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Bacterial soft rot

Also known as Erwinia soft rot, Pectobacterium soft rot

DiseaseHarmful
Water-soaked bacterial rot on an orchid leaf Photo: Scot Nelson · CC0

In short

Bacterial soft rot is a fast, wet decay caused mostly by Pectobacterium and Erwinia bacteria, and sometimes Pseudomonas. The bacteria dissolve plant tissue into a mushy, foul-smelling mass. On orchids such as Phalaenopsis it can destroy a plant within two to three days.

How it looks

The first sign is usually a water-soaked, translucent spot on a leaf, stem base, bulb or rhizome. It spreads quickly with a clearly advancing water-soaked edge; the tissue behind it turns soft, mushy and watery. . Infected tissue has a strong rotten or fishy smell, which is one of the best clues. On Dieffenbachia the lower leaves yellow and spots enlarge rapidly, sometimes leaving holes where the centres fall out. On orchids the rot runs from leaves into pseudobulbs and roots.

Signs

Rapidly spreading water-soaked patches; mushy, collapsing leaves or stem bases; oozing tissue; a foul smell; whole leaves or growths collapsing within days.

Life cycle

The bacteria survive in soil, plant debris and infected plant material. They enter through wounds and natural openings and spread by contact, contaminated tools, splashing water and sometimes insects. Once inside they release enzymes that break down cell walls.

What favours it

Warm, wet, poorly ventilated conditions. The bacteria grow from roughly 0–32 °C and multiply fastest at about 21–27 °C (70–80 °F), with high humidity; these are ordinary tropical conditions, so good airflow and dry leaves matter year-round. Wounds, water standing in leaf axils or crowns, and waterlogged media increase the risk.

Prevention

  • Water in the morning and keep water out of crowns and leaf axils; avoid misting susceptible plants.
  • Improve air circulation and space plants apart.
  • Avoid wounding plants; handle leaves, roots and bulbs gently.
  • Use fresh, well-drained potting mix, never garden soil, and disinfect tools and pots between plants.
  • Start from clean, healthy plants or tissue-cultured stock.

Treatment

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

  1. Act immediately: isolate the plant and cut out all soft tissue well into firm, healthy tissue with a sterilised blade, sterilising again between cuts.
  2. Let cut surfaces dry and keep the plant drier and well ventilated while it recovers.
  3. Discard plants where the rot has reached the crown, main stem or most of the plant; bin, do not compost, the debris.
  4. Copper-based bactericides may slow spread on remaining tissue but will not cure infected parts; follow the product label.

Pros and cons

Cons

  • Moves extremely fast; plants can be lost in days.
  • No curative treatment once tissue is infected.
  • Can spread to neighbouring plants through contact, water and tools.

Look-alikes

Crown and stem rot (fungal)
Fungal rots such as Fusarium stem rot can look similar; fungal rots are usually firmer and slower and lack the strong foul smell. Confirm before choosing a treatment.
Root rot
Root rot starts below ground and shows as wilting and yellowing; soft rot usually shows first as water-soaked spots on leaves or stem bases.
Cold damage
Chilled tissue can look water-soaked but appears across many leaves at once after cold exposure and does not smell of rot.

Affected plants

Very wide host range: orchids (especially Phalaenopsis), Dieffenbachia, and many vegetables and tropical crops such as potato, sweet potato, carrot, cabbage, onion, pepper, tomato and banana.

Sources

  1. Bacterial soft rot — Wikipedia
  2. Bacteria — American Orchid Society
  3. PP286: Dieffenbachia Diseases: Identification and Control — UF/IFAS
  4. Houseplant Problems — UC Statewide IPM Program

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