Plant care guide
Alocasia Brown Spots: Wet Roots, Leaf Spot, or Scorch?
Updated
Why does my Alocasia have brown spots?
Brown spots on Alocasia usually come from wet roots, a fungal or bacterial leaf spot, sun scorch, dry air at the leaf edges, or heavy spider mite feeding. The shape, edge and position of the spots tell them apart.
What to do first
Look at where the spots sit and whether they have a yellow halo or a water-soaked edge. Check the mix and roots, move the plant out of direct sun, keep water off the leaves, and remove badly spotted leaves.
Brown spots on a big Alocasia leaf feel like a warning, and sometimes they are. But a brown spot is a symptom, not a diagnosis. NC State Extension notes that root rot and leaf spots on Alocasia can follow overwatering. The RHS Alocasia growing guide adds sun scorch and dry air. The fix depends on which one you have, so read the spots before you treat.
Read the spots
| What you see | Most likely cause | Check next |
|---|---|---|
| Brown spots with a yellow margin, sometimes with rings or tiny black dots | Fungal leaf spot | Wet leaves, crowding, poor airflow |
| Water-soaked spots of even size, with a yellow halo or sticky ooze | Bacterial leaf spot | Splashing water, cold, crowding |
| Browning in the middle of the leaf with wet mix and yellowing | Wet roots | Mix moisture, smell, root color |
| Dry, pale or brown patches on the side facing a window | Sun scorch | Direct sun through glass |
| Crisp brown edges and tips | Dry air or uneven watering | Room humidity, salt crust on the mix |
| Fine speckling that turns tan or brown | Spider mites | Leaf undersides with a hand lens |
Leaf spot diseases
Clemson HGIC’s houseplant diseases and disorders guide describes two patterns:
- Fungal leaf spots are small brown spots with yellowish margins. They may show a target pattern or tiny black dots in the dead tissue. Spots can merge until the whole leaf dies.
- Bacterial leaf spots are water-soaked and usually uniform in size. They may have a yellow halo or sticky ooze. In wet conditions they enlarge and run together. In dry air they stop growing and turn reddish-brown.
The RHS guide to leaf damage on houseplants also links dark spots with yellow margins to fungal leaf spot. NC State lists bacterial stem rot among the overwatering problems of Alocasia × mortfontanensis, the plant sold as A. × amazonica or ‘Polly’.
Both spread with water. Clemson warns that splashing water spreads leaf spot fungi.
Wet roots
The RHS notes that overwatering often causes browning in the middle of the leaves. University of Maryland Extension’s page on root rots of indoor plants lists yellowing, browning and dieback of leaves, with brown to black, soft roots.
Suspect the roots when spots appear on several leaves at once, the mix stays wet, and stalks soften near the base. The RHS warns that Alocasia roots rot most easily when overwatered in winter, while the plant is resting.
Scorch, dry air and cold
The RHS says strong sunlight can scorch Alocasia foliage. NC State gives the same warning for A. × mortfontanensis. The RHS leaf-damage guide notes that sun damage shows as pale spots on exposed surfaces. Look at the side of the leaf that faces the window.
Brown leaf edges are a different pattern. The RHS lists low humidity as a cause of browning on Alocasia leaf edges. Clemson adds hot, dry air, improper watering and salt buildup as causes of brown tips and margins.
Cold also damages leaves. The RHS advises keeping Alocasia above 10 °C (50 °F) at all times and at least 16 °C (60 °F) in the growing season. White or straw-colored spots after watering are a separate problem. Clemson links those to cold water on the leaves.
Spider mites
Iowa State University Extension’s houseplant spider mite article notes that light infestations look like tiny yellowish-green speckles. Heavy infestations turn foliage greenish-yellow, then tan or brown. NC State notes that Alocasia is prone to spider mites. Check the undersides for moving dots and fine webbing before you treat for disease.
What to do
- Isolate the plant if you suspect leaf spot or mites.
- Remove badly spotted leaves. Clemson recommends removing and destroying infected plant material. Cut at the base with clean scissors and wear gloves, because Alocasia sap irritates skin.
- Keep water off the foliage. Water the mix, not the leaves. Clemson advises against splashing water on leaves and crowding plants.
- Improve airflow. Space plants so leaves do not touch.
- Check the mix and roots. If only a few roots are brown and soft, Clemson advises cutting them out and repotting in fresh, sterile mix. The RHS suggests watering only once the top 5 cm (2 in) of mix is dry in the growing season.
- Move the plant out of direct sun. The RHS recommends bright but indirect light.
- Treat only after removing infected parts. Clemson lists copper soap and Bacillus amyloliquefaciens sprays to reduce future leaf spot. Choose one labeled for houseplants and follow the label.
Existing brown spots will not disappear. Judge progress by new leaves that open clean.
When spots are normal
The RHS notes that older leaves on healthy houseplants naturally turn brown and die back over time. One old leaf browning while new leaves stay clean is usually turnover. Many leaves spotting at once is not.
For a broader comparison, see our guide to brown spots on houseplant leaves and the root rot guide. The Alocasia humidity guide covers crisp edges, and the Alocasia hub lists our other Alocasia pages.
Related guides
Other alocasia problems we cover: corm and pup propagation, root rot, brown tips, overwatering.
References
- NC State Extension Gardener Plant Toolbox: Alocasia
- NC State Extension Gardener Plant Toolbox: Alocasia × mortfontanensis
- RHS: Alocasia Growing Guide
- RHS: Leaf Damage on Houseplants
- Clemson Cooperative Extension HGIC: Houseplant Diseases & Disorders
- Iowa State University Extension and Outreach: Spider Mites on Houseplants
- University of Maryland Extension: Root Rots of Indoor Plants
How this guide was put together: NC State Extension, RHS, Clemson Cooperative Extension HGIC, Iowa State University Extension, and University of Maryland Extension guidance.