Plant care guide
Orchid Brown Spots: Bacterial Spot, Sunburn or Cold? Peroxide?
Updated
Why does my Orchid have brown spots?
Soft, water-soaked brown or black spots on a moth orchid leaf are usually bacterial brown spot, the most common Phalaenopsis disease. Spots that appear suddenly on the highest, sunniest part of the leaf are sunburn, and sunken, pitted patches after a chill are cold damage. Hydrogen peroxide appears in American Orchid Society advice for bacterial spots, not for burns.
What to do first
Isolate the plant and press a spot gently. If it is soft or watery, cut it out with a sterile blade well into healthy tissue, keep the leaves dry and improve air movement. If it is dry and on the sunny side, move the plant to indirect light.
A brown spot on a moth orchid leaf is alarming, because one of the causes can travel through the leaf fast. Others are scars from sun or cold that will not spread. Before you reach for the peroxide bottle, work out which one you have.
Sort the spot
| What you see | Likely cause |
|---|---|
| Small, soft, water-soaked blister, dirty green, sometimes with a yellow or pale halo, turning brown or black and sunken | Bacterial brown spot |
| Water-soaked spots spreading very fast, leaf turning mushy, foul smell | Bacterial soft rot |
| Rot at the centre where new leaves emerge, after water sat there | Crown rot |
| Sudden dry black or tan patch on the highest, most exposed part of the leaf | Sunburn |
| Pitting, sunken areas and surface lesions after cold glass, a draught or cold water | Cold damage |
The American Orchid Society’s Phalaenopsis problems article gives these descriptions. It calls bacterial brown spot the most common disease of Phalaenopsis.
Bacterial brown spot
Oregon State University’s PNW disease handbook names the cause as Acidovorax avenae subsp. cattleyae, formerly Pseudomonas cattleyae. On susceptible Phalaenopsis, a soft, water-soaked spot can become a brown or black cavity and reach the growing point. The bacterium spreads in splashing water, on infected plants and on unsanitised tools. Warm, wet conditions and high nitrogen feeding make it worse.
The AOS bacterial diseases page advises removing infected tissue, improving air movement and avoiding overhead watering and misting.
What the sources say about hydrogen peroxide
This is the part of the query with the least published support, so we keep to what the sources say.
- Bacterial brown spot. The AOS problems article suggests pouring peroxide over the infected tissue, and removing the tissue if the spot keeps enlarging. It also lists bactericides such as copper compounds, used to label directions. It does not give a peroxide strength.
- Soft rot. The AOS says to cut out infected tissue with a sterile instrument first, then pour peroxide over the wound.
- Crown rot. The AOS suggests tilting the plant so water drains from the crown, or adding a few drops of peroxide to the crown.
- Sunburn and cold. The AOS gives no peroxide step. Damaged leaves still photosynthesise; remove only badly damaged ones.
The one peer-reviewed study we found tested peroxide on roots, not leaves. In HortTechnology (2021), Goossen and Williams dipped Phalaenopsis roots in 3%, 6% or 12% peroxide for three minutes. The 6% and 12% dips damaged root health permanently. The 3% dip caused minor damage and did not kill algae. The authors advise caution and better watering practice instead. Do not soak the root system to treat a leaf spot.
What to do
- Isolate the orchid from other orchids.
- Cut out soft spots with a blade sterilised between cuts. The PNW handbook lists 10% bleach, trisodium phosphate or Physan 20 for tools.
- Keep the leaves dry. The AOS advises against overhead watering. Water the mix, and tip out any water left in the crown.
- Improve air movement and stop misting.
- Feed less nitrogen while the disease is active, as the PNW handbook advises.
- Discard a badly infected plant. The handbook advises destroying severely infected plants.
Prevent sun and cold spots
University of Maryland Extension’s moth orchid care page recommends indirect light from an east or west window. It notes that too much light turns leaves yellow-green with pink or reddish margins. Clemson’s Phalaenopsis care guide gives 16–29 °C (60–85 °F) and warns to keep leaves off cold glass in winter.
For limp, creased leaves, see our orchid wrinkled leaves guide. For roots, see orchid root rot.
References
- American Orchid Society: Growing Phalaenopsis. What Can Go Wrong?
- American Orchid Society: Bacteria (Orchid Pests and Diseases)
- Oregon State University, PNW Plant Disease Management Handbook: Orchid, Bacterial Brown Spot
- Goossen R, Williams KA (2021). Characterizing the Phytotoxic Effects of Hydrogen Peroxide Root Dips on Hybrid Phalaenopsis Orchid Plants. HortTechnology 31(6):810–816
- University of Maryland Extension: Care of Phalaenopsis Orchids (Moth Orchids)
- Clemson Cooperative Extension HGIC: General Care for Phalaenopsis Orchids
How this guide was put together: American Orchid Society, Oregon State University's PNW Plant Disease Management Handbook, University of Maryland Extension and Clemson Extension guidance, and a HortTechnology study on hydrogen peroxide.