fighting off infections. They’re like humans in this regard. A healthy human is perfectly able to fight off many infections without medication – they just need rest and time.
It’s the same with plants, except even after they contract the infection, you can set them up so they can better fight them off.
In an ideal world, we’d put the plant in a terrarium (a clear plastic box will do) with a light so that we can keep it isolated from other plants and reduce the chances of the infection spreading, adding a grow light for energy and warmth.
The humidity should remain high by itself, and adding extra substrate might make it too high, so just leave it in the pot with nothing else in the box.
If it’s cold, it’s easier to heat a glass box (a heat mat will work) than it is an entire room.
If you don’t have this kind of setup, then you need to find a warm spot that gives your plant optimal light. Putting it somewhere that you’ll go past regularly is also a good shout – being forgotten about will NOT help.
- Repot your plant only if necessary
Repotting your plant isn’t usually the best thing to do to a stressed plant because it can induce shock, but bacteria can be living in the soil, so fresh soil can be an easy way to get rid of any fugitives.
Be sure to check the roots whilst you have your plant out of the pot, even if you’re pretty sure it isn’t root rot. If the roots are white and healthy, then I would put your plant back and don’t stress it out further.
If you have brown mushy roots and saturated soil, remove as much soil and dead growth as you can, and repot it in fresh soil. Make sure the pot has drainage holes, and that the soil isn’t too dense.
- Use a topical treatment
There are LOADS of products you can buy to treat bacterial infections, and you probably have a couple of them already.
- neem oil
- Hydrogen peroxide
- Castile soap
These can all be used to wipe down the leaves and see off any bacterial spores. Keep repeating this twice a week until the infection is gone.
In short, to reduce the chance of your house plant getting a bacterial infection, you need to treat it well, water it properly, make sure it’s warm enough, and keep the leaves clean.
You can get specific products for more specific issues like rust fungus, which is usually treated with a copper fungicide.
Rust fungus is one of the more PITA fungi, because it thrives in hot and humid conditions. Luckily it tends to prefer outdoor conditions so it’s more of a pain to outdoor gardeners and farmers.
Fungicides can be great when you have no other options, but it’s worth noting that fungi are a vast and ancient group of organisms that are extremely good at adapting to whatever life throws at them – they can become immune to fungicides so prevention is definitely better than cure.
Bacterial infections in house plants are most likely to turn up in winter when humidity is high and temperatures are cold.
Apart from, of course, the ones that thrive in the heat.
The advice is simple but not exactly easy:
- Make sure the plant has the light and humidity it needs to thrive
- Don’t overwater
- Keep the plant warm
- Don’t let the leaves get too wet
- Check for pests regularly