Spots on orchid leaves: fungal or bacterial?

Not every spot is an infection. Rule these out first:

• Sunburn. A pale or bleached patch on the side facing the window, turning brown or black, after a move into stronger light. Our yellow leaves guide shows how it looks.
• Cold damage. Pitted, sunken or water-soaked patches after cold nights, cold water or cold window glass. See cold damage.
• Scale insects. Small yellow speckles on top of the leaf, with insects underneath. See our orchid pests guide.
• Harmless speckles. The New York Botanical Garden (NYBG) says some Oncidiums get tiny black speckles when cold water hits their leaves, and they're nothing to worry about. The AOS says black spots on older Zygopetalum leaves aren't infectious, and NC State ties them to poor airflow.

Infections come from bacteria, fungi and water molds. The American Orchid Society (AOS) says both bacterial and fungal diseases spread in splashing water, and that wet leaves and high humidity help them along. The two need different treatment, which is why telling them apart matters.

The Pacific Northwest Handbook is clear that chemicals alone don't control bacterial brown spot: dry leaves, airflow and clean tools come first. With that said, here's what the sources list:

• For bacterial spots, the AOS suggests spraying the rest of the plant, and nearby plants, with a bactericide after you cut out the infection. The Connecticut station lists copper products among those registered there. One AOS writer warns against using copper on Dendrobiums or on plants in bloom.
• For fungal spots, the AOS says repeated fungicide sprays can slow Phyllosticta and protect healthy plants, though getting rid of it completely is very hard. The Connecticut station advises a fungicide as soon as anthracnose shows.

Whatever you use, read the whole label. It should list orchids or ornamentals and indoor use, and it tells you the dose, how often to repeat and any safety steps. Spray outdoors in the shade if you can, and keep the plant out of sun until it dries.

Pets. The moth orchid itself is non-toxic to cats and dogs, per the ASPCA. Fungicides and bactericides are the risk. Keep pets away from treated plants until the leaves are fully dry, store products out of reach, and if your pet chews a treated plant and seems unwell, call your vet.

• Dry by nightfall. Water before noon so the plant dries before the evening cool-down, says the AOS. Tilt the plant if water collects in the crown. Our watering guide shows how.
• Skip misting and overhead watering. The AOS and the Pacific Northwest Handbook both link them to bacterial disease.
• Keep air moving. Space plants so their leaves don't touch.
• Clean your tools. Disinfect blades between plants (Pacific Northwest Handbook, Connecticut).
• Quarantine new plants. The AOS suggests one to two months apart from your collection, since Phyllosticta is highly contagious.
• Go easy on nitrogen. The Pacific Northwest Handbook says high nitrogen favors bacterial brown spot.
• Avoid repotting in a heat wave. The AOS links repotting in very hot weather to soft rot.
• Clean the pebble tray. The NYBG suggests scrubbing humidity trays with 10% bleach every two months.
• Clear away dead leaves and flowers. The AOS advises removing dead plant material to keep fungi in check.

The AOS and the Connecticut station describe petal blight, caused by Botrytis, as small round spots on petals and sepals that spread, sometimes with gray fuzz in cool, damp air. The AOS says it isn't fatal to the plant and spreads on airborne spores. One AOS writer notes that the fungus is common everywhere and can't be wiped out, so the goal is to make conditions less friendly:

• Cut off spotted flowers.
• Improve airflow and lower humidity around the plant.
• Keep flowers dry: water the mix, early in the day.
• Keep nights a little warmer: the AOS suggests above about 17 to 18 °C (mid-60s °F) for moth orchids.