Why are there spots on my leaves?

Photo by gryffyn m on Unsplash

It is common for most plant owners to find a leaf spot or two in their plant collection. While these blemishes might be unsightly, most of the time they’re pretty benign and easily removed with a little pruning. 

However, leaf spots can also indicate underlying health issues with your plants. They might be a sign that you need to change your plant care regimen. Alternatively, your houseplant may have been infected by a pathogen.

Like many plant symptoms, leaf spots can have several causes. Figuring out what lies behind the blemishes will rely on a little detective work.

Fungal infection

Ian Alexander https://commons.wikimedia.org/wiki/File:Leaf_Spot_on_Oak_in_Gunnersbury_Triangle.jpg

Spots associated with fungal infections are dark brown or rust with yellow outlines. Some plants get infected by particular species of fungus – for example, Fusarium, a type of Dracaena, gets infected by the Fusarium moniliforme fungus. You can treat this by clipping damaged leaves, keeping the foliage completely dry, and spraying the plant with a mix of mild liquid soap and water. This soapy solution is a really effective way to treat a range of houseplant pests so its helpful to keep a bottle of the stuff around.

Cold water damage

White or yellow spots or rings on foliage may indicate that the water you are using on the plant is too cold. Orchids are particularly fussy about water temperature. 

To prevent cold water spots, you should make sure you are watering and misting your plants with water room temperature. Plants generally appreciate lukewarm water as it is less likely to damage the plant at its roots.

Sun burn

whologwhy https://commons.wikimedia.org/wiki/File:LEAF_BACKLIGHTING_(8149293600).jpg

Dark spots on leaves may be the result of strong, direct sunlight hitting the leaves. Only some plants can withstand full direct sunlight. Most of the tropical houseplants that you buy will prefer a partially shaded spot or somewhere they can get bright but filtered light.

You can prevent sun burn spots by moving your plant away from a bright window to a more suitably lit position.

Bacterial infection

Scot Nelson https://commons.wikimedia.org/wiki/File:Panax_Bacterial_leaf_blight_(22064601690).jpg

Leaf spots caused by bacterial rot look like watery, dark blotches that are sometimes lined with yellow. On Dieffenbachia, bacterial leaf spots may have translucent centers. Eventually, the leaves will turn yellow, die, and fall from the plant.

Bacterial infections happen when bacteria enters the leaf, rotting the tissue from the inside. The chances of plants contracting this increases under humid conditions.

To treat, you’ll need to isolate the plant to stop the bacterial spread. If you have been keeping it in a heavily humid environment, move it to a drier space. Next, cut off any rotting pieces of leaf with a sterilized knife or scissors. Stop misting the leaves completely and keep the plant as dry as possible to prevent recurrence. 

Mealy bugs

If the spots are white and can be removed from the leaf, it’s likely you’re dealing with this common houseplant pest. These aren’t leaf spots per se but given how damaging these critters are to plants, you should be aware of how to treat them.

Mealybugs are tiny insects that produce a sticky, cotton-like substance on the leaves. If you leave this untreated, the insects will feed on the sap of your plant until it dies.

To treat, immediately separate infested plants to stop the bugs spreading. Wipe off the bugs manually using cotton swabs or similar dipped in rubbing alcohol. Spray the plant with organic insect soap spray each week for several weeks. This will kill any remaining bugs on the plant.

Edema

Tiny yellow, red, or black spots on the surface of leaves accompanied by raised corky bumps on the underside of the leaf are known as edema. These blister-like spots occur when environmental conditions are not suitable for the plant, especially when over-watering is combined with high humidity. It can also happen when slow-growing plants are given too much fertilizer.

Edema is much less serious than the pest-related spots covered above. They are simply an indication that you need to alter your plant care routine. It will be useful to read up on the care needs of the plant in question to see where you might be going wrong.