Why are the spots on my moth orchid leaves turning brown and sunken?

Brown spots on a moth orchid may be bacterial brown spot, which the American Orchid Society calls the most common disease of Phalaenopsis. Fungal disease, cold damage and salt buildup can also leave dark marks. Check whether the spot is soft and water-soaked, whether it oozes, and how cold your plant and its water have been.

Updated 27 October 2026

Is it bacterial brown spot?

Bacterial brown spot is the first thing to rule in.

The American Orchid Society describes a small, soft, water-soaked blister that may have a yellowish or pale green halo. The spot starts dirty green. Then it grows, merges with other spots, and eventually turns brown or black, dried up and sunken.

The spot can ooze liquid full of bacteria, particularly once the disease reaches the leaf tip. The society says the bacteria enter the plant through wounds or the stomata.

University of Maryland Extension adds more clues for bacterial leaf spots on indoor plants:

• Collapsed tissue is usually soft and mushy, with a rotten smell.
• Spots are often seen between the veins and may be contained by the veins, so they look angular or V-shaped.
• Other signs include yellow margins around the lesions, small blister-like spots that merge, or spots where the center falls out.

Take a clear photo of the spot as soon as you see it. Our guide to photographing a sick plant shows how, and a photo helps you see if the spot grows.

What else can cause dark spots on a moth orchid?

Moth orchid spots can also come from fungus, cold or salt. Each one leaves different clues.

• Fungal disease. The American Orchid Society says irregular dark spots on leaves point to a fungal infection, usually one of the soft rots. University of Maryland Extension says fungal leaf spots often look like many tan to reddish brown to black, roughly circular spots. They may run together into large irregular lesions. If they attack the newest leaves near the top, the growing point may be killed.
• Ringed lesions. The American Orchid Society says grayish-brown lesions with rings are usually caused by a virus when they are the size of a penny or dime. Larger lesions, or ones that keep spreading, point to fungus.
• Botrytis on flowers. The American Orchid Society says this fungal infection causes very small black or light brown spots on the flowers. The spots may grow and cover the whole flower.
• Cold damage. The American Orchid Society says cold damage can come from low temperatures or from watering with cold water. It collapses cell tissue and causes lesions, pitting, sunken areas and discoloration. University of Maryland Extension says cold shock can happen at temperatures potentially under 50°F (10°C).
• Salt buildup. The American Orchid Society says black leaf tips on their own are caused by too much salt buildup. It is usually fertilizer salts, but very hard water can also be the cause.

How do I tell the spots apart?

Match what you see to the table. If more than one row fits, treat the moth orchid as having a possible bacterial problem first and keep watching.

Plant Doctor, an AI plant diagnosis app for iPhone, can look at three photos and suggest the most likely cause, along with a step-by-step care plan.

What should I do first about the spots?

Start with the gentlest steps. Change one thing at a time so you can tell what helps.

1. Remove infected tissue. The American Orchid Society says to sanitize the plant by removing infected tissue when the cause is Pseudomonas bacterial brown spot. University of Maryland Extension says individual leaves with spots can be picked off and destroyed. For fungal spots, the American Orchid Society says to remove infected parts if possible and watch for continued infection.
2. Water early in the day. University of Maryland Extension says this lets leaf surfaces dry quickly.
3. Keep the crown dry. The American Orchid Society says water standing in the crown lets bacteria multiply and can rot the growing tip. Tilt the plant so the water drains out.
4. Add air movement. The American Orchid Society says to increase air movement for bacterial leaf spots. University of Maryland Extension says to space plants so air can circulate between them and reduce moisture.
5. Use lukewarm water. The American Orchid Society says water that is too cold or too hot can cause temperature shock.
6. Think twice about sprays. University of Maryland Extension says fungicides labeled for houseplants can be used. It also says that in most home conditions, removing infected parts and drying conditions should help. If you do choose a product, follow the product label and pick one labeled for houseplants.

University of Maryland Extension says systemically infected plants cannot be cured and should be discarded. If the spots spread fast or you can’t tell what you have, ask a local nursery or extension service.

Could my care routine be adding to the problem?

Your moth orchid’s watering and potting medium can matter, because the American Orchid Society says the roots can rot from watering too often or from a potting medium that is breaking down. Spots on leaves do not always mean root trouble, but check the roots too. Our guide to root rot covers what rotten roots look like.

• Watering. University of Maryland Extension says that at normal home temperatures, moth orchids should not need water more than once a week. It also says aerial roots turn from dull silver or white to pale green when you have applied enough water. For more help, see how to tell if a plant needs water.
• Repotting. The American Orchid Society says moth orchids normally need repotting every year or two. University of Maryland Extension lists these clues: the plant has outgrown its pot, or the bark has broken down so it looks like mulch, drains poorly and holds too much moisture. When you repot, cut off dead, discolored or damaged roots. Read should you repot a struggling plant before you decide.
• Light. University of Maryland Extension says a moth orchid with the right amount of light has light green leaves. Low light gives dark green leaves that feel very stiff.

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