Why are there spots on my houseplant's leaves?

Spots on your houseplant’s leaves are a common sign that indicates something is wrong with your plant. This article explains six causes of spots on leaves: sunlight damage, fungal damage, bacterial damage, over-watering, and mineral deficiencies.

The most common cause of spots in houseplants is sunlight damage. However, if your plant has been in a shaded position you can eliminate sunburn as the reason for the spots. In this case, they are likely due to a fungal or bacterial infection. Both fungal and bacterial infections produce similar looking brown spots on the leaves so try several treatments to see what works.

A perhaps less common cause of spots on houseplants is mineral deficiency in the plant from under-fertilization. Garden centers often use fertilizer-enriched soils so plants tend to have enough minerals in the soil that they come in to last them several months, or even more if you bought your houseplant during the dormant season when it is not growing vigorously. Mineral deficiencies are more common in outdoor plants and neglected older plants.  

Different types of dark leaf spots and their causes

Amorphous brown blotches, possibly with a yellow halo 

Sunburnt Jade plant 1sock www.flickr.com/photos/1sock/7482378916/ 

This is most likely caused by too much exposure to sunlight, particularly on leafy tropical plants. Although you might assume these exotic plants want as much light as possible, the native habitats of monstera, philodendron, and parlor palm are dark, humid rainforest floors. You need to replicate these conditions as far as possible. Place the plants in filtered sunlight. Use net curtains or blinds if it’s next to a window or place next to a window facing a large tree. These brown marks are permanent.

Small brown spots, or black spots with a brown halo. Round, possibly with a yellow or purple halo. The spots grow larger and merge. 

Fungal leaf spot often look like frog eyes Scot Nelson live.staticflickr.com/5561/15116621216_3e51591197_b.jpg 

This may indicate a fungal leaf infection where the spores of leaf spot fungus land on the leaves, germinate, and attack the plant. It will rarely be lethal to a healthy plant but you should remove infected parts quickly. Spray with baking soda spray or bacterial fungicide. Disinfect any scissors/knives you use on the infected plant so you don’t spread the infection.

Fungal leaf infections produce similar symptoms to bacterial leaf spot infections (see below).

Spots with rounded edges, first appearing translucent, possibly with a yellow halo

Bacterial leaf spot. This infection is common on staghorn ferns and lady’s slipper orchids. The spots are very similar to those caused by fungal leaf infection (see above) so it is difficult to identify what is causing them. As a precaution, you should spray the plant with bacterial fungicide or baking soda as you would for a fungal leaf infection. You should also remove infected plant parts and keep foliage dry. Disinfect any scissors/knives you use on the infected plant so you don’t spread the infection.

Spots on upper surface of the leaf with bumps below each spot on under the leaf

Orange markings on the underside of a leaf infected with rust fungus Katja Schulz www.flickr.com/photos/treegrow/39006550150 

This is a rust fungus infection. Remove infected parts as soon as possible and spray with bacterial fungicide.

Tiny hard brown bumps that can’t be removed from the leaves

Sandra Jensen, Cornell University, www.forestryimages.org/browse/detail.cfm?imgnum=5503297

This may be edema, a condition where cork tissue develops in response to overwatering. The bumps are permanent and the only way to prevent more is to maintain an appropriate watering schedule for your plant.

Black, brown, yellow, or purple spots along with other symptoms like discoloration of leaves (purple or yellow) and wilting

Discoloration typical of zinc deficiency. Howard F. Schwartz, Colorado State University www.ipmimages.org/browse/detail.cfm?imgnum=5364032 

Spots on leaves can indicate low levels of phosphorus, iron, magnesium, manganese and calcium in the soil. Purple or bronze spots can indicate potassium deficiency. Spots indicating mineral deficiency will often look similar to those that appear as a result of fungal and bacterial infection.

If you haven’t fertilized your plant for several months then it might be worth checking whether the dark spots are accompanied by other symptoms of mineral deficiency. You can correct mineral deficiencies with a regular fertilizing regimen using the appropriate fertilizer for your plant.