Brown Spots on Leaves of Indoor Plants, and What Each Kind Means

• Problems
• Watering

Photo by grom900 on Pixabay

Brown spots on indoor plant leaves come from fungus, bacteria, scorch, salts or pests. How to read the spot, what to do first, and when to cut the leaf off.

Read the spot before you act

Brown spots look alike from across the room and have very different causes. Three details separate them: the shape and edge of the spot, where on the plant it appears, and what the leaf feels like.

1. 1

   Look at the edge

   A yellow ring around a small brown spot is the classic sign of leaf spot. A pale, bleached or papery patch with no ring is more likely scorch or dry air.

2. 2

   Note where it is

   Spots on the leaves facing the window point to sun. Spots on the lowest, oldest leaves point to roots or old age. Spots on the leaves touching a neighbour or a cold pane point to damp or cold.

3. 3

   Touch it

   Dry and crisp is scorch, salts or dry air. Soft, wet or slightly sunken is rot or bacteria. Sticky or gritty means look underneath for pests.

4. 4

   Check the pot

   Push a finger a few centimetres (an inch or two) into the compost. Wet days after watering, or a sour smell, means the roots are the real problem.

5. 5

   Turn the leaf over

   Fine webbing, tiny moving dots or a silvery stipple on the underside mean a pest is behind the marks, not a disease.

Leaf spot: small, ringed and spreading

This is the one most people mean by brown spots. Clemson's extension service describes fungal spots as "small, brown spots with yellowish margins on the leaves". Penn State adds that they are oval, brown or black, "not limited by leaf veins", which is how you tell them from damage that stops at a vein.

Bacterial leaf spot looks similar. Clemson says the spots enlarge and run together in wet conditions, so a leaf that stays damp can end up with one large blotch.

Both need water sitting on the leaf, and the fungal kind spreads by spores. Penn State notes that airborne spores spread to healthy leaves, so one spotted plant can seed the rest of a shelf.

What to do, in order:

1. Isolate the plant. Move it away from others while you sort it out.
2. Remove the worst leaves. Penn State advises pruning and discarding infected parts as soon as you detect a problem, and Clemson says to pick off and destroy infected leaves. Put them in the bin, not on the compost heap.
3. Stop wetting the foliage. Clemson says to avoid splashing water on the foliage since this spreads the fungi. Water at the base of the plant, and stop misting. Our guide to bottom watering covers a good way to keep leaves dry.
4. Give it air. Crowded plants in a still room hold moisture on their leaves for longer. A little space between pots does a lot.
5. Clean up the pot. Remove fallen leaves from the compost surface, because they carry spores.

Waterlogged roots: big, soft, brown patches

If the brown areas are large and soft rather than small and ringed, look down, not at the leaf. Penn State describes root rot as a soil-borne fungus that causes wilting and blackened, mushy roots. Clemson is direct about the cause: do not overwater, since too much water increases the occurrence of root rot.

The leaves often show it before the roots do. Brown blotches appear on the lower leaves, yellowing starts at the base of the plant, and the whole thing looks limp even though the compost is wet. If this sounds right, our guides to how often to water houseplants and why houseplant leaves turn yellow will help you find what went wrong.

Slide the plant out and look. Firm, pale roots are healthy. Brown or black, soft and slipping roots are rotting. Cut those away with clean scissors, repot into fresh potting mix in a pot with drainage holes, and water once, lightly. Then wait. A plant with fewer roots needs much less water than before.

Scorch: dry, pale and facing the window

Sunlight through glass is stronger than it feels, particularly on a plant that spent the winter further back in the room. Scorch gives a dry, pale to brown, papery patch on the leaves nearest the glass, often with a sharp edge and no yellow ring. It also appears when a plant is moved suddenly into brighter light, or when a drop of water on the leaf acts as a tiny lens.

Move the plant back from the glass, or hang a sheer curtain, and the new leaves come in clean. The scorched patches stay. Which window is "sunny" depends on where you live: it is the south-facing side in the northern hemisphere and the north-facing side in the southern, so go by the window that gets the most direct sun.

Salts and dry air: crisp brown edges

When the brown is at the edge or tip rather than in the middle of the leaf, the cause is often dry air or mineral salts. Clemson says white to grey crusty deposits on the soil surface or rim of pots are a sign of salt buildup. If you see that crust, flush the pot with plenty of plain water and let it drain fully. Our guide to brown tips on houseplant leaves goes through this cause in detail, along with dry air and thirst.

Pests that look like spots

Some "spots" are insect damage. Spider mites leave a fine stipple of pale or brown dots with webbing under the leaf, and spider mites love warm, dry rooms. Thrips leave silvery, scratchy patches, covered in our guide to thrips on houseplants. Scale insects sit on the leaf as brown bumps that can be rubbed off with a fingernail, and sticky leaves are a clue to several pests at once. If you can wipe the spot away or it feels sticky, work through the guide to sticky leaves on indoor plants.

Which plants mark first

Stop guessing what the spot is

The hard part with brown spots is that the same-looking mark can be a fungus, a sunburn or a root problem, and the treatments are opposites. Folly's Plant Doctor is built for exactly this. Photograph the leaf and it matches the spot against 500+ diseases and pests, then gives you the likely cause, what it looks like, what causes it, how to prevent it and what to do first. It is free on every plan.

When a health scan finds a problem it can treat, "Fix my plant" on Pro asks a few quick questions about what the plant grows in, how the soil feels and when it was last watered, and builds a day-by-day recovery plan with check-ins and reminders. Your usual watering reminders pause while the plan runs, so you are not tempted to water a plant that needs to dry out.

On Folly Pro, Smart Care schedules also help prevent the most common cause behind these spots. They set each plant's watering interval from 15+ attributes and show every factor, with the percentage it added or removed, so you can see why a plant is due and not just trust a number. For a related problem, see our guide to why houseplant leaves turn yellow.