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Orchid black rot

Also known as Black rot of orchids, Phytophthora cactorum, Pythium ultimum

DiseaseHarmful
Blackened, rotting Cattleya leaf with a yellow margin (the kind of lesion black rot causes; cause of this example not confirmed) Photo: Pogrebnoj-Alexandroff · CC BY 3.0

In short

Black rot is a fast-moving rot of orchids caused by water moulds, Phytophthora cactorum and Pythium ultimum. It spreads in water, so wet leaves, splashing and constantly damp media are the danger. Cattleya orchids are especially susceptible.

How it looks

Small blackish spots or lesions on leaves, pseudobulbs, roots or new growths that enlarge rapidly and can spread through the whole plant; the affected tissue is soft and exudes water when pressed, and the leaf around it may turn yellow. In seedlings, infection at the media line causes damping off.

Signs

Spreading black patches, yellowing, stunting and wilting, rotting roots and new growths, and rapid collapse of seedlings or whole plants if not stopped.

Life cycle

The organisms produce swimming spores (zoospores) that need free water to move and infect. They spread by splashing water, wet foliage, contaminated pots and media, and infected plants. Pythium also forms resting spores in the medium.

What favours it

Extended damp or rainy periods, water sitting on leaves and in new growths, overwatering, poor drainage, poor air circulation, and reuse of unsterilised pots and media.

Prevention

  • Keep leaves and new growths dry, and water early so plants dry before night.
  • Give strong air movement, for example with fans.
  • Use clean or sterilised pots and fresh media; do not reuse old media.
  • Isolate any plant with suspicious black patches immediately.

Treatment

Start with the first step and move down only if the problem persists.

  1. Isolate the plant and stop overhead watering.
  2. Cut out all infected tissue with a sterile blade, well into healthy green tissue, sterilising the tool between cuts.
  3. Dust or paste the wound with a drying protectant; cinnamon powder and copper-based pastes are used by orchid growers.
  4. Systemic or protectant oomycete fungicide groups (for example phosphonates, phenylamides or etridiazole-type products) are used by growers; follow the product label.
  5. Discard plants that are badly rotted, along with their media.

Pros and cons

Cons

  • Can kill a plant within days if left unchecked.
  • Spreads easily between plants on a shared bench through splashing water.

Look-alikes

Bacterial soft rot
Bacterial rots are usually watery, translucent and foul-smelling; laboratory testing separates them reliably.
Cold damage
Cold damage appears after a chill as sunken, dark patches on several plants at once and does not keep spreading.
Sunburn
Sunburn is dry, bleached or brown and confined to the exposed part of the leaf.

Affected plants

Many orchids, especially Cattleya and its hybrids; monopodial orchids such as Vanda and Phalaenopsis are also affected.

Sources

  1. Black Rot — American Orchid Society
  2. Black rot on orchids — Wikipedia

Written in our own words from these sources. Check a product label before using any treatment.