Written by
Carmen
Your houseplant has been looking healthy and happy for a while, but suddenly you notice black spots on its leaves. Annoying, but fortunately not always a reason to panic. There are a few common causes, and with the right approach your plant will usually bounce back quickly. In this article, we explain the difference between black sooty deposits and actual black spots, what causes the problem and how to fix it.
Black sooty deposits or black spots: what’s the difference?
These two are often confused, but they’re actually two different problems. Black sooty deposits, also known as sooty mould, form a thin, soot-like layer on the leaf. This layer is a fungus that grows on the sticky honeydew left behind by insects such as aphids, scale insects or whiteflies. In short, it’s basically their mess that attracts the fungus. The fungus doesn’t attack the plant itself and can easily be wiped off the leaf with a damp cloth, so there’s no need to panic and certainly no reason to give up on your plant straight away.
Black spots are a different story. These spots aren’t on the leaf but inside the leaf tissue, so you can’t wipe them away, no matter how hard you scrub. This is also known as leaf spot disease and is caused by fungi or bacteria that thrive particularly well in damp conditions. The spots often start small and range from brown to black, sometimes with a lighter border around them, and can gradually spread across the leaf.
Did you know?
Sooty mould gets its name from the soot-like layer the fungus leaves on the leaf, as if someone has sprinkled a little ash over it. Without honeydew from insects, this fungus can’t develop.
Why does my houseplant have black spots on its leaves?
There are several things that can cause black spots on houseplants. These are the most common.
Too much water.
Overwatering can cause root rot, making it harder for the plant to absorb water and nutrients. This often shows up as dark, damp-looking spots on the leaves. A case of well-intentioned love going just a little too far.
High humidity combined with poor air circulation.
The fungi and bacteria that cause leaf spot disease thrive in humid conditions where there’s very little air movement. Keeping plants close together increases the risk. Cosy for you, less so for your plant.
Cold or draughts.
A stream of cold air, for example near a draughty door or window, can damage parts of the leaf. This often appears as dark, sunken spots.
Sunburn.
Strong, direct sunlight can scorch the leaf tissue, resulting in brown to black spots that are often a little drier.
Contaminated tools or water on the leaves.
If you use dirty pruning shears, or regularly get water on the leaves rather than the potting soil, fungi and bacteria can spread more easily.
How to treat black spots on your houseplant
- First, remove the affected leaves with clean, sharp scissors so the fungus or bacteria can’t spread further through the plant.
- Throw these leaves away rather than leaving them near the plant, because that’s exactly the kind of thing fungi secretly love.
- Then adjust your care routine: allow the potting soil to dry out properly between waterings, water at the base of the plant rather than on the leaves, and make sure there’s enough space and air circulation around the plant.
- Is the plant sitting in full sun or close to a draughty spot? Move it somewhere a little more sheltered.
- If the problem keeps coming back, a fungicide suitable for houseplants may help and is available from well-stocked garden centres.
Which houseplants often suffer from black spots?
Some houseplants are more prone to black spots than others, usually because they have dense foliage. Think Ivy, Begonia, Calathea, Moth Orchid and Ficus. With these plants, it’s especially important to ensure good air circulation and the right balance of water and light.
Want to know more about healthy houseplants?
Not sure whether your plant could use a little extra attention? Take a look at our care tips for each houseplant too, so you know exactly what your green housemate needs to get back to its best. You can also read 10 frequently asked questions about houseplants.
