WHY DOES THIS HAPPEN?
There can be a number of reasons why brown or black spots appear on your plant - usually on its leaves. Causes can include: a draft of cold air combined with too much water, sunburn due to too excessive light exposure, overwatering/lack of drainage, dark light conditions or underwatering. Additionally, it should also be kept in mind that variegated leaves (leaves that have more than one color) are more sensitive, meaning that the lighter colored parts of the leaves can become spotted and/or easily turn black or brown due to their sensitivity.
Brown spots can also be caused by a bacterial or fungal infection known as Leaf Spot Disease.
It can be confusing, especially when the possible causes seem to contradict each other, but it’s a good idea to try and narrow down what could be the possible cause of the spots in your particular plant. You can look out for variations such as the shape or color of the spots, or where they appear on the leaves.
IF THE SPOTS APPEAR ON THE TOP OF THE LEAF:
If your plant’s leaves have dry brownish spots, often accompanied by scorched leaf tips or sometimes also pale leaves, it’s likely that it is being exposed to too much direct light. This can be quite harsh for houseplants - particularly sensitive tropical plants that would naturally grow in the rainforest, where they’re protected from sunlight by forest foliage. Therefore, you could try moving it to a darker, more sheltered location.
IF THE SPOTS APPEAR ON THE UNDERSIDE OF THE LEAF:
Spots on the underside of the leaf can often be an indicator of pests, especially when combined with other symptoms. For example, spider mites can cause small brown speckles to appear all over the leaf, along with webbing on the underside of leaves. Other sap-feeding pests, such as thrips and mealybugs, can also cause spots to appear.
IF THE SPOTS APPEAR ON THE LOWER AND INNER BRANCHES:
This is most likely a fungal infection, which may have arisen due to overwatering and/or poor drainage. Fungal spots can vary in size, shape, and color, whereas bacterial spots can be a bit easier to identify - they’re typically small, dry, and may have a yellow circle around the spot. If left untreated, it can progress to developing a hole in the leaf.
IF YOU’RE STILL UNSURE:
If you’re finding it tricky to determine what the cause of spots on your plant is, then you can try a process of elimination. Check on your watering schedule to make sure that you’re not watering too often or not frequently enough, and keep an eye out for additional symptoms. For instance, if you suspect that the cause could be overwatering/poor drainage, you can take a look at your plant’s roots - if they’re discolored or mushy then it could be a sign of root rot caused by too much water.